Repairability of underfill encapsulated flip-chip packages

Underfill encapsulation has been proven to yield dramatic improvement in the fatigue reliability of flip-chip mounted packages on ceramics as well as organic chip carriers. However, lack of reworkability of the underfill epoxy has limited its application to single chip modules or a few simpler multi-chip packages. Reworkability is needed for all expensive multi-chip packages in order to replace the defective chip with a new one. The rework process for an encapsulated flip chip package is complicated due to factors such as localized chip removal, solder replenish, new chip joining, encapsulation and establish reliability. In this context, the authors have done technical evaluations on a few selected chip repair schemes, such as mechanical chip-grinding, mold release layer, and reworkable epoxy underfill materials. They were studied on metallized ceramic pin grid array modules used as test vehicles. Thermal cycling reliability experiments as well as rework feasibility evaluations were done on test vehicles. Results are briefly presented in this paper.