Wafer level application of reworkable underfill

Underfill adhesives play a major role in the reliability of modern flip chip packages. Although these materials have been the focus of much research and development, the method of their application has remained nearly unchanged. Underfill is still dispensed at each die. Significant time and cost savings can be realized by using a process that applies underfill concurrently to multiple dies. The preliminary development for a wafer level underfill process is described in this paper. Screen printing was selected as the initial application method because it is well established in the electronics industry. The equipment is inexpensive compared to other application methods. Process development began with the selection of an underfill material. Nine candidate materials were screen printed onto silicon wafers according to a designed test matrix, and the resulting films were compared for thickness, quality, and uniformity. Two of the materials were selected for use in the next phase of process development based on their ranking. The results of this research are being used in the ongoing development of a reworkable underfill.