Void-Free Underfill Process With Variable Frequency Microwave for Higher Throughput in Large Flip Chip Package Application

Moisture voiding in underfill materials can cause reliability issues for the flip chip packages. The bake-out step included in the assembly process flow to avoid this problem can be not completely efficient for some large die size packages. This is due to complex substrate circuit designs and time delays subsequent to the bake-out step. The current work proposes using the variable frequency microwave cure to eliminate the moisture voiding of flip chip large packages assembled without any bake-out step. Results showed, for a given ramp rate, a decrease in voiding formation with decreasing VFM cure temperature. It was also found that, at low final cure temperatures, the hold steps promoted the voids formation more than the ramp steps. At high final cure temperatures both ramp and hold steps induced voids formation, growth and coalescence. Another interesting observation was that a slower ramp rate reduced void formation even at high cure temperature. Based on the voiding evolution study done here, two optimized cure profiles were proposed, one comprising a two-step approach and another using a one-step cure with a low ramp rate of 2 °C/ min. These optimized VFM profiles demonstrated good adhesion and reliability results while providing a void-free underfill process without the need for a time-consuming bake-out step.