Solder-TIMs (thermal interface materials) for superior thermal management in power electronics

Abstract

The use of High Power devices are increasing exponentially given the need for increased current switching and conversion rates. High power devices are used in diverse end applications including traction (trains, powertrain management in an Electric Vehicle), LEDs (high brightness, high power), Diode Pumped Solid State Lasers, computing and graphics. Higher power translates to more heat being generated, and this heat needs to be removed from the power die and dissipated out of the package. Keeping the power die cooler is the key to increased functionality of the device and extending the life of the device. The choice of the right Thermal Interface Materials (TIMs) used to transport heat away from the power source is therefore crucial in preventing the power die from overheating. This paper discusses the work done on Solder-TIMs that have been developed for high-power applications and how they compare to thermal grease. Solder-TIMs as the name indicates are solid solders (typically 100% Indium / Indium-containing alloys) that are very soft versus typical thermal greases that are silicone-based with metal (Ag) particles). With higher power and higher heat dissipation needs, regular grease is not able to make the cut and superior performing Solder-TIMs are better suited to take the heat away from the die due to the following reasons: (a) Thermal grease has a low bulk thermal conductivity of 3–12 W/m.K compared to 100In that has a very high bulk thermal conductivity of 87 W/m.K; (b) During device usage over time, thermal grease tends to pump-out and migrate away from the center of the die (due to the diaphragm effect) and this means that the center of the power die gets hotter and this could lead to premature failures. On the other hand, there is no pump-out with a Solder-TIM. 100In is extremely soft (4X softer than lead) and this softness helps fill the interface gaps thus reducing thermal interface resistance. In addition, over time, the malleability of the solder helps fill the interface gaps even better. So thermal interface resistance with a Solder-TIM decreases over time as opposed to thermal grease where the thermal interface resistance increases over time; (c) Over time, grease tends to bake-out and dry (becomes powdery), thereby increasing thermal resistance and reducing heat-dissipation effectiveness. With Solder-TIMs, there is no bake-out; (d) Grease is messy when applying versus a solid solder that can be packaged in tape & reel and picked & placed. The Solder-TIM tested was the Heat-Spring® which is a foil made of In/In-containing alloys, with a proprietary altered surface for reduced thermal interface resistance. The Heat-Spring® needs only compression force, does not need to be melted/reflowed, does not need a flux and therefore eliminates voiding associated with flux and reflow, does not need any special substrate metallization. The Solder-TIM was compared to industry-used thermal greases. Testing regimes included (i) Bake test: at 90 deg C for 1500, 3000 hours; (ii) Power Cycling: 1000 cycles, 0–50W; (iii) Change in interface resistance and solder thickness for (a) T=1000 hours; (b) Thermal Cycling- 1000 cycles, −10/+95C; (c) HAST: 85C/85% RH for 1000 hours. In all the tests, Solder-TIMs consistently outperformed thermal grease by achieving low thermal interface resistances especially over time and prevented the power die from overheating.