Thermoelectric Module Reliability

Thermoelectric modules are highly reliable due to their solid state construction and lack of moving parts. In applications involving control of a constant temperature where DC power is continuously applied, thermoelectric reliability is extremely high, with the mean time between failure (MTBF) typically greater than 200,000 hours. Overall system design plays a key role in thermoelectric module reliability. The following factors play a roll in overall thermoelectric system reliability:

1) Mechanical Design
Thermoelectric modules have high mechanical strength under compression and can typically survive compressive loads of up to 1000 psi. Thermoelectric modules do not perform well under shear or tensile loading. As a result, the mechanical design of any thermoelectric system must ensure that only compressive loads are applied to the modules. Solid State Cooling System’s extensive expertise in the mechanical design of thermoelectric systems ensures that premature thermoelectric module failure from improper mechanical loading will not occur.

2) Thermal Contact
Thermoelectric modules must reject the heat they transfer plus the electrical energy applied to them. This requires intimate thermal contact between the TE module hot side and its heat sink. Poor thermal contact will result in overheating of the TE module, which will significantly reduce performance and could lead to premature module failure.

3) Moisture
Moisture is the nemesis of thermoelectric modules. Moisture inside a thermoelectric module can lead to reduced performance and premature failure of the device. Moisture can both corrode materials inside the module and provide a ground path between the module’s electrically hot components and the heat sink, leading to an electric short. Both scenarios will lead to premature module failure. Solid State Cooling Systems overlays a triple seal on its products to ensure moisture never enters the modules. First, the thermoelectric module is sealed with a highly moisture resistant acrylic epoxy. Next, the heat sink/cold plates are sealed with a silicone RTV. Finally, the entire thermoelectric assembly, bolts and all, are coated with another silicone barrier. In-house tests show that Solid State Cooling Systems thermoelectric heat exchangers can operate for over 6 months completely submerged in water!

4) Thermal Cycling
Thermoelectric module reliability is reduced when the material interfaces inside the module are cycled in temperature. Cycling these interfaces, such as between the Bi2Te3 crystals and copper inter-connecting pads, or between the copper pads and the ceramic covers, can lead to material fatigue and structural failure due to the different rates of thermal expansion between these materials.

There are two ways to cycle thermoelectric module interface temperatures: cycling the operating temperature or cycling the operating current. The more rapid the cycling, the sooner the failure.
Cycling the operating temperature is typically a process requirement, but optimum system design and module selection can maximize the number of cycles before failure. In one study, by iTi Ferrotec, a two stage 150 °C rated thermoelectric module assembly cycled from -55 °C to 125 °C produced a MTBF of 8100 cycles while the same assembly using 200 °C rated modules yielded an MTBF of 17,500 cycles. In another test, single stage 200 °C rated modules were cycled from +30 °C to +100 °C. None of the modules tested failed at less than 25,000 cycles, with an MTBF for the group of 68,000 cycles.

Cycling the operating current of thermoelectric modules causes changes in internal temperatures even when the system temperature is kept constant. In one test, 150 °C rated thermoelectric modules cycled on and off every 15 seconds (7.5 seconds on, 7.5 seconds off) at a constant temperature exhibited an MTBF of 30 million cycles. In a separate test, thermoelectric modules cycled between cooling and heating at a constant system temperature exhibited an MTBF of 16 million cycles.

Tests have shown this cycling phenomenon exists until switching frequencies reach approximately 1000 Hz (1000 cycles per second). As a result, the choice of control schemes can substantially impact module reliability. On/off or pulse width modulation at frequencies of less than 1000 Hz can have a major impact on module reliability. For example, a 1 Hz (one cycle per second) pulse width modulation control at a constant system temperature will turn on and off 31.5 million times in one year, when operated 24 hrs/day, 365 days per year. Such a thermoelectric system would have an MTBF of less than one year. The same system using a variable current supply would have an MTBF of about 200,000 hours, or 22.8 years. As a result, on/off control (such as thermostats) and pulse width modulated control less than 1000 Hz is not recommended for high reliability systems. Such systems must use a variable output supply, such as Solid State Cooling Systems Switchback 6600 power supply.