Moisture induced swelling and sorption characteristics of four types of epoxy molding compounds used in the packaging of semiconductor devices were experimentally investigated. The hygroscopic strain with respect to moisture content was found to be linear except at the initial stages of desorption where anomalous trends were observed. The swelling coefficient values obtained from the "stable" swelling regions were found to range from 0.3 to 0.6 (%L/%M) at 85/spl deg/C for the four types of molding compounds tested. It was also found that the swelling coefficient increased with temperature for all four types of molding compounds tested. The significance impact of hygroscopic mismatch strain was investigated by comparing the strains induced due to mismatch of the coefficient of hygroscopic swelling and the coefficient of thermal expansion for each type of molding compound when attached to a copper lead frame. The hygroscopic and thermal mismatch strains were compared using the swelling coefficient and CTE values for each type of molding compound and adjacent material (i.e., copper lead frame). Hygroscopic mismatch strains were found to be highly significant relative to the thermal mismatch strains, and they should be accounted for in the reliability modeling of packages subjected to accelerated testing. The effect of hygroscopic mismatch strains is often ignored in the reliability tests and models. In this study it is found that the hygroscopic strains can be comparable to, if not higher than, thermal mismatch strains.

A reliable technique for characterizing the hygroscopic swelling of materials has been developed and used to characterize a number of packaging materials. Using these data, hygroscopic stress modeling were performed. The hygroscopic stress induced through moisture conditioning was found to be significant compared to the thermal stress during solder reflow. Hygroscopic stress in over-molded wire bond PBGA and molded Flip Chip PBGA was found to be 1.3 times to 1.5 times that of thermal stress. Hygroscopic swelling of the underfill in FCPBGA was found to be the main failure driver during autoclave test. Autoclave performance of FCPBGA package assembled with different underfills and chips were analyzed. Excellent correlation was found between autoclave performance and the hygroscopic swelling characteristics of the underfills.