Effect of geometric surface features on void formation: application to the underfill process

A study of the capillary flow process used to place underfill material in a chip-to-substrate gap is reported. The motion of an advancing liquid front has been recorded in an experimental study on the role of geometric surface features and gap spacing characteristic of flip-chip applications. A gallery of images is presented to characterize the hydrodynamic void formation that can occur during the underfill flow process. Patterns of trace lines and solder bump cutouts have been used to construct model surfaces. The motion of the liquid front and the probability of void formation is a complex function of the surface pattern geometry. For example, at small gap spacing fluid fingers advance on trace peaks. Reconnection of finger tips creates air voids in the trough between the parallel traces. Additional examples of void formation are reported here.