Effect of nonlinear hygro-thermal and residual stresses on the interfacial fracture in plastic IC packages

This study presents a multi-disciplinary approach in order to investigate the influence of hygroscopic stresses and stress relaxation in Epoxy Molding Compounds (EMC) on the apparent interfacial fracture toughness of Cu/EMC interfaces. Bi-material beams were designed and produced via transfer molding similarly to the process used in state-of-the-art semiconductor packaging. An empirical method was used to investigate the so-called ldquostress-freerdquo temperature of the package using warpage measurement of the beams at elevated temperatures. Cure shrinkage was measured by introducing a fitting parameter to the Finite Element (FE) analysis to match the simulation results to the experