Using Peltier modules for thermal management of electronic systems
Coping with the heat generated by electronic components is a never-ending problem. The era of the discrete transistor, promising low-power circuit designs, has largely been superseded by microelectronic circuits that integrate not just thousands but millions of transistors.
While the power loss due to the inefficiency of an individual transistor may be small, the total sum of these losses from a complex IC such as a microcontroller can be substantial. By the time you have designed several ICs and various other devices into a piece of electronic equipment, you are back to needing to find a way to deal with the resulting heat.
This is especially true when customers demand ever greater equipment functionality, requiring more and more devices to be packed into the same, or sometimes even smaller, space. Such increased system density can be self-defeating though if, for example, the clock speed of a processor has to be lowered to keep power dissipation within thermal limits.
Well-established and proven methods of extracting excess heat from electronic equipment primarily rely on the principles of conduction and convection. Conduction provides the means for moving heat from the locations where it is generated to somewhere else in the system and then ultimately into the ambient environment.
For example, heat generated in an IC might be conducted through the circuit board into the enclosure of the equipment, or into a heat sink to be dissipated to the surrounding air by convection. In some systems natural convection is sufficient, but often the addition of a fan to provide forced-air cooling is necessary.
However, forced air cooling is not always an option for thermal management. Some systems are closed and have no means to vent cooling air, while in other situations the noise associated with cooling fans may not be acceptable. Thermoelectric modules provide such an alternative and are, in effect, solid state heat pumps that can be used for both cooling and heating.
What is a Peltier thermoelectric module?
The thermoelectric effect will be known to most engineers from its application in thermocouples where it is used to measure temperature. This effect, discovered by Thomas Seebeck in the early 19th century, causes a current to flow when there is a temperature difference between the junctions of two dissimilar conductors.
The Peltier effect, discovered by Jean Peltier a decade later, demonstrated the reverse principle, enabling heat to be emitted or absorbed by passing current through two dissimilar conductors. However, practical application of the Peltier effect became possible only through advances made in semiconductor technology from the mid-20th century and only recently have modern techniques enabled efficient thermoelectric modules.
The implementation of a Peltier thermoelectric module uses N-type and P-type Bismuth Telluride semiconductor materials connected to a power source and sandwiched between thermally conducting metallised ceramic substrates. The pairs of P/N semiconductor pellets are electrically connected in series, but thermally arranged in parallel to maximise thermal transfer between the hot and cold ceramic surfaces of the module (see Figure 1).
Figure 1. The structure of a Peltier module uses an array of doped semiconductor pellets
Applying a dc voltage causes the positive and negative charge carriers to absorb heat from one substrate surface and transfer and release it to the substrate on the opposite side (see figure 2). Therefore, the surface where energy is absorbed becomes cold and the opposite surface, where the energy is released, becomes hot. Reversing the polarity reverses the hot and cold sides.
Figure 2. The Peltier principle using N-type and P-type Bismuth Telluride semiconductor materials
The advantages of Peltier modules
As stated at the outset, the prime motivation for using Peltier modules is that they are ideal for situations where forced air cooling is not an option, e.g. in sealed equipment/environments. Other key benefits they offer include:
- Precise temperature control and a fast temperature response:
For any given module operating with a known temperature difference between its hot and cold surfaces, there are well-defined relationships that determine the supply current that needs to be applied to achieve the required heat absorption. Fast feedback circuits allow temperatures to be controlled to within a fraction of a degree.
- Compact form factor and lightweight
- Peltier modules can be extremely compact, with height profiles as low as 3mm. This feature is particularly attractive for applications where size and weight are a concern.
- Capable of sub-ambient cooling
- Because Peltier modules provide active cooling to remove heat they are able to achieve temperatures below ambient. For this reason, manufacturers typically provide performance data for a hot surface temperature of 27°C as well as 50°C.
- High reliability due to solid-state construction with no moving parts
- Unlike forced-air cooling systems that use fans whose bearings have a limited life, Peltier modules have no moving parts that can wear out. When operating with a constant temperature difference, a typical MTBF (mean time between failures) figure might be 100,000 hours.
- Environmentally friendly
- Because Peltier modules do not use refrigerants there is no risk to the environment either from emissions during operation or when equipment is disposed of at the end of its life.
- Can be used for cooling or heating
By reversing the current flow, Peltier modules can be used to pump heat into a system rather than extract heat. Indeed, they can also be used as thermoelectric generators to harvest energy from waste heat.
arcTEC™ Structure – An Advanced Construction Technique to Combat Thermal Fatigue
A known disadvantage of conventionally manufactured thermoelectric coolers is thermal fatigue, which can affect the integrity of the solder bonds between the electrical interconnect (copper) and the P/N semiconductor elements, as well as the solder or sinter bonds between the interconnect and ceramic substrate, as shown in figure 3. While these bonding techniques normally create strong mechanical, thermal and electrical bonds, they are inflexible, and when subjected to the repeated heating and cooling cycles that are typical of normal Peltier module operation, they can degrade and eventually fail.
Figure 3. Peltier module structure with conventional solder and sinter bonds
The arcTEC™ structure is an advanced construction technique for Peltier modules, devised and implemented by CUI to combat the effects of thermal fatigue. In the arcTEC structure, the conventional solder bond between the copper electrical interconnect and the ceramic substrate on the cold side of the module is replaced by a thermally conductive resin. This resin provides an elastic bond within the module that allows for the expansion and contraction that occurs during the repeated thermal cycling of normal Peltier module operation. The elasticity of this resin reduces stresses within the module while achieving a better thermal connection and a superior mechanical bond, and shows no marked drop-off in performance over time.
Figure 4. CUI’s arcTEC structure replaces the cold ceramic to copper bond with resin and uses SbSn solder in place of conventional BiSn solder for the copper to semiconductor bonds
Along with the resin bond, modules with the arcTEC structure use SbSn solder to replace the BiSn solder typically used between the P/N semiconductor elements and the copper interconnect – see figure 4. With its much higher melting point of 235°C compared to 138°C for BiSn, SbSn solder offers superior resistance to thermal fatigue and a better shear strength.
arcTEC structure delivers improved reliability and thermal performance
Bond failure within Peltier modules manifests itself as an increase in resistance and is compounded by repeated thermal cycling. As the life expectancy of a module is dependent on the quality of these bonds, then the change in resistance with the number of thermal cycles is a useful predictor of failure. It further demonstrates the stark difference between modules built with and without the arcTEC structure, as can be seen from the results presented in figure 5.
Figure 5. Reliability of the arcTEC structure versus modules with standard construction
The other advance offered by the arcTEC structure is the use of P/N elements made from a premium silicon that are up to 2.7 times larger than those employed by other modules. This ensures a more uniform cooling performance, avoiding the uneven temperatures that contribute to the risk of a shorter working life, while delivering a greater than 50% improvement in cooling time compared to competing modules – a performance gap that widens as the number of thermal cycles increases (see figure 6).
Figure 6. Comparison between the IR temperature distribution of a conventional Peltier module (top) and a module built using the arcTEC structure (bottom)
Conclusion
Thermoelectric modules are another tool at the disposal of design engineers who have to battle the excess heat generated by increasingly complex integrated circuits and other electronic components that are confined in ever-smaller spaces. Faced with sealed environments, where forced-air cooling has been rendered ineffective, the Peltier module becomes the ideal solution. Furthermore, thermoelectric modules enable precise temperature control and allow sub-ambient cooling.
While the benefits of thermoelectric modules that act as heat pumps to remove heat are becoming more popular, the reduction in life expectancy due to thermal fatigue from repeated heating and cooling cycles presents a problem for conventional thermoelectric coolers. This problem arises because of the strong but inflexible bonds that are necessary to connect the internal elements of the module to make it work.
However, thanks to the arcTEC structure implemented in CUI’s line of high performance Peltier modules, this problem has met its match. Delivering substantially better reliability, in excess of 30,000 thermal cycles, and a greater than 50% improvement in cooling time compared to competing devices, CUI’s Peltier modules with the arcTEC structure have your thermal management needs covered where forced-air cooling is not an option.
For more information on Peltier Devices, visit http://www.cui.com/catalog/components/thermal-management/peltier-devices
