Effect of underfill entrapment on the reliability of flip-chip solder joint

The application of underfill materials to fill up the room between the chip and substrate is known to substantially improve the thermal fatigue life of flip chip solder joints. Nowadays, no-flow underfill materials are gaining much interest over traditional underfill as the application and curing of this type of underfill can be undertaken before and during the reflow process and thus aiding high volume throughput. However there is always a potential chance of entrapping no-flow underfill in the solder joints. This work, attempts to find out the extent of underfill entrapment in the solder joints and its reliability effect on the flip chip packages. Some unavoidable underfill entrapments at the edges of the joint between solder bumps and substrate pads are found for certain solder joints whatever bonding conditions are applied. It is interesting to report for the first time that partial underfill entrapment at the edges of the solder joint seems to have no adverse effect on the fatigue lifetime of the samples since most of the first solder joint failure in the no-flow flip chip samples during thermal cycling are not at the site of solder interconnection with underfill entrapment. Our modeling results show good agreement with the experiment that shows underfill entrapment can actually increase the fatigue lifetime of the no-flow flip chip package.