On the effect of partial underfilling on the fatigue life of flip-chip micro-solder bumps in a heterogeneously integrated TSI package using finite element simulations

The requirement of partial underfilling of flip-chips in certain heterogeneously integrated packages could adversely affect the fatigue life of the micro-solder bumps. We performed numerical simulations to show that the underfilling of the surrounding regions of micro-bumps is necessary to have a comparable fatigue life as those of the full underfill case. The effect of underfill away from the micro-bumps is insignificant on their fatigue life. In the case of partial underfilling with some bumps without surrounding underfill, they become critical with significantly lower life as compared to the micro-bumps surrounded by underfill.