Thermoelectric Technical Reference — Introduction to Thermoelectric Cooling
Ferrotec's Thermoelectric Technical Reference Guide is a comprehensive technical explanation of thermoelectrics and thermoelectric technology.
1.0 Introduction to Thermoelectric Cooling
1.1 A thermoelectric (TE) cooler, sometimes called a thermoelectric module or Peltier cooler, is a semiconductor-based electronic component that functions as a small heat pump. By applying a low voltage DC power source to a TE module, heat will be moved through the module from one side to the other. One module face, therefore, will be cooled while the opposite face simultaneously is heated. It is important to note that this phenomenon may be reversed whereby a change in the polarity (plus and minus) of the applied DC voltage will cause heat to be moved in the opposite direction. Consequently, a thermoelectric module may be used for both heating and cooling thereby making it highly suitable for precise temperature control applications.
1.1.1 To provide the new user with a general idea of a thermoelectric cooler's capabilities, it might be helpful to offer this example. If a typical single-stage thermoelectric module was placed on a heat sink that was maintained at room temperature and the module was then connected to a suitable battery or other DC power source, the "cold" side of the module would cool down to approximately -40°C. At this point, the module would be pumping almost no heat and would have reached its maximum rated "DeltaT (DT)." If heat was gradually added to the module's cold side, the cold side temperature would increase progressively until it eventually equaled the heat sink temperature. At this point the TE cooler would have attained its maximum rated "heat pumping capacity" (Qmax).
1.2 Both thermoelectric coolers and mechanical refrigerators are governed by the same fundamental laws of thermodynamics and both refrigeration systems, although considerably different in form, function in accordance with the same principles.
In a mechanical refrigeration unit, a compressor raises the pressure of a liquid and circulates the refrigerant through the system. In the evaporator or "freezer" area the refrigerant boils and, in the process of changing to a vapor, the refrigerant absorbs heat causing the freezer to become cold. The heat absorbed in the freezer area is moved to the condenser where it is transferred to the environment from the condensing refrigerant. In a thermoelectric cooling system, a doped semiconductor material essentially takes the place of the liquid refrigerant, the condenser is replaced by a finned heat sink, and the compressor is replaced by a DC power source. The application of DC power to the thermoelectric module causes electrons to move through the semiconductor material. At the cold end (or "freezer side") of the semiconductor material, heat is absorbed by the electron movement, moved through the material, and expelled at the hot end. Since the hot end of the material is physically attached to a heat sink, the heat is passed from the material to the heat sink and then, in turn, transferred to the environment.
1.3 The physical principles upon which modern thermoelectric coolers are based actually date back to the early 1800's, although commercial TE modules were not available until almost 1960. The first important discovery relating to thermoelectricity occurred in 1821 when a German scientist, Thomas Seebeck, found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals provided that the junctions of the metals were maintained at two different temperatures. Seebeck did not actually comprehend the scientific basis for his discovery, however, and falsely assumed that flowing heat produced the same effect as flowing electric current. In 1834, a French watchmaker and part time physicist, Jean Peltier, while investigating the "Seebeck Effect," found that there was an opposite phenomenon whereby thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit. Twenty years later, William Thomson (eventually known as Lord Kelvin) issued a comprehensive explanation of the Seebeck and Peltier Effects and described their interrelationship. At the time, however, these phenomena were still considered to be mere laboratory curiosities and were without practical application.
In the 1930's Russian scientists began studying some of the earlier thermoelectric work in an effort to construct power generators for use at remote locations throughout the country. This Russian interest in thermoelectricity eventually caught the attention of the rest of the world and inspired the development of practical thermoelectric modules. Today's thermoelectric coolers make use of modern semiconductor technology whereby doped semiconductor material takes the place of dissimilar metals used in early thermoelectric experiments.
1.4 The Seebeck, Peltier, and Thomson Effects, together with several other phenomena, form the basis of functional thermoelectric modules. Without going into too much detail, we will examine some of these fundamental thermoelectric effects.
1.4.1 SEEBECK EFFECT: To illustrate the Seebeck Effect let us look at a simple thermocouple circuit as shown in Figure (1.1). The thermocouple conductors are two dissimilar metals denoted as Material x and Material y.
In a typical temperature measurement application, thermocouple A is used as a "reference" and is maintained at a relatively cool temperature of Tc. Thermocouple B is used to measure the temperature of interest (Th) which, in this example, is higher than temperature Tc. With heat applied to thermocouple B, a voltage will appear across terminals Tl and T2. This voltage (Vo), known as the Seebeck emf, can be expressed as: Vo = axy x (Th - Tc)
Vo is the output voltage in volts
axy is the differential Seebeck coefficient between the two materials, x and y, in volts/oK
Th and Tc are the hot and cold thermocouple temperatures, respectively, in oK
1.4.2 PELTIER EFFECT: If we modify our thermocouple circuit to obtain the configuration shown in Figure (1.2), it will be possible to observe an opposite phenomenon known as the Peltier Effect.
If a voltage (Vin) is applied to terminals Tl and T2 an electrical current (I) will flow in the circuit. As a result of the current flow, a slight cooling effect (Qc) will occur at thermocouple junction A where heat is absorbed and a heating effect (Qh) will occur at junction B where heat is expelled. Note that this effect may be reversed whereby a change in the direction of electric current flow will reverse the direction of heat flow. The Peltier effect can be expressed mathematically as:
Qc or Qh=pxy x I
Where: pxy is the differential Peltier coefficient between the two materials, x and y, in volts I is the electric current flow in amperes Qc, Qh is the rate of cooling and heating, respectively, in watts
Joule heating, having a magnitude of I x R (where R is the electrical resistance), also occurs in the conductors as a result of current flow. This Joule heating effect acts in opposition to the Peltier effect and causes a net reduction of the available cooling.
1.4.3 THOMSON EFFECT: When an electric current is passed through a conductor having a temperature gradient over its length, heat will be either absorbed by or expelled from the conductor. Whether heat is absorbed or expelled depends upon the direction of both the electric current and temperature gradient. This phenomenon, known as the Thomson Effect, is of interest in respect to the principles involved but plays a negligible role in the operation of practical thermoelectric modules. For this reason, it is ignored.
Thermoelectric Technical Reference — Basic Principles of Thermoelectric Materials
Ferrotec's Thermoelectric Technical Reference Guide is a comprehensive technical explanation of thermoelectrics and thermoelectric technology.
2.0 Basic Principles of Thermoelectric Modules & Materials
2.1 THERMOELECTRIC MATERIALS: The thermoelectric semiconductor material most often used in today's TE coolers is an alloy of Bismuth Telluride that has been suitably doped to provide individual blocks or elements having distinct "N" and "P" characteristics. Thermoelectric materials most often are fabricated by either directional crystallization from a melt or pressed powder metallurgy. Each manufacturing method has its own particular advantage, but directionally grown materials are most common. In addition to Bismuth Telluride (Bi2Te3), there are other thermoelectric materials including Lead Telluride (PbTe), Silicon Germanium (SiGe), and Bismuth-Antimony (Bi-Sb) alloys that may be used in specific situations. Figure (2.1) illustrates the relative performance or Figure-of-Merit of various materials over a range of temperatures. It can be seen from this graph that the performance of Bismuth Telluride peaks within a temperature range that is best suited for most cooling applications.
APPROXIMATE FIGURE-OF-MERIT(Z)FOR VARIOUS TE MATERIALS
Figure (2.1) Performance of Thermoelectric Materials at Various Temperatures
2.1.1 BISMUTH TELLURIDE MATERIAL: Crystalline Bismuth Telluride material has several characteristics that merit discussion. Due to the crystal structure, Bi2Te3 is highly anisotropic in nature. This results in the material's electrical resistivity being approximately four times greater parallel to the axis of crystal growth (C-axis) than in the perpendicular orientation. In addition, thermal conductivity is about two times greater parallel to the C-axis than in the perpendicular direction. Since the anisotropic behavior of resistivity is greater than that of thermal conductivity, the maximum performance or Figure-of-Merit occurs in the parallel orientation. Because of this anisotropy, thermoelectric elements must be assembled into a cooling module so that the crystal growth axis is parallel to the length or height of each element and, therefore, perpendicular to the ceramic substrates.
