Underfill swelling and temperature-humidity performance of flip chip PBGA package

Two mechanisms for autoclave failure of flip chip PBGA package have been id entified : Transverse hygroscopic swelling of underfill has been found to b e responsible for the first failure mechanism typified by randomised solder joint failure associated with localised underfill-chip delamination around the solder joints. The localised delamination may coalesce into a larger d elamination near the centre of the chip. In-plane hygroscopic swelling of u nderfill and substrate is responsible for the second failure mechanism. It results in extensive underfill-chip delamination initiating from chip edge before solder joint failure. Vast difference in performance has been found for flip chip package subject ed to 85 degreesC/85%RH and 121 degreesC/100%RH conditioning respectively. The compressive hygrothermal strain around the solder is believed to have c ontributed to the excellent performance during 85 degreesC/85%RH test.