High power/voltage IGBT modules with SiC and GaN dies are widely adopted by power inverter manufacturers, allowing for higher switching speeds to improve energy efficiency and battery range for EVs. The increasing power density in the design necessitates superior thermal management solutions, where thermal interface materials (TIM) play a key role in ensuring long-term reliability and stable performance.
In inverters, 50% of the total thermal resistance is coming from the interface between IGBT modules and heatsink, where thermal interface material is applied. Hence It is essential to minimize the TIM resistance while maintaining long-term reliability under high power and switch frequency conditions.
The key considerations of Thermal interface material selection can be roughly outlined as follows:
Excellent Thermal Performance
- Low thermal resistance down to 0.04 ˚C·cm2/W
- High thermal conductivity (up to 8 W/mK)
- Optimal Surface Wetting & Low Contact resistance
- Great conformability to fill up microscopic voids and cavities
High Reliability and Long-life
- No bleed-out, pump-out, and flow-out issues
- No aging effects including dry-out over time and degraded thermal performance
- Stable thermal impedance across accelerated aging test
Manufacturability
- Easy assembly & broad process window
- High yield with low/zero material waste
- Pad and paste form factor
Phase change materials, specifically, excel at those requirements and become the most popular TIM product for the power modules application. They adopt a non-silicone-based polymer matrix with steric structure and chemical bonds with fillers, which limits filler migration and largely reduces the risk of material mobility issues like pump out. Additionally, they go through phase change with a dramatic viscosity drop at temperatures above 45°C. This helps to wet the contact surface, fill the mini gaps, squeeze out the air voids between the heatsink and heat source, and ultimately achieve a very low thermal impedance.
Reliability test types
Besides thermal impedance and bond line thickness, the performance of those reliability tests is the key criterion when selecting PCM products for inverter manufacturers. The typical reliability tests that PCM needs to go through for power module applications are:
(1) High-temperature baking
- Test standard: JESD22-A103
- Test conditions: 1000hrs @150°C
Objective: Accelerate changes in TIM’s material and performance characteristics relative to prolonged and elevated temperature
(2) 85°C & 85%RH double 85 tests
- Test Standard: IEC 61215
- Test conditions: 85°C, 85%RH, 1000 hrs
Objective: Accelerate the corrosive impact of high humidity and temperature on the thermal performance of the test structure
(3) Highly-accelerated stress test
- Test standard: JESD22-A110-B
- Test conditions: 130°C, 85%RH, 96 hrs
Objective: Accelerate the corrosive impact of high humidity and temperature on the thermal performance of the test structure
(4) Temperature cycling test
- Test Standard: JESD22-A104C
- Test condition: -55 °C to 125 °C (TCB), 1000 cycles
Objective: Determine the resistance of TIM to extremes of high and low temperatures, and its ability to withstand cyclical stresses
Thermal cycling and storage tests are more of a temperature and mechanical stress concern. High-temperature storage tests provide information about how the components will perform under long-term high-temperature conditions. Thermal cycling tests provide information about how the components will perform under conditions of repeated temperature changes.
HAST and double 85 also include humidity factors. Double 85 tests are typically used to evaluate the long-term reliability of electronic components under normal operating conditions. HAST tests, on the other hand, are used to evaluate the reliability of electronic components under extreme conditions, such as those that may be encountered in extreme environments or during transportation. The stress level on the component during the HAST test is higher than during the Double 85 tests.
Although the test conditions differ from each other to simulate various operating and environmental stress for devices, the mechanism of PCM failure is similar. Please see our blog article Phase Change Material Failure Mechanism In Reliability Tests
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 competition as can be seen in the PTM7950 Reliability Report.
Honeywell PTM7XXX series also offers stability among vertical applications, passes automotive standard vibration tests, and is currently widely used in automotive applications such as power inverters.
Are you interested in learning more about Power module thermal interface materials? Contact us and one of our Sales engineers will get back to you as soon as possible.