There is one other interesting characteristic of Bismuth Telluride that also is related to the material's crystal structure. Bi2Te3 crystals are made up of hexagonal layers of similar atoms.
While layers of Bismuth and Tellurium are held together by strong covalent bonds, weak van der Waals bonds link the adjoining [Te¹] layers. As a result, crystalline Bismuth Telluride cleaves readily along these [Te¹][Te¹] layers, with the behavior being very similar to that of Mica sheets. Fortunately, the cleavage planes generally run parallel to the C-axis and the material is quite strong when assembled into a thermoelectric cooling module.
2.1.2 Bismuth Telluride material, when produced by directional crystallization from a melt, typically is fabricated in ingot or boule form and then sliced into wafers of various thicknesses. After the wafer's surfaces have been properly prepared, the wafer is then diced into blocks that may be assembled into thermoelectric cooling modules. The blocks of Bismuth Telluride material, which usually are called elements or dice, also may be manufactured by a pressed powder metallurgy process.
2.2 THERMOELECTRIC COOLING MODULES: A practical thermoelectric cooler consists of two or more elements of semiconductor material that are connected electrically in series and thermally in parallel. These thermoelectric elements and their electrical interconnects typically are mounted between two ceramic substrates. The substrates serve to hold the overall structure together mechanically and to insulate the individual elements electrically from one another and from external mounting surfaces. After integrating the various component parts into a module, thermoelectric modules ranging in size from approximately 2.5-50 mm (0.1 to 2.0 inches) square and 2.5-5mm (0.1 to 0.2 inches) in height may be constructed.
Figure (2.2) Schematic Diagram of a Typical Thermoelectric Cooler
2.2.1 Both N-type and P-type Bismuth Telluride thermoelectric materials are used in a thermoelectric cooler. This arrangement causes heat to move through the cooler in one direction only while the electrical current moves back and forth alternately between the top and bottom substrates through each N and P element. N-type material is doped so that it will have an excess of electrons (more electrons than needed to complete a perfect molecular lattice structure) and P-type material is doped so that it will have a deficiency of electrons (fewer electrons than are necessary to complete a perfect lattice structure). The extra electrons in the N material and the "holes" resulting from the deficiency of electrons in the P material are the carriers which move the heat energy through the thermoelectric material. Figure (2.2) shows a typical thermoelectric cooler with heat being moved as a result of an applied electrical current (I). Most thermoelectric cooling modules are fabricated with an equal number of N-type and P-type elements where one N and P element pair form a thermoelectric "couple." The module illustrated in Figure (2.2) has two pairs of N and P elements and is termed a "two-couple module".
Heat flux (heat actively pumped through the thermoelectric module) is proportional to the magnitude of the applied DC electric current. By varying the input current from zero to maximum, it is possible to adjust and control the heat flow and temperature.
3.0 Applications for Thermoelectric Coolers
3.1 Applications for thermoelectric modules cover a wide spectrum of product areas. These include equipment used by military, medical, industrial, consumer, scientific/laboratory, and telecommunications organizations. Uses range from simple food and beverage coolers for an afternoon picnic to extremely sophisticated temperature control systems in missiles and space vehicles.
Unlike a simple heat sink, a thermoelectric cooler permits lowering the temperature of an object below ambient as well as stabilizing the temperature of objects which are subject to widely varying ambient conditions. A thermoelectric cooler is an active cooling module whereas a heat sink provides only passive cooling.
Thermoelectric coolers generally may be considered for applications that require heat removal ranging from milliwatts up to several thousand watts. Most single-stage TE coolers, including both high and low current modules, are capable of pumping a maximum of 3 to 6 watts per square centimeter (20 to 40 watts per square inch) of module surface area. Multiple modules mounted thermally in parallel may be used to increase total heat pump performance. Large thermoelectric systems in the kilowatt range have been built in the past for specialized applications such as cooling within submarines and railroad cars. Systems of this magnitude are now proving quite valuable in applications such as semiconductor manufacturing lines.
3.2 Typical applications for thermoelectric modules include:
- Black Box Cooling
- CCD (Charged Couple Devices)
- CID (Charge Induced Devices)
- Cold Chambers
- Cold Plates
- Compact Heat Exchangers
- Constant Temperature Baths
- Dew Point Hygrometers
- Electronics Package Cooling
- Electrophoresis Cell Coolers
- Environmental Analyzers
- Heat Density Measurement
- Ice Point References
- Immersion Coolers
- Integrated Circuit Cooling
- Inertial Guidance Systems
- Infrared Calibration Sources and Black Body References
- Infrared Detectors
- Infrared Seeking Missiles
- Laser Collimators
- Laser Diode Coolers
- Long Lasting Cooling Devices
- Low Noise Amplifiers
- Microprocessor Cooling
- Microtome Stage Coolers
- NEMA Enclosures
- Night Vision Equipment
- Parametric Amplifiers
- Photomultiplier Tube Housing
- Power Generators (small)
- Precision Device Cooling (Lasers and Microprocessors)
- Refrigerators and on-board refrigeration systems (Aircraft, Automobile, Boat, Hotel, Insulin, Portable/Picnic, Pharmaceutical, RV)
- Restaurant Portion Dispenser
- Self-Scanned Arrays Systems
- Semiconductor Wafer Probes
- Stir Coolers
- Thermal Viewers and Weapons Sights
- Thermal Cycling Devices (DNA and Blood Analyzers)
- Thermostat Calibrating Baths
- Tissue Preparation and Storage
- Vidicon Tube Coolers
- Wafer Thermal Characterization
- Water and Beverage Coolers
- Wet Process Temperature Controller
5.0 Heat Sink Considerations
5.1 Rather than being a heat absorber that consumes heat by magic, a thermoelectric cooler is a heat pump which moves heat from one location to another. When electric power is applied to a TE module, one face becomes cold while the other is heated. In accordance with the laws of thermodynamics, heat from the (warmer) area being cooled will pass from the cold face to the hot face. To complete the thermal system, the hot face of the TE cooler must be attached to a suitable heat sink that is capable of dissipating both the heat pumped by the module and Joule heat created as a result of supplying electrical power to the module.
A heat sink is an integral part of a thermoelectric cooling system and its importance to total system performance must be emphasized. Since all operational characteristics of TE devices are related to heat sink temperature, heat sink selection and design should be considered carefully.
A perfect heat sink would be capable of absorbing an unlimited quantity of heat without exhibiting any increase in temperature. Since this is not possible in practice, the designer must select a heat sink that will have an acceptable temperature rise while handling the total heat flow from the TE device(s). The definition of an acceptable increase in heat sink temperature necessarily is dependent upon the specific application, but because a TE module's heat pumping capability decreases with increasing temperature differential, it is highly desirable to minimize this value. A heat sink temperature rise of 5 to 15°C above ambient (or cooling fluid) is typical for many thermoelectric applications.
Several types of heat sinks are available including natural convection, forced convection, and liquid-cooled. Natural convection heat sinks may prove satisfactory for very low power applications especially when using small TE devices operating at 2 amperes or less. For the majority of applications, however, natural convection heat sinks will be unable to remove the required amount of heat from the system, and forced convection or liquid-cooled heat sinks will be needed.
Heat sink performance usually is specified in terms of thermal resistance (Q):
Ts - Ta
Qs = Thermal Resistance in Degrees C per Watt
Ts = Heat Sink Temperature in Degrees C
Ta =Ambient or Coolant Temperature in Degrees C
Q = Heat Input to Heat Sink in Watts
5.2 Each thermoelectric cooling application will have a unique heat sink requirement and frequently there will be various mechanical constraints that may complicate the overall design. Because each case is different, it is virtually impossible to suggest one heat sink configuration suitable for most situations. We have several off the shelf heat sinks and liquid heat exchangers appropriate for many applications but encourage you to contact our engineering department.
Note that when combining thermoelectric cooling modules and heat sinks into a total thermal system, it normally is NOT necessary to take into account heat loss or temperature rise at the module to heat sink junctions. Module performance data presented herein already includes such losses based on the use of thermal grease at both hot and cold interfaces. When using commercially available heat sinks for thermoelectric cooler applications, it is important to be aware that some off-the-shelf units do not have adequate surface flatness. A flatness of 1mm/m (0.001 in/in) or better is recommended for satisfactory thermal performance and it may be necessary to perform an additional lapping, flycutting, or grinding operation to meet this flatness specification.
