Measurement of Hygroscopic Swelling in Mold Compounds and Its Effect on PEM Reliability

Hygroscopic stresses arise in a plastic encapsulated microcircuit (PEM) when the mold compound swells upon absorbing moisture and the lead frame, die paddle, and silicon die, do not experience swelling. Similar to the thermal stress produced by the mismatch of coefficients of thermal expansion between adjacent materials, the hygroscopic stress increases as the hygroscopic swelling coefficient of the mold compound increases. Accurate measurement of hygroscopic swelling is essential in assessing the effect of hygroscopic stresses on package reliability. In this paper, a whole-field experimental method to characterize the hygroscopic swelling of mold compounds is proposed. The method is implemented to determine the hygroscopic swelling and the corresponding coefficient of hygroscopic contraction of five commercial molding compounds. A comparison between the hygroscopic and thermally induced mismatch strains in PEMs is presented and its implication on PEMs reliability is discussed.