Two mechanisms for autoclave failure of flip chip PBGA packages have been identified. Transverse hygroscopic swelling of underfill has been found to be 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 delamination near the centre of the chip. In-plane hygroscopic swelling of underfill and substrate is responsible for the second failure mechanism. It results in extensive underfill-chip delamination initiating from the chip edge before solder joint failure. A vast difference in performance has been found for flip chip packages subjected to 85/spl deg/C/85%RH and 121/spl deg/C/100%RH conditioning, respectively. The compressive hygrothermal strain around the solder is believed to have contributed to the excellent performance during 85/spl deg/C/85%RH test.