5.2.1 NATURAL CONVECTION HEAT SINKS: Natural convection heat sinks normally are useful only for low power applications where very little heat is involved. Although it is difficult to generalize, most natural convection heat sinks have a thermal resistance (Qs) greater than 0.5°C/watt and often exceeding 10°C/watt. A natural convection heat sink should be positioned so that (a) the long dimension of the fins is in the direction of normal air flow, vertical operation improves natural convection and (b) there are no significant physical obstructions to impede air flow. It also is important to consider that other heat generating components located near the heat sink may increase the ambient air temperature, thereby affecting overall performance.
5.2.2 FORCED CONVECTION HEAT SINKS: Probably the most common heat-sinking
method used with thermoelectric coolers is forced convection. When compared to natural convection heat sinks, substantially better performance can be realized. The thermal resistance of quality forced convection systems typically falls within a range of 0.02 to 0.5°C/watt. Many standard heat sink extrusions are available that, when coupled with a suitable fan, may be used to form the basis of a complete cooling assembly. Cooling air may be supplied from a fan or blower and may either be passed totally through the length of the heat sink or may be directed at the center of the fins and pass out both open ends. This second air flow pattern, illustrated in Figure (5.l), generally provides the best performance since the air blown into the face of the heat sink creates greater turbulence resulting in improved heat transfer. For optimum performance, the housing of an axial fan should be mounted a distance of 8-20mm (0.31-0.75") from the fins. Other configurations may be considered depending on the application.
Figure (5.1) Forced Convection Heat Sink System Showing Preferred Air Flow
The thermal resistance of heat sink extrusions often is specified at an air flow rate stated in terms of velocity whereas the output of most fans is given in terms of volume. The conversion from volume to velocity is:
Velocity = Volume / Cross-sectional Area of Air Passage
or: Linear Feet per Minute = Cubic Feet per Minute / Area in Square Feet
or: Linear Meters per Minute = Cubic Meters per Minute / Area in Square Meters
5.2.3 LIQUID COOLED HEAT SINKS: Liquid cooled heat sinks provide the highest thermal performance per unit volume and, when optimally designed, can exhibit a very low thermal resistance. Although there are many exceptions, the thermal resistance of liquid cooled heat sinks typically falls between 0.01 and 0.1°C/watt. Simple liquid heat sinks can be constructed by soldering copper tubing onto a flat copper plate or by drilling holes in a metal block through which water may pass. With greater complexity (and greater thermal performance), an elaborate serpentine water channel may be milled in a copper or aluminum block that later is sealed-off with a cover plate. We offer several liquid-type heat sinks that may be used advantageously in thermoelectric systems. With other commercial heat sinks, always check the surface flatness prior to installation. While liquid cooling may be considered undesirable and/or unsatisfactory for many applications, it may be the only viable approach in specific situations.
6.0 Installation of Thermoelectric Modules
This section of the technical reference guide explaines the techniques that can used to install or mount a thermoelectric module or peltier cooler including:
» Bonding with Epoxy
» Mounting Pads and other Material
6.1 Important Installation Considerations
Techniques used to install thermoelectric modules in a cooling system are extremely important. Failure to observe certain basic principles may result in unsatisfactory performance or reliability. Some of the factors to be considered in system design and module installation include the following:
- Thermoelectric modules have high mechanical strength in the compression mode but shear strength is relatively low. As a result, a TE cooler should not be designed into a system where it serves as a significant supporting member of the mechanical structure.
- All interfaces between system components must be flat, parallel, and clean to minimize thermal resistance. High conductivity thermal interface material is often used to ensure good contact between surfaces.
- The "hot" and "cold" sides of standard thermoelectric modules may be identified by the position of the wire leads. Wires are attached to the hot side of the module, which is the module face that is in contact with the heat sink. For modules having insulated wire leads, when the red and black leads are connected to the respective positive and negative terminals of a DC power supply, heat will be pumped from the module's cold side, through the module, and into the heat sink. Note that for TE modules having bare wire leads, the positive connection is on the right side and the negative connection is on the left when the leads are facing toward the viewer and the substrate with the leads attached presented on the bottom.
- When cooling below ambient, the object being cooled should be insulated as much as possible to minimize heat loss to the ambient air. To reduce convective losses, fans should not be positioned so that air is blowing directly at the cooled object. Conductive losses also may be minimized by limiting direct contact between the cooled object and external structural members.
- When cooling below the dew point, moisture or frost will tend to form on exposed cooled surfaces. To prevent moisture from entering a TE module and severely reducing its thermal performance, an effective moisture seal should be installed. This seal should be formed between the heat sink and cooled object in the area surrounding the TE module(s). Flexible foam insulating tape or sheet material and/or silicone rubber RTV are relatively easy to install and make an effective moisture seal. Several methods for mounting thermoelectric modules are available and the specific product application often dictates the method to be used. Possible mounting techniques are outlined in the following paragraphs.
6.1.1 HEIGHT TOLERANCE: Most thermoelectric cooling modules are available with two height tolerance values, +/-0.3mm (+/-0.010") and +/-0.03mm (0.001"). When only one module is used in a thermoelectric subassembly, a +/-0.3mm tolerance module generally is preferable since it provides a slight cost advantage over a comparable tight-tolerance module. For applications where more than one module is to be mounted between the heat sink and cooled object, however, it is necessary to closely match the thickness of all modules in the group to ensure good heat transfer. For this reason, +/-0.03mm (+/-0.001") tolerance modules should be used in all multiple-module configurations.
The most common mounting method involves clamping the thermoelectric module(s) between a heat sink and flat surface of the article to be cooled. This approach, as illustrated in Figure (6.1), usually is recommended for most applications and may be applied as follows:
a) Machine or grind flat the mounting surfaces between which the TE module(s) will be located. To achieve optimum thermal performance mounting surfaces should be flat to within 1mm/m (0.001 in/in).
b) If several TE modules are mounted between a given pair of mounting surfaces, all modules within the group must be matched in height/thickness so that the overall thickness variation does not exceed 0.06mm (0.002"). Module P/N with a "B" ending should be specified.
|c) Mounting screws should be arranged in a symmetrical pattern relative to the module(s) so as to provide uniform pressure on the module(s) when the assembly is clamped together. To minimize heat loss through the mounting screws, it is desirable to use the smallest size screw that is practical for the mechanical system. For most applications, M3 or M3.5 (4-40 or 6-32) stainless steel screws will prove satisfactory. Alternately, nonmetallic fasteners can be used, e.g., nylon. Smaller screws may be used in conjunction with very small mechanical assemblies. Belleville spring washers or split lock-washers should be used under the head of each screw to maintain even pressure during the normal thermal expansion or contraction of system components.
d) Clean the module(s) and mounting surfaces to ensure that all burrs, dirt, etc., have been removed.
e) Coat the "hot" side of the module(s) with a thin layer (typically 0.02mm / 0.001" or less thickness) of thermally conductive grease and place the module, hot side down, on the heat sink in the desired location. Gently push down on the module and apply a back and forth turning motion to squeeze out excess thermal grease. Continue the combined downward pressure and turning motion until a slight resistance is detected. Ferrotec America recommends and stocks American Oil and Supply (AOS) type 400 product code 52032.
f) Coat the "cold" side of the module(s) with thermal grease as specified in step (e) above. Position and place the object to be cooled in contact with the cold side of the module(s). Squeeze out the excess thermal grease as previously described.
g) Bolt the heat sink and cooled object together using the stainless steel screws and spring washers. It is important to apply uniform pressure across the mounting surfaces so that good parallelism is maintained. If significantly uneven pressure is applied, thermal performance may be reduced, or worse, the TE module(s) may be damaged. To ensure that pressure is applied uniformly, first tighten all mounting screws finger tight starting with the center screw (if any). Using a torque screwdriver, gradually tighten each screw by moving from screw to screw in a crosswise pattern and increase torque in small increments. Continue the tightening procedure until the proper torque value is reached. Typical mounting pressure ranges from 25 - 100 psi depending on the application. If a torque screwdriver is not available, the correct torque value may be approximated by using the following procedure:
In a crosswise pattern, tighten the screws until they are "snug" but not actually tight. In the same crosswise pattern, tighten each screw approximately one quarter turn until the spring action of the washer can be felt.
h) A small additional amount of thermal grease normally is squeezed out soon after the assembly is first clamped together. In order to insure that the proper screw torque is maintained, wait a minimum of one hour and recheck the torque by repeating step (g) above.
i) CAUTION: Over-tightening of the clamping screws may result in bending or bowing of either the heat sink or cold object surface especially if these components are constructed of relatively thin material. Such bowing will, at best, reduce thermal performance and in severe cases may cause physical damage to system components. Bowing may be minimized by positioning the clamping screws close to the thermoelectric module(s) and by using moderately thick materials. However, if hot and/or cold surfaces are constructed of aluminum which is less than 6mm (0.25") thick or copper which is less than 3.3mm (0.13") thick, it may be necessary to apply screw torque of a lower value than specified in step (g) above.
