RELIABILITY STUDIES OF FLIP CHIP PACKAGE WITH REFLOWABLE UNDERFILL

Reflowable underfill is originated to simplify the Flip-Chip assembly process. In this study, JEDEC level 3 moisture sensitivity preconditioning followed by thermal cycling (TCB,-55oC~125oC) was conducted. Two reflowable underfill mainly differentiated by curing kinetics and adhesion strength, namely Underfill A and Underfill C, were evaluated. Failure analysis was carried out via C-SAM and cross section. Unit failure that was electrically identified in present study was defined as occurrence of bridging and/or open circuit. The results showed that for packages assembled with Underfill A that has superior adhesion with silicon nitride passivation, all units can pass preconditioning but bridging commenced to appear at 750 cycles. Open circuitry was encountered between 1750 and 2000 cycles. While for Underfill C with fast cure characteristics and relatively weaker adhesion compared to Underfill A, bridging and open circuitry started to surface at 500 and 1250 cycles, respectively. Failure analysis indicated that bridging mainly resulted from solder bump extrusion through micro voids trapped between adjacent bumps during assembly process; meanwhile the gap between neighboring bumps is too close to prevent convergence of two extruded bumps. Open circuitry is originated by several factors such as insufficient wetting, underfill delamination, die crack, underfill crack etc. On the whole, no underfill delamination and die crack have been observed before 1000 cycles in present study. Most importantly, no solder crack that is the direct reflection of CTE mismatch has been observed throughout the whole reliability test. This fact suggests that assemblies with relatively-higher-CTE reflowable underfill still can meet general reliability standard