A study of underfill dispensing process

The underfill dispensing process has been studied through material characterization and application of statistical tools. Coefficient of Planar Penetrance (COPP) introduced by Schwiebert and Leong3 was used to evaluate the flow performance of three types of underfills. It is dependent on the viscosity, the surface tension, and the wetting angles of the underfill / substrate materials. In addition, the estimated flow times for that underfill can be calculated using Schwiebert and Leong's model with know die size. With the underfill material selected based on COPP, the temperature setting of various heating stations of a dispensing machine was evaluated using the statistical method targeting to set-up the mass production equipment at the optimum operating point. The C-Mode Scanning Acoustics Microscopy (C-SAM) was used to assess the ability of the set-up to provide a void free process. The results show that the actual flow times obtained are about 30 % higher than the predicted ones. It was also observed that void formation occurred when there was insufficient underfill filling in the gap between the die and the substrate.