TE Module Installation Using the Clamping Method The proper bolt torque for TE module assemblies can be determined by the following relationship:
T=((Sa x A)/N) x K x d
|torque on each bolt|
|cycling 25-50 psi, static 50-75 psi.|
|total surface area of module(s)|
|number of bolts used in assembly|
|torque coefficient (use K=0.2 for steel, K=0.15 for nylon)|
|nominal bolt diameter|
For steel fasteners, we typically recommend either:
6-32 d=.138 in (.350 cm)
4-40 d=.112 in (.284 cm)
The following recommended torque is calculated for nine 9500/065/018 modules held by four 4-40 steel fasteners:
T=((75 lbs/in.2 x (.44" x .48") x 9)/4)x 0.2 x .112 in. = 0.8 in-lbs.
6.3 BONDING WITH EPOXY
A second module mounting method that is useful for certain applications involves bonding the module(s) to one or both mounting surfaces using a special high thermal-conductivity epoxy adhesive. Since the coefficients of expansion of the module's ceramics, heat sink and cooled object vary, we do not recommend bonding with epoxy for larger modules. Please consult your applications engineer for guidance. Note: Unless suitable procedures are used to prevent outgassing, epoxy bonding is not recommended if the TE cooling system is to be used in a vacuum. For module mounting with epoxy:
a) Machine or grind flat the mounting surfaces between which the TE module(s) will be located. Although surface flatness is less critical when using epoxy, it is always desirable for mounting surfaces to be as flat as possible.
b) Clean and degrease the module(s) and mounting surfaces to insure that all burrs, oil, dirt, etc., have been removed. Follow the epoxy manufacturer's recommendations regarding proper surface preparation.
c) Coat the hot side of the module with a thin layer of the thermally conductive epoxy and place the module, hot side down, on the heat sink in the desired location. Gently push down on the module and apply a back and forth turning motion to squeeze out excess epoxy. Continue the combined downward pressure and turning motion until a slight resistance is detected.
d) Weight or clamp the module in position until the epoxy has properly cured. Consult the epoxy manufacturer's data sheet for specific curing information. If an oven cure is specified, be sure that the maximum operating temperature of the TE module is not exceeded during the heating procedure. Note that most TE cooling modules manufactured by Ferrotec America have maximum operating temperatures of either 150°C for the ValueTEC™ Series or 200°C for the SuperTEC™ Series.
Thermoelectric modules that have metallized external faces may be soldered into an assembly provided that reasonable care is taken to prevent module overheating. Soldering to a rigid structural member of an assembly should be performed on one side of the module only (normally the hot side) in order to avoid excessive mechanical stress on the module. Note that with a module's hot side soldered to a rigid body, however, a component or small electronic circuit may be soldered to the module's cold side provided that the component or circuit is not rigidly coupled to the external structure. Good temperature control must be maintained within the soldering system in order to prevent damage to the TE module due to overheating. Our thermoelectric modules are rated for continuous operation at relatively high temperatures (150 or 200°C) so they are suitable in most applications where soldering is desirable. Naturally these relative temperatures should not be exceeded in the process. Since the coefficients of expansion of the module ceramics, heat sink and cooled object vary, we do not recommend soldering modules larger than 15 x 15 millimeters. Soldering should not be considered in any thermal cycling application. For module mounting with solder, the following steps should be observed:
a) Machine or grind flat the mounting surface on which the module(s) will be located. Although surface flatness is not highly critical with the soldering method, it is always desirable for mounting surfaces to be as flat as possible. Obviously, the heat sink surface must be a solderable material such as copper or copper plated material.
b) Clean and degrease the heat sink surface and remove any heavy oxidation. Make sure that there are no burrs, chips, or other foreign material in the module mounting area.
c) Pre tin the heat sink surface in the module mounting area with the appropriate solder. The selected solder must have a melting point that is less than or equal to the rated maximum operating temperature of the TE device being installed. When tinning the heat sink with solder, the heat sink's temperature should be just high enough so that the solder will melt but in no case should the temperature be raised more than the maximum value specified for the TE device.
d) Apply soldering flux to the TE module's hot side and place the module on the pre tinned area of the heat sink. Allow the module to "float" in the solder pool and apply a back and forth turning motion on the module to facilitate solder tinning of the module surface. A tendency for the module to drag on the solder surface rather than to float is an indication that there is an insufficient amount of solder. In this event, remove the module and add more solder to the heat sink.
e) After several seconds the module surface should be tinned satisfactorily. Clamp or weight the module in the desired position, remove the heat sink from the heat source, and allow the assembly to cool. When sufficiently cooled, degrease the assembly to remove flux residue.
6.5 Mounting Pads And Other Material
There are a wide variety of products available designed to replace thermally conductive grease as an interface material. Perhaps the most common are silicon-based mounting pads. Originally for use in mounting semiconductor devices, these pads often exhibit excessive thermal resistance in thermoelectric applications. Since the pads allow for rapid production and eliminate cleanup time, they are popular in less demanding applications. Leading manufacturers in this area include The Bergquist Company and the Chomerics Division of Parker Hannifin Corporation.
7.0 Power Supply Requirements
7.1 Thermoelectric coolers operate directly from DC power suitable power sources can range from batteries to simple unregulated "brute force" DC power supplies to extremely sophisticated closed-loop temperature control systems. A thermoelectric cooling module is a low-impedance semiconductor device that presents a resistive load to its power source. Due to the nature of the Bismuth Telluride material, modules exhibit a positive resistance temperature coefficient of approximately 0.5 percent per degree C based on average module temperature. For many noncritical applications, a lightly filtered conventional battery charger may provide adequate power for a TE cooler provided that the AC ripple is not excessive. Simple temperature control may be obtained through the use of a standard thermostat or by means of a variable-output DC power supply used to adjust the input power level to the TE device. In applications where the thermal load is reasonably constant, a manually adjustable DC power supply often will provide temperature control on the order of +/- 1°C over a period of several hours or more. Where precise temperature control is required, a closed-loop (feedback) system generally is used whereby the input current level or duty cycle of the thermoelectric device is automatically controlled. With such a system, temperature control to +/- 0.1°C may be readily achieved and much tighter control is not unusual.
7.2 Power supply ripple filtering normally is of less importance for thermoelectric devices than for typical electronic applications. However we recommend limiting power supply ripple to a maximum of 10 percent with a preferred value being < 5%.
7.2.1 Multistage cooling and low-level signal detection are two applications which may require lower values of power supply ripple. In the case of multistage thermoelectric devices, achieving a large temperature differential is the typical goal, and a ripple component of less than two percent may be necessary to maximize module performance. In situations where very low level signals must be detected and/or measured, even though the TE module itself is electrically quiet, the presence of an AC ripple signal within the module and wire leads may be unsatisfactory. The acceptable level of power supply ripple for such applications will have to be determined on a case-by-case basis.
7.3 Figure (7.1) illustrates a simple power supply capable of driving a 71-couple, 6-ampere module. This circuit features a bridge rectifier configuration and capacitive-input filter. With suitable component changes, a full-wave-center-tap rectifier could be used and/or a filter choke added ahead of the capacitor. A switching power supply, having a size and weight advantage over a comparable linear unit, also is appropriate for powering thermoelectric devices.
