Re-building the Underfill: Performance of percolating fillers at package scale

This paper addresses both, the thermal and the thermo-mechanical performance of percolating thermal underfill applied flip-chip packages. We present a thermal test platform in flip-chip package design allowing the thermal conductivity of any underfill to be measured at package scale. We give details about design technology and current fabrication status. We benchmarked the thermal performance of the platform with percolated thermal underfill against the capillary thermal underfill case. In a layout with peripheral solder bumps the performance benefit related to heat dissipation of an overall system with percolating thermal underfill can be over 50% for liquid cooling and over 25% for enforced convection. The improvement for natural convection, typical in mobile environment, is 5% in the best considered case. We also present a study of the thermo-mechanical performance of the flip chip test platform during thermo-shock cycling by the aid of finite element (FE) tools at the critical regions of interest. We benchmarked the percolating thermal underfill against capillary mechanical underfill and capillary thermal underfill. For that investigation we used an effective material merging a visco-elastic polymer and a visco-plastic solder for saving computation time. In the percolating thermal underfill applied package, we've found slightly higher risk of delamination at the die | underfill interface but very low fillet loads compared to conventional underfill. The risk of solder fatigue is significantly lower and the risk of die cracking is 25% higher for percolating than for the capillary underfills.