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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°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.