Visualization experiment on pump-out phenomena of thermal grease under thermal cycling

Abstract

In order to reduce thermal resistance of power module system, Thermal Interface Materials (TIMs) have been used at the interface between baseplate of power module and heat sink. TIMs have an important role to reduce thermal resistances due to solid surfaces with roughness or waviness. Thermal grease is a primary candidate to fill the gap between them. The grease can be pumped out of the gap during thermal cycling while the grease has the advantages of reduced cost compared to the other TIMs and TIM’s thickness. New reliability test of the grease is required for thermal management as the pump-out phenomena cause an increase of the thermal contact resistances. This study shows a visualization test method to evaluate pump-out phenomena of the grease during thermal cycling. A bimetal plate consisting of copper and invar is used to simulate thermal deformation of baseplate in power module. The grease is filled between the bimetal plate and Al thick plate, and then thermal cycling test is conducted. A 25 MHz ultrasonic imaging is used to visualize the grease layer. The ultrasonic images show a sequence of the grease behavior after the thermal cycling. The ultrasonic imaging results indicate that voids occur in non-curing grease under repeated pressure while cracks are detected in curing grease. It is also found that the curing grease layer applied in power devices can be designed by the evaluation of initial thermal contact resistances at the interface. Sensitivity analysis of correlated factors shows pump-out phenomena are strongly affected by curing of grease.

Keywords

• thermal interface material
• themal grease
• thermal cycling
• pump-out
• ultrasonic imaging