Wafer level underfill entrapment in solder joint during thermocompression: Simulation and experimental validation

The purpose of this paper is to model and understand wafer-level underfill (WLUF) flow behavior during thermocompression in order to predict the risk of residual polymer entrapment between top and bottom interconnects, which drastically reduces electrical yield. In the first part of this paper, the fluidic equation describing remaining polymer entrapment thickness has been established. It depends on layout such as interconnect diameter, thermocompression parameters such as force and time as well as WLUF initial thickness and viscosity. Secondly, comparison with experimental results has been made. This work highlighted that residual polymer thickness cannot only be predicted by WLUF flow ability phenomenon. Procedure installation by Fourier transform infrared spectroscopy analysis has allowed verifying that WLUF cross-linkage did not happen during thermocompression. However, it has been demonstrated that plastic deformations of the solder joint could occur and dramatically increased residual underfill thicknesses in solder joint during thermocompression.