Effect of Silicone Gel on the Reliability of Heavy Aluminum Wire Bond for Power Module During Thermal Cycling Test

Abstract

Ultrasonic heavy aluminum wire bonding whose wire diameter is greater than 100 microns has been the dominant interconnection technology in IGBT modules to implement the electrical connection among chips, DBC substrates and electrode terminals. During the switching on/off operation, the stress concentrates in the bond wire induced by the coefficient of expansion (CTE) mismatch between Al wire and silicon chip will lead to wire failure, such as heel crack or lift off. This paper mainly investigates the effect of silicone gel on the reliability of heavy aluminum wire bond during thermal cycling test. Various cases with different quantity of silicone gel or without silicone gel are conducted by the approach of finite element method. Prony series expression based on the Maxwell model was used to define the viscoelastic behavior of silicone gel. The results showed that when the bonding wires were covered with soft and viscous silicone gel, the main mechanism of wire bond failure was heel crack. Besides, the reliability of wire was closely related to the coverage of silicone gel. The recommended coating method which uses less amount of silicone gel to reach better effect is to spread silicone gel just under the wire heel, or add more gel to cover over the foot meanwhile.