Simple Power Supply to Drive a 71-Couple, 6-Ampere TE Module
7.4 A typical analog closed-loop temperature controller is illustrated in Figure (7.2). This system is capable of closely controlling and maintaining the temperature of an object and will automatically correct for temperature variations by means of the feedback loop. Many variations of this system are possible including adaptation to digital and/or computer control.
Block Diagram of a Typical Closed-Loop Temperature Controller
8.0 Thermal System Design Considerations
8.1 The first step in the design of a thermoelectric cooling system involves making an analysis of the system's overall thermal characteristics. This analysis, which may be quite simple for some applications or highly complex in other cases, must never be neglected if a satisfactory and efficient design is to be realized. Some of the more important factors to be considered are discussed in the following paragraphs. Although we have made certain simplifications that may horrify the pure thermodynamicist, the results obtained have been found to satisfy all but those few applications that approach state-of-the-art limits.
Please note that design information contained in this manual is presented for the purpose of assisting those engineers and scientists who wish either to estimate cooling requirements or to actually develop their own cooling systems. For the many individuals who prefer not to become involved in the details of the thermoelectric design process, however, we encourage you to contact us for assistance. Ferrotec America is committed to providing strong customer technical support and our engineering staff is available to assist in complex thermoelectric-related design activities.
8.2 ACTIVE HEAT LOAD: The active heat load is the actual heat generated by the component, "black box" or system to be cooled. For most applications, the active load will be equal to the electrical power input to the article being cooled (Watts = Volts x Amps) but in other situations the load may be more difficult to determine. Since the total electrical input power generally represents the worst case active heat load, we recommend that you use this value for design purposes.
8.3 PASSIVE HEAT LOAD: The passive heat load (sometimes called heat leak or parasitic heat load) is that heat energy which is lost or gained by the article being cooled due to conduction, convection, and/or radiation. Passive heat losses may occur through any heat-conductive path including air, insulation, and electrical wiring. In applications where there is no active heat generation, the passive heat leak will represent the entire heat load on the thermoelectric cooler.
Determination of the total heat leak within a cooling system is a relatively complicated issue but a reasonable estimate of these losses often can be made by means of some basic heat transfer calculations. If there is any uncertainty about heat losses in a given design, we highly recommend that you contact our engineering staff for assistance and suggestions.
8.4 HEAT TRANSFER EQUATIONS: Several fundamental heat transfer equations are presented to assist the engineer in evaluating some of the thermal aspects of a design or system.
8.4.1 HEAT CONDUCTION THROUGH A SOLID MATERIAL: The relationship that describes the transfer of heat through a solid material was suggested by J.B. Fourier in the early 1800's. Thermal conduction is dependent upon the geometry and thermal conductivity of a given material plus the existing temperature gradient through the material. Although thermal conductivity varies with temperature, the actual variation is quite small and can be neglected for our purposes. Mathematically, heat transfer by conduction may be expressed as:
||Heat Conducted Through the Material
||Thermal conductivity of the material
||Cross-sectional area of the material
||Thickness of length of the materials
||Temperature difference between cold and hot sides of the material
8.4.2 HEAT TRANSFER FROM AN EXPOSED SURFACE TO AMBIENT BY CONVECTION: Heat leak to or from an uninsulated metal surface can constitute a significant heat load in a thermal system. Isaac Newton proposed the relationship describing the transfer of heat when a cooled (or heated) surface is exposed directly to the ambient air. To account for the degree of thermal coupling between the surface and surrounding air, a numerical value (h), called the Heat Transfer Coefficient, must be incorporated into the equation. Heat lost or gained in this manner may be expressed mathematically as: Q=(h)(A)(DT)
||Heat transferred to or from ambient
||Heat transfer coefficient.
For still air use a value of:
For turbulent air use a value of:
4 to 5
15 to 20
23 to 28
85 to 113
||Area of the exposed surface
||Temperature difference between the exposed surface and ambient
8.4.3 HEAT TRANSFER THROUGH THE WALLS OF AN INSULATED ENCLOSURE: Heat leak to or from an insulated container combines an element of thermal conduction through the insulating material with an element of convection loss at the external insulation surfaces. Heat lost from (or gained by) an insulated enclosure may be expressed mathematically as:
||Heat conducted through the enclosure
||Thermal conductivity of the insulation
||External surface area of the enclosure
||Thickness of the insulation
||Temperature difference between the inside and outside of the enclosure
||Heat transfer coefficient
For still air use a value of:
For turbulent air use a value of:
4 to 5
15 to 20
23 to 28
85 to 113
8.4.4 TIME NEEDED TO CHANGE THE TEMPERATURE OF AN OBJECT: Determination of the time required to thermoelectrically cool or heat a given object is a moderately complicated matter. For good accuracy, it would be necessary to make a detailed analysis of the entire thermal system including all component parts and interfaces. By using the simplified method presented here, however, it is possible to make a reasonable estimate of a system's thermal transient response.
||Time period for temperature change
||Weight of material
||Specific heat of the material
||Temperature change of the material
||Heat transferred to or from material
Note (1): 1 Watt = 0.239 calories/second
Note (2):Thermoelectric modules pump heat at a rate that is proportional to the temperature difference (DT) across the module. In order to approximate actual module performance, the average heat removal rate should be used when determining the transient behavior of a thermal system. The average heat removal rate is:
Q = 0.5 (Qc at DTmin + Qc at DTmax)
Where: Qc at DTmin is the amount of heat a thermoelectric module is pumping at the initial object temperature when DC power is first applied to the module. The DT is zero at this time and the heat pumping rate is at the highest point.
Qc at DTmax is the amount of heat a thermoelectric module is pumping when the object has cooled to the desired temperature. The DT is higher at this time and the heat pumping rate is lower.
8.4.5 HEAT TRANSFER FROM A BODY BY RADIATION: Most thermoelectric cooling applications involve relatively moderate temperatures and small surface areas where radiation heat losses usually are negligible. Probably the only situation where thermal radiation may be of concern is that of a multistage cooler operating at a very low temperature and close to its lower limit. For such applications, it sometimes is possible to attach a small radiation shield to one of the lower module stages. By fabricating this shield so that it surrounds the upper stage and cooled object, thermal radiation losses may be reduced substantially.
As an indication of the magnitude of heat leak due to thermal radiation, consider the following. A perfect black-body having a surface area of 1.0 cm2 and operating at -100°C (173K) will receive approximately 43 milliwatts of heat from its 30°C (303K) surroundings. Be aware that the accurate determination of radiation loss is a highly complicated issue and a suitable heat transfer textbook should be consulted for detailed information. A very simplified estimation of such losses may be made, however, by using the equation given below.
QR=(s)(A) (e) (Th4 – Tc4)
||Radiation heat loss
||1.714 x 10-9
|5.67 x 10-8 |
||Area of the exposed surface
||Emissivity of exposed surfaces
||Absolute temperature of warmer surface
||Absolute temperature of colder surface
8.4.6 R-VALUE OF INSULATION: The R-value of an insulating material is a measure of the insulation's overall effectiveness or resistance to heat flow. R-value is not a scientific term, per se, but the expression is used extensively within the building construction industry in the United States. The relationship between R-value, insulation thickness, and thermal conductivity may be expressed by the equation:
x = Thickness of the insulation in inches
k = Thermal conductivity of the insulation in BTU/hr-ft-°F
Note: Insulation R-value normally is based on insulation thickness in inches. Since thermal conductivity values in Appendix B are expressed in feet, the k value in the equation's denominator has been multiplied by 12.
8.5 THERMAL INSULATION CONSIDERATIONS: In order to maximize thermal performance and minimize condensation, all cooled objects should be properly insulated. Insulation type and thickness often is governed by the application and it may not be possible to achieve the optimum insulation arrangement in all cases. Every effort should be made, however, to prevent ambient air from blowing directly on the cooled object and/or thermoelectric device.
Figures (8.1) and (8.2) illustrate the relationship between the heat leak from an insulated surface and the insulation thickness. It can be seen that even a small amount of insulation will provide a significant reduction in heat loss but, beyond a certain point, greater thicknesses give little benefit. The two heat leak graphs show heat loss in watts per square unit of surface area for a one degree temperature difference (DT) through the insulation. Total heat leak (Qtot) in watts for other surface areas (SA) or DT's may be calculated by the expression:
Qtot = Qleak x SA x DT
Heat Leak from an Insulated Surface in Metric Units
Heat Leak from an Insulated Surface in English Units
9.0 Thermoelectric Module Selection
9.1 Selection of the proper TE Cooler for a specific application requires an evaluation of the total system in which the cooler will be used. For most applications it should be possible to use one of the standard module configurations while in certain cases a special design may be needed to meet stringent electrical, mechanical, or other requirements. Although we encourage the use of a standard device whenever possible, Ferrotec America specializes in the development and manufacture of custom TE modules and we will be pleased to quote on unique devices that will exactly meet your requirements.
The overall cooling system is dynamic in nature and system performance is a function of several interrelated parameters. As a result, it usually is necessary to make a series of iterative calculations to "zero-in" on the correct operating parameters. If there is any uncertainty about which TE device would be most suitable for a particular application, we highly recommend that you contact our engineering staff for assistance.
Before starting the actual TE module selection process, the designer should be prepared to answer the following questions:
- At what temperature must the cooled object be maintained?
- How much heat must be removed from the cooled object?
- Is thermal response time important? If yes, how quickly must the cooled object change temperature after DC power has been applied?
- What is the expected ambient temperature? Will the ambient temperature change significantly during system operation?
- What is the extraneous heat input (heat leak) to the object as a result of conduction, convection, and/or radiation?
- How much space is available for the module and heat sink?
- What power is available?
- Does the temperature of the cooled object have to be controlled? If yes, to what precision?
- What is the expected approximate temperature of the heat sink during operation? Is it possible that the heat sink temperature will change significantly due to ambient fluctuations, etc.?
Each application obviously will have its own set of requirements that likely will vary in level of importance. Based upon any critical requirements that can not be altered, the designer's job will be to select compatible components and operating parameters that ultimately will form an efficient and reliable cooling system. A design example is presented in section 9.5 to illustrate the concepts involved in the typical engineering process.
9.2 USE OF TE MODULE PERFORMANCE GRAPHS: Before beginning any thermoelectric design activity it is necessary to have an understanding of basic module performance characteristics. Performance data is presented graphically and is referenced to a specific heat sink base temperature. Most performance graphs are standardized at a heat sink temperature (Th) of +50°C and the resultant data is usable over a range of approximately 40°C to 60°C with only a slight error. Upon request, we can supply module performance graphs referenced to any temperature within a range of -80°C to +200°C.
9.3 To demonstrate the use of these performance curves let us present a simple example. Suppose we have a small electronic "black box" that is dissipating 15 watts of heat. For the electronic unit to function properly its temperature may not exceed 20°C. The room ambient temperature often rises well above the 20°C level thereby dictating the use of a thermoelectric cooler to reduce the unit's temperature. For the purpose of this example we will neglect the heat sink (we cannot do this in practice) other than to state that its temperature can be maintained at 50°C under worst-case conditions. We will investigate the use of a 71-couple, 6-ampere module to provide the required cooling.
9.3.1 GRAPH: Qc vs. I This graph, shown in Figure (9.1), relates a module's heat pumping capacity (Qc) and temperature difference (DT) as a function of input current (I). In this example, established operating parameters for the TE module include Th = 50°C, Tc = 20°C, and Qc = 15 watts. The required DT = Th-Tc = 30°C.
It is necessary first to determine whether a single 71-couple, 6-ampere module is capable of providing sufficient heat removal to meet application requirements. We locate the DT=30 line and find that the maximum Qc value occurs at point A and with an input current of 6 amperes. By extending a line from point A to the left y-axis, we can see that the module is capable of pumping approximately 18 watts while maintaining a Tc of 20°C. Since this Qc is slightly higher than necessary, we follow the DT=30 line downward until we reach a position (point B) that corresponds to a Qc of 15 watts. Point B is the operating point that satisfies our thermal requirements. By extending a line downward from point B to the x-axis, we find that the proper input current is 4.0 amperes.
Heat Pumping Capacity Related to Temperature Differential as a Function of Input Current for a 71-Couple, 6-Ampere Module
9.3.2 GRAPH: Vin vs. I This graph, shown in Figure (9.2), relates a module's input voltage (Vin) and temperature difference (DT) as a function of input current (I). In this example, parameters for the TE module include Th = 50°C, DT = 30°C, and I = 4.0 amperes. We locate the DT=30 line and, at the 4.0 ampere intersection, mark point C. By extending a line from point C to the left y-axis, we can see that the required module input voltage (Vin) is approximately 6.7 volts.
Input Voltage Related to Temperature Differential as a
Function of Input Current for a 7I-Couple, 6-Ampere Module
9.3.3 GRAPH:COP vs. I This graph, shown in Figure (9.3), relates a module's coefficient of performance (COP) and temperature differential (DT) as a function of input current (I). In this example, parameters for the TE module include Th = 50°C, DT = 30°C, and I = 4.0 amperes.
We locate the DT=30 line and, at the 4.0 ampere intersection, mark point D. By extending a line from point D to the left y-axis, we can see that the module's coefficient of performance is approximately 0.58.
Coefficient of Performance Related to Temperature Differential as a
Function of Input Current for a 71-Couple, 6-Ampere Module
Note that COP is a measure of a module's efficiency and it is always desirable to maximize COP whenever possible. COP may be calculated by:
9.4 An additional graph of Vin vs. Th, of the type shown in Figure (9.4), relates a module's input voltage (Vin) and input current (I) as a function of module hot side temperature (Th). Due to the Seebeck effect, input voltage at a given value of I and Th is lowest when DT=O and highest when DT is at its maximum point. Consequently, the graph of Vin vs. Th usually is presented for a DT=30 condition in order to provide the average value of Vin.
Input Voltage Related to Input Current as a Function of
Hot Side Temperature for a 71-Couple, 6-Ampere Module
9.5 DESIGN EXAMPLE: To illustrate the typical design process let us present an example of a TE cooler application involving the temperature stabilization of a laser diode. The diode, along with related electronics, is to be mounted in a DIP Kovar housing and must be maintained at a temperature of 25°C. With the housing mounted on the system circuit board, tests show that the housing has a thermal resistance of 6°C/watt. The laser electronics dissipate a total of 0.5 watts and the design maximum ambient temperature is 35°C.
It is necessary to select a TE cooling module that not only will have sufficient cooling capacity to maintain the proper temperature, but also will meet the dimensional requirements imposed by the housing. An 18-couple, 1.2 ampere TE cooler is chosen initially because it does have compatible dimensions and also appears to have appropriate performance characteristics. Performance graphs for this module will be used to derive relevant parameters for making mathematical calculations. To begin the design process we must first evaluate the heat sink and make an estimate of the worst-case module hot side temperature (Th). For the TE cooler chosen, the maximum input power (Pin) can be determined from Figure (9.5) at point A.
- Max. Module Input Power (Pin) = 1.2 amps x 2.4 volts = 2.9 watts
- Max. Heat Input to the Housing = 2.9 watts + 0.5 watts = 3.4 watts
- Housing Temperature Rise = 3.4 watts x 6°C/watt = 20.4°C
- Max. Housing Temperature (T,) = 35°C ambient + 20.4°C rise = 55.4°C Since the hot side temperature (Th) of 55.4°C is reasonably close to the available Tin = 50°C performance graphs, these graphs may be used to determine thermal performance with very little error.
Vin vs. I Graph for an 18-Couple, I.2 Ampere Module
Now that we have established the worst-case Th value it is possible to assess module performance.
Module Temperature Differential (DT) = Th - Tc = 55.4 - 25 = 30°C
Qc vs. I Graph for an 18-Couple, 1.2 Ampere Module
From Figure (9.6) it can be seen that the maximum heat pumping rate (Qc) for DT=30 occurs at point B and is approximately 0.9 watts. Since a Qc of only 0.5 watts is needed, we can follow the DT=30 line down until it intersects the 0.5 watt line marked as point C. By extending a line downward from point C to the x-axis, we can see that an input current (I) of approximately 0.55 amperes will provide the required cooling performance. Referring back to the Vin vs. I graph in Figure (9.5), a current of 0.55 amperes, marked as point D, requires a voltage (Vin) of about 1.2 volts. We now have to repeat our analysis because the required input power is considerably lower than the value used for our initial calculation. The new power and temperature values will be:
- Max. Module Input Power (Pin) = 0.55 amps x 1.2 volts = 0.66 watts
- Max. Heat Input to the Housing = 0.66 watts + 0.50 watts = 1.16 watts
- Housing Temperature Rise = 1.16 watts x 6°C/watt = 7°C
- Max. Housing Temperature (Th) = 35°C ambient + 7°C rise = 42°C
Module Temperature Differential (DT) = Th-Tc = 42°C-25°C = 17°C
It can be seen that because we now have another new value for Th it will be necessary to continue repeating the steps outlined above until a stable condition is obtained. Note that calculations usually are repeated until the difference in the Th values from successive calculations is quite small (often less that 0.1°C for good accuracy). There is no reason to present the repetitive calculations here but we can conclude that the selected 18-couple TE module will function very well in this application. This analysis clearly shows the importance of the heat sink in any thermoelectric cooling application.
9.6 USE OF MULTIPLE MODULES: Relatively large thermoelectric cooling applications may require the use of several individual modules in order to obtain the required rate of heat removal. For such applications, TE modules normally are mounted thermally in parallel and connected electrically in series. An electrical series-parallel connection arrangement may also be used advantageously in certain instances. Because heat sink performance becomes increasingly important as power levels rise, be sure that the selected heat sink is adequate for the application.
10.0 Reliability of Thermoelectric Cooling Modules
10.1 INTRODUCTION: Thermoelectric cooling modules are considered to be highly reliable components due to their solid-state construction. For most applications they will provide long, trouble-free service. There have been many instances where TE modules have been used continuously for twenty or more years and the life of a module often exceeds the life of the associated equipment. The specific reliability of thermoelectric devices tends to be difficult to define, however, because failure rates are highly dependent upon the particular application. For applications involving relatively steady-state cooling where DC power is being applied to the module on more-or-less continuous and uniform basis, thermoelectric module reliability is extremely high. Mean Time Between Failures (MTBFs) in excess of 200,000 hours are not uncommon in such cases and this MTBF value generally is considered to be an industry standard. On the other hand, applications involving thermal cycling show significantly worse MTBFs especially when TE modules are cycled up to a high temperature.
The publishing of thermoelectric module reliability data entails some risk because there are numerous application parameters and conditions that will affect the end result. Although reliability data is valid for the conditions under which a test was conducted, it is not necessarily applicable to other configurations. Module assembly and mounting methods, power supply and temperature control systems and techniques, and temperature profiles, together with a host of external factors, can combine to produce failure rates ranging from extremely low to very high. In an effort to provide users with certain basic information about thermoelectric module life and to aid engineers in designing systems for optimum reliability, we instituted several test programs to acquire the necessary reliability data. Test results to date are presented for several situations that may be useful to end-users having similar or related applications. This data will be shared on a case-by-case basis depending on application and availability.
General requirements for the proper installation of thermoelectric modules may be found in Section 6 of this technical manual. It is important that modules are installed in accordance with these general requirements in order to minimize the possibility of premature module failure due to faulty assembly techniques. Some installation related factors that can affect module reliability include:
a) Thermoelectric modules exhibit relatively high mechanical strength in a compression mode but shear strength is comparatively low. A TE cooler should not be designed into a system where it serves as a major supporting member of the mechanical structure. Furthermore, in applications where severe shock and vibration will be present, a thermoelectric cooling module should be compression-mounted, i.e., installed by the clamping method. When properly mounted, thermoelectric coolers have successfully met the shock and vibration requirements of aerospace, military, and similar environments.
b) Although the maximum recommended compression loading for thermoelectric modules is 15 kilograms per square centimeter (200 pounds per square inch) of module surface area, tests have shown that well over 75 kilograms per square centimeter (1000 pounds per square inch) compression normally can be applied to most of our modules without causing failure. It is important to ensure that when modules are installed using the clamping method, sufficient pressure is maintained so that a module is not "loose" whereby it may easily be moved by applying a small sideways or lateral force. Loose modules may be a particular problem when several modules are grouped together in the same cooling assembly. In this situation, the lack of adequate clamping pressure may result in both reduced cooling performance and early module failure. When multiple modules are mounted in an array, modules with a close height tolerance of +/- .03mm (.001") are recommended. In all cases, clamping pressure must be applied uniformly and mating surfaces must be flat (see section 6 for Installation Guidelines).
c) A large unsupported mass should not be directly bonded to a module's cold surface to prevent the possible fracture of module components when subjected to significant mechanical shock. Where a large mass is involved, thermoelectric modules should be clamped between the heat sink and either the mass itself or an intermediate "cold plate" on which the mass is mounted. In this arrangement, the clamping screws will effectively increase shear strength of the overall mechanical system.
d) Moisture should not be allowed to enter the inside of a thermoelectric module in order to prevent both a reduction in cooling performance and the possible corrosion of module materials through electro-chemical action or electrolysis. When cooling below the dew point, a moisture seal should be provided either on the module itself or between the heat sink and cooled object in the area surrounding the TE module. An electronic-grade silicone rubber RTV may be used to directly seal a thermoelectric module. Flexible closed-cell foam insulating tape or sheet material, possibly combined with RTV to fill small gaps, may be used for a seal between the cold object and the heat sink.
e) When an application will involve large temperature changes or thermal cycling, thermoelectric modules should not be installed using solder or epoxy whereby an object is rigidly bonded to the module. Unless the thermal coefficients of expansion of all system components are similar, rigid bonding combined with temperature cycling often will result in early module failure due to the induced thermal stresses. Rigid bonding to the module's hot side generally is less of a problem because the hot side temperature tends to be relatively constant during operation. When significant temperature variation or temperature cycling is involved, we strongly recommend that modules be mounted by clamping (compression) using a flexible mounting material such as thermal grease or foils of graphite or indium. In addition, rigid mounting to both sides of modules is not recommended for devices larger than about 15mm (5/8") square.
Temperature control methods also have an impact on thermoelectric module reliability. Linear or proportional control should always be chosen over ON/OFF techniques when prolong life of the module is required.
10.2 MODULE RELIABILITY RELATED TO HIGH TEMPERATURE EXPOSURE
Thermoelectric module failures typically may be classified into two groups: catastrophic failures and degradation failures. Degradation failures tend to be long-term in nature and usually are caused by changes in semiconductor material parameters or increases in electrical contact resistance. High temperature exposure may lead to material parameter changes and, therefore, reduced thermoelectric performance. A test was conducted to study this effect. Several groups of ValueTEC™ Series (6300 Series) and SuperTEC™ Series (9500 Series) TE modules were subjected to long-term, continuous exposure to an elevated temperature of 150°C in a normal air atmosphere. During the test period, relevant module parameters were regularly measured and recorded. One parameter that is a good indicator of overall module performance is the maximum temperature differential (DTmax). This parameter was tracked over a 42-month period with the average value being shown the graph of Figure (10.1). It can be seen that a small (2.5%) decline in DTmax, with a decreasing rate of change, occurred in the first 12 months of high temperature exposure. In the remaining 30 months, however, the additional reduction in DTmax was only about 1.3% as semiconductor material characteristics stabilized. Note that for this test, there was no measurable difference in results between the ValueTEC™ Series (150°C rated) or the SuperTEC™ Series (200°C rated) modules.
10.3 MODULE RELIABILITY RELATED TO THERMAL CYCLING
The continuous thermal cycling of thermoelectric modules over a wide temperature range effectively constitutes a module "torture test," especially when the modules are raised to a relatively high temperature at one end of the cycle. Except for a few unusual applications, module failure rates are higher for this mode of operation than for any other operating condition. The basis for most thermal cycling failures is the unavoidable mismatch of thermal expansion coefficients of the various module components and materials. Such failures tend to be catastrophic in nature but some degradation normally may be observed prior to failure.
It is necessary, at this point, to define thermal cycling. Many thermoelectric applications involve the periodic raising and lowering of the control temperature, sometimes over a fairly wide temperature range. Although there often is not a well defined line between a cycling and noncycling application, thermal cycling usually is considered to be an operation where the temperature is regularly, and more or less continuously, raised and lowered over a long period of operation. A cycling application tends to suggest automatic or machine control of the temperature excursion as opposed to manual control. If the temperature of an apparatus is temperature-cycled up and down a few times each day, this generally is not considered to be a temperature-cycling application for the purpose of this discussion. If you are uncertain about the status of your particular application, please do not hesitate to contact us for assistance.
At least four factors relate to failure rate in thermal cycling including (1) the total number of cycles, (2) the total temperature excursion over the cycle, (3) the upper temperature limit of the cycle, and (4) the rate of temperature change. Highest reliability and module life is seen when the number of cycles is small, the temperature excursion or range is narrow, the upper temperature limit is relatively low and the rate of temperature change is minimalized. (Conversely, a large number of cycles over a wide temperature range with a rapid rate of change and a high temperature value on the up cycle results in significantly lower module life.) It is important to note that absolute module life is dependent upon the total number of cycles rather that the total time required to accrue those cycles. Consequently, when discussing thermal cycling, MTBF is best stated in terms of number of cycles instead of hours; we will take the liberty of using MTBF in this manner in the following discussion.
The type of module used in thermal cycling applications also is important in respect to failure rate. Modules rated at higher maximum operating temperatures provide substantially better life than do lower rated devices. This is true even though the upper temperature of the cycle is well below the maximum rated module temperature. In one application involving a two-stage thermoelectric assembly that was being cycled between -55°C and +125°C, a 150°C rated module provided a MTBF of 8100 cycles while a module rated at 200°C exhibited a MTBF of 17,500 cycles. Modules rated at even lower maximum operating temperatures should only be used for relatively low temperature cycling applications. In general, we recommend the SuperTEC series modules (rated for 200°C) be used for thermal cycling applications exceeding 90°C.
It should be mentioned that two other factors also may affect thermal cycling MTBFs. Physically smaller modules having fewer couples appear to provide improved life as do modules having larger elements or "dice." Sufficient data is available to suggest that modules having a size of 30mm (1.17") square or less exhibit better reliability in thermal cycling applications than do physically larger modules. Thermally induced mechanical stresses are greater in larger modules and such modules generally have a greater number of couples resulting in many more individual solder connections which may become fatigued by thermal stress.
In order to better define module failure rates under high temperature thermal cycling conditions, a test was conducted involving the continuous cycling of SuperTEC™ Series modules between +30°C and +100°C. Modules were mounted on a forced convection heat sink and covered with an insulated aluminum plate. Polarity of the applied DC power was alternately reversed to provide active heating and cooling and the cover-plate temperature was measured to determine cycling limits. The total time period of the cycle was 5 minutes (2.5 minutes from 30°C to 100°C and 2.5 minutes from 100°C to 30°C) resulting in 288 cycles per day or 2016 cycles per week. Module parameters were measured weekly and a failure was indicated by a sharp rise in electrical resistance.
Modules showed a slow and predictable rise in electrical resistance until reaching a point where a rapid resistance increase occurred indicating failure. All modules achieved a minimum of 25,000 cycles without failure, see Figure (10.2), and the test was continued until 50% of the modules failed. MTBF of the module group was calculated to be 68,000 cycles. Once again it is important to note that mounting methods, and overall assembly details are important factors when the application involves thermal cycling. Some applications have been tested between 5°C and 95°C exhibiting MTBF's over 100,000 cycles.
Figure (10.2) Before leaving the subject of thermal cycling it might be worthwhile to mention a practical use for this process. Because of the resulting mechanical stresses within a thermoelectric module, thermal cycling has been shown to be an effective "burn-in" technique. By subjecting thermoelectric devices to a well controlled cycling program, it is possible to identify potentially unsatisfactory modules thereby reducing the likelihood of infant mortality failures. There obviously is some cost associated with this operation but it may be useful when extremely high reliability is required.
10.4 MODULE RELIABILITY RELATED TO ON/OFF POWER CYCLING
As discussed previously, the accepted industry standard for thermoelectric module MTBF is 200,000 hours minimum. This MTBF value is based on relatively steady-state module operation where system power is occasionally (typically a few times per day) turned on and off. In some applications power is turned on and off more frequently especially where thermostatic temperature control is used. A test was conducted using ValueTEC™ Series modules to study the effects of ON/OFF power cycling at a relatively constant temperature. Modules were mounted between a pair of forced convection cooled heat sinks using thermal grease at the module/heat sink interfaces. Full rated current was supplied to the modules for a period of 7.5 seconds followed by a 7.5 second "off" period that resulted in one complete ON/OFF cycle every 15 seconds. The input current to each module was monitored and a failure was indicated by an appreciable current decrease resulting from an increase in module electrical resistance. The test was run until an arbitrary total of 25,000 hours or approximately 6 million cycles was accrued. For these test conditions, the calculated MTBF was 125,000 hours or 3x107 on off cycles.
CAUTION: Most conventional thermostats inherently have moderately large open/close temperature differentials. This condition may effectively set up a thermal cycling situation where the temperature of the TE module is continuously varying between the upper and lower differential limits. Since thermal cycling is known to reduce the life of thermoelectric modules, the use of traditional ON/OFF thermostatic temperature control schemes is not recommended for high-reliability applications.
10.5 ENVIRONMENTAL CONSIDERATIONS
Thermoelectric modules often are installed in systems that are subject to significant shock, vibration, and/or other potentially detrimental environmental conditions. As mentioned earlier in this report, modules will withstand moderate compression forces but shear strength is relatively low. However, when thermoelectric modules are properly mounted within a mechanical subassembly, they will withstand substantial mechanical stress without failure.
Ferrotec's modules have been subjected to a number of environmental/mechanical test conditions and have successfully met those conditions without failure. Such tests include:
|High Temperature Operations and Storage:
||150°C for 30,000+ hours|
|Low Temperature Operations and Storage:
||-40°C for 1000+ hours|
||(a) 100°C (15 sec)/100°C (15 sec), 10 cycles |
(b) 150°C (5 min)/-65°C (5 min)/ 150°C, 10 cycles
(c) MIL-STD-202, Method 107
Range for ValueTEC™ Series modules: -55oC to +85oC
Range for SuperTEC™ Series modules: -65oC to 150oC
||(a) 100G, 200G 2 6msec; 500G, 1000G @1 sec 3-axis, three shocks each axis |
(b) MIL-STD-202, Method 213, Test Condition I
||(a) 10/55/10 Hz,1 minute cycle, 9.1G, 3-axis, 2-hours each axis |
(b) MIL-STD-202, Method 204A, Test Condition B, 15G Peak
10.6 STANDARD QUALITY CONTROL PROCEDURES
Thermoelectric device manufacturers have independently developed quality control and test procedures to insure that products meet published specifications and exhibit acceptable standards of workmanship. While few formal standards (Military Specifications, etc.) exist within the industry, there have emerged certain minimum recognized criteria to which most major thermoelectric manufacturers adhere. However, if users have particular concerns about quality-related issues that may affect their specific application, it generally is desirable for users to discuss their concerns with individual thermoelectric manufacturers.
Ferrotec America's test and quality program has evolved from many years of industry experience covering an extensive range of thermoelectric cooling applications. General aspects of this program include 100% electrical and mechanical testing/inspection of products prior to shipment; in-process testing and screening using either 100% inspection or sampling inspection as per MIL-STD-105; and the use of statistical process control techniques on various critical operations. The overall quality assurance program is structured in accordance with MIL-Q-9858A.
10.7 CONCLUDING REMARKS
In the foregoing discussion we have emphasized the great dependence of thermoelectric module reliability on application conditions. By following some basic guidelines, and with knowledge of how certain factors tend to affect module life, it should be possible for designers to optimize system reliability. While some may wish to perform a comprehensive analysis and model all relevant parameters, many users having unusual requirements or nontraditional configurations often turn to an empirical approach for determining the reliability of their specific application.