Low Modulus Polyimide Coating on Wedge Bond to Improve Wire Bond Robustness in Thermal Cycling

Aluminum (Al) wires are widely used in high power devices as interconnection materials. Al wire wedge bond crack in thermal cycling test (TCT) is a major failure in reliability test. Al wedge bond crack is mainly caused by high stress level due to mismatch of epoxy molding compound (EMC) coefficient of thermal expansion (CTE) with other assembly components. In this paper, a polyimide (PI) coating on wire wedge to reduce stress on wedge bond is studied. A simulation model is first set up to evaluate the plastic strain on wedge bond with PI coating and w/o PI coating on wedge bond area. Simulation results show plastic strain on wedge bond per cycle is reduced by 24% in TCT with PI coating on wedge bond, because flexible PI coating as a stress buffer layer in between EMC and wire reduces the strain stress on Al wedge bond and decreases the risk of wedge degradation or crack. The simulation results illustrate that adding a flexible insulating PI coating on wedge bond is an effective method to improve wire bond robust in TCT. To verify the effectiveness of the PI coating, IC devices are assembled and evaluated in TCT. After 3000 cycles, devices with PI coating show no wedge crack, while those without PI coating exhibit significant wedge cracking. At the same time, delamination is also improved for the devices with PI coating. The results demonstrate that PI coating is one solution to improve wire bond robustness and as well as delamination in parallel in thermal cycling.