As discussed in our previous blog article, Reliability Tests Of Thermal Interface Material In Power Module Applications, there are several typical reliability tests that PCM needs to pass in power module applications, such as thermal cycling test (TCT), high-temperature storage (HTS), HAST or double 85 tests, etc. Although the test conditions differ from each other to simulate various operating and environmental stress for devices, the mechanisms of Phase change material failure are similar.
There are several factors that could cause PCM to fail in power inverters:
High Temperature
PCM is designed to operate within a specific temperature range, and if the power inverter exceeds this range, the PCM may become ineffective or even fail. At high temperatures, the chemical structure of the PCM can break down. Another effect of high-temperature bake tests on PCM is that they can cause mechanical stress on the PCM. It’s worth noting that the specific effects of high-temperature bake tests on PCM will depend on the type of PCM being used and the conditions of the test. For example, some paraffin-based PCMs can withstand temperatures up to 150°C and some salt hydrates-based PCMs can withstand temperatures up to 180°C.
Mechanical Stress
The repeated expansion and contraction of the PCM due to temperature changes can cause mechanical stress on the PCM, which can lead to cracking and failure. Additionally repeated thermal cycling can cause mechanical stress on the PCM layer, which can lead to leakage, pump out, bleed out, or other failures. This can cause the PCM to no longer be in contact with the heat-generating components or the heat sink. To prevent mechanical stress, it’s important to choose PCMs that have a high thermal expansion coefficient and low modulus of elasticity, which will help to reduce the stress caused by temperature changes.
It’s also important to design and test the power inverter to ensure that the PCM is properly assembled and that it is compressed correctly to make good contact with the heat-generating components and the heat sink.
Chemical Degradation
While PCM is being applied to the components, there are many environmental factors playing a role in PCM chemical degradation and further resulting in failures.
- Corrosion: PCM can be sensitive to certain types of corrosion, such as oxidation, which can occur when the PCM comes into contact with certain metals, such as aluminum or copper. This can result in the formation of an oxide layer on the surface of the PCM, causing the PCM to degrade over time.
- Contamination: Dust, oil, or other chemicals, can reduce PCM’s thermal conductivity and effectiveness.
- Compatibility: PCM can be sensitive to certain types of materials, such as certain types of thermal grease or other thermal interface materials, which can cause chemical reactions that can degrade the PCM over time.
- Hydration: Hydration of PCM refers to the absorption of water by the PCM, which can occur if the power inverter is operated in humid environments or during the shipment and storage of PCM. It can cause the chemical structure of the PCM to change, leading to a reduction in its thermal conductivity. Besides, the absorbed water can cause the PCM to swell, which can cause mechanical stress on the PCM. Furthermore, the water can cause the PCM to freeze at lower temperatures, which can cause the PCM to crack and fail. If the PCM is stored under a humid environment (>65%RH), the water can cause the PCM to increase its viscosity, which can make it more difficult for the PCM to flow. It’s also worth noting that hydration can cause the PCM to corrode and react with other materials in the inverter, such as the metal of the heat sink.
To prevent the hydration of PCM, it’s important to choose PCMs that are hydrophobic and keep the power inverter in a dry environment, or use protective coatings on the PCM to reduce the effects of water. Additionally, it’s important to monitor the performance of the PCM over time and replace it if necessary.
Cyclic loading
Repeated exposure to temperature changes can cause the PCM to degrade over time. As the PCM is repeatedly heated and cooled, its chemical structure can begin to break down, reducing its thermal conductivity and effectiveness.
Time Aging
PCM has a limited life span, and if it is used beyond its recommended lifetime, it may fail. All of the above are aging factors.
Setup Issues
There are also some setup issues that could also result in failure:
- Leakage: PCM is a liquid at certain temperatures, and if it leaks out of its container or encapsulation, it may not be able to function properly. Thus, PCM is more suitable for a horizontal installed position.
- Compression: PCM needs enough pressure (40 psi) to make good contact with the heat-generating components and the heat sink. This pressure also helps PCM to spread well and minimize the air gap during the phase change stage.
- Overheating Spots: If the power inverter is not designed properly, or if there are other issues that cause overheating spots, the PCM may not be able to dissipate the heat effectively.
- Air bubbles: Power cycling can cause the formation of air bubbles in the PCM, which can cause the PCM to fail.
It’s worth noting that many of these factors can be mitigated with proper design, testing, and maintenance of the power inverter. Additionally, choosing the right PCM for the specific requirements of the power inverter, as well as regular monitoring of the system’s temperature and performance, can help to ensure the reliability of the PCM.
Honeywell PTM7950 is a super highly thermally conductive Phase Change Material (PCM). It is designed to minimize thermal resistance at interfaces, maintain excellent performance through reliability testing, and provide scalable applications at a competitive cost. Based on a novel polymer PCM system, PTM7950 exhibits excellent interface wettability during typical operating temperature ranges, resulting in extremely low surface contact resistance.
Honeywell PTM7950 is one of the most successful TIM products used in high-power inverters and high-performance computing devices. Ultra-low thermal impedance with minimum achievable BLT and superior long-term reliability make PTM7950 stand out from the competitors (Check the PTM7950 Reliability Report). PTM7XXX series also presents stability among vertical applications and pass automotive standard vibration test (Check the Vibration & Shock Test Report), thus PTM7XXX is currently widely used in automotive applications such as power inverters.
If you are in the process of selecting a Phase change material or you are having reliability issues with your existing TIM, Contact us and one of our Sales engineers will get back to you as soon as possible.
