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Important Factors for Improved Peltier Module Reliability

Przez: Jeff Smoot, VP of Apps Engineering and Motion Control at CUI Devices

Przekazane przez: Północnoamerykańscy redaktorzy Digi-Key

Peltier modules or thermoelectric coolers (TEC) have seen a rise in popularity due to their reliable solid-state construction and precise temperature control. Their basic operation works by transferring heat from one side of the module to the other when electrical power is applied. As with any component in design, Peltier module reliability is important, so having a baseline knowledge of their implementation and construction can greatly aid designers in their proper application. To assist engineers in understanding this growing technology, this article will provide a brief review of Peltier module construction as well as the common failure mechanisms to avoid in order to improve overall reliability.

Basic construction

As solid-state devices with no moving parts, thermoelectric coolers can operate within a wide temperature range. From a high-level, Peltier modules are comprised of semiconductor pellets placed between two electrically insulating but thermally conductive ceramic plates. The semiconductor pellets are also doped to carry either a positive or negative charge. Plated on the inner surfaces of each ceramic are conductive metal patterns, which the semiconductor pellets are soldered to and configured in a way that puts them electrically in series and mechanically in parallel. These electrical and mechanical configurations ultimately lead to the basic thermal principles of Peltier devices, where heat is absorbed from the cold side ceramic and expelled by the hot side ceramic.

For more information on these basic principles and construction, CUI Devices’ whitepaper serves as a further resource on this topic.

Diagram of general Peltier module constructionFigure 1: General Peltier module construction (Image source: CUI Devices)

Common failure mechanisms

Mechanical fracturing of the semiconductor pellets or the associated solder joints represent the most common failure mechanisms of Peltier modules. Although these fractures do not initially develop throughout the entire pellet or solder joint, a complete failure can occur if the fracture spreads completely across either of those two areas. However, these fractures can be detected prior to a total failure by noting a rise in the series resistance of the Peltier module that reduces its overall efficiency.

Tension and shear forces

Peltier modules are commonly used in applications where the cold side of the TEC module is placed on an object to be cooled with a heatsink on the hot side to improve heat dissipation. However, if the heatsink and object to be cooled are attached to the ceramic plates without any supportive mechanical structure, large shear or tension forces could occur across the TEC module. As Peltier modules are not meant to withstand these types of loads, these forces could break the module or lead to another mechanical failure.

Diagram of shear or tension forces in a common Peltier module assemblyFigure 2: Demonstration of shear or tension forces in a common Peltier module assembly (Image source: CUI Devices)

To combat these shear or tension forces, many Peltier modules are clamped in between the object and a heatsink because of a Peltier module’s ability to withstand large compressive forces from the clamps. In turn, the clamps are then able to absorb any shear or tension forces from the object and heatsink.

Diagram of common stresses on a Peltier moduleFigure 3: Common stresses on a Peltier module (Image source: CUI Devices)

Compression forces

While clamping does combat many of the negative forces on a Peltier module, it can create its own problems if not properly implemented. When clamping heatsinks and objects to a Peltier module, it is imperative that even clamping forces are applied to minimize the torque stresses on the TEC module and reduce the chance of damage. These uneven clamping forces can create torques as well as compressive forces that lead to mechanical failure.

Diagram of proper and improper clamping of a Peltier moduleFigure 4: Proper and improper clamping of a Peltier module (Image source: CUI Devices)

Thermal cycles

The ceramic plates and semiconductor pellets of thermoelectric coolers each have associated coefficients of thermal expansion (CTE). As the TEC module goes through its thermal cycles of heating and cooling, a mismatch of the ceramic and semiconductor CTEs can lead to mechanical stresses that cause fractures in the semiconductor pellets and solder joints. In addition to changes in a Peltier module’s absolute temperature, thermal gradients and rapid rates of change in a module’s temperature can also lead to mechanical stresses due to the CTEs. Operating a TEC module at extreme temperatures, with large temperature gradients, and at high temperature slew rates also produces mechanical stresses and an increased chance of device failure.

External contaminants

Exposure to external contaminants are another path to mechanical failures for the semiconductor pellets, solder joints, and metalized conduction patterns of a Peltier module. To minimize exposure to these contaminants, it is common for a sealant to be applied around the perimeter of the TEC module, between the two ceramic plates. Of the typical sealant methods, silicone rubber is widely used for its mechanical compliance. However, it can be ineffective as a vapor barrier in extreme operating conditions. To overcome this shortfall, epoxy can be used as an alternative sealant in high vapor environments, but it lacks the mechanical compliance of silicone rubber. Ultimately, deciding between the trade-offs of each sealant will depend on the end application and its operating conditions.

Reliability improvements

To combat the mechanical stresses that can lead to cracks in the solder joints and semiconductor pellets of a Peltier module, CUI Devices has developed the arcTEC™ structure. This unique construction is implemented in CUI Devices’ high-performance Peltier modules for improved reliability, cycle life, and performance. The arcTEC structure counteracts the effects of thermal fatigue by replacing the solder joints with an electrically conductive resin on the cold side of the TEC module that is more mechanically compliant than solder. This helps to minimize the stresses and fracturing that can occur in traditional Peltier module structures. The rest of the solder joints are then replaced with a high temperature antimony solder (SbSn, 235°C) that tolerates mechanical stress better than the more traditional, lower temperature bismuth solder (BiSn, 138°C). CUI Devices’ Peltier modules also implement the silicone rubber vapor barrier mentioned earlier for added mechanical compliance with epoxy and other vapor barriers available upon request.

To demonstrate the improved reliability of the arcTEC structure, the graph below plots resistance against the number of thermal cycles. As increasing changes in resistance lead to an increased chance of failure, one can clearly see the more stable performance of CUI Devices’ arcTEC structure compared to standard Peltier modules over a higher number of thermal cycles.

Graph demonstrating the more reliable performance of the arcTEC structureFigure 5: Graph demonstrating the more reliable performance of the arcTEC structure (Image source: CUI Devices)

Conclusion

There are many factors that can lead to improved or degraded Peltier module performance and reliability, including mechanical installation, operating conditions, and external contaminants. When selecting a Peltier module, it is important to observe proper mounting practices and operating parameters. CUI Devices’ arcTEC structure, found in many of its Peltier modules, can help mitigate some of these common failure mechanisms and improve overall reliability. With a range of Peltier devices in a variety of sizes and thermal ratings, CUI Devices has multiple options to meet an engineer’s thermal management needs.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

Informacje o autorze

Jeff Smoot, VP of Apps Engineering and Motion Control at CUI Devices

Article provided by Jeff Smoot of CUI Devices.

Informacje o wydawcy

Północnoamerykańscy redaktorzy Digi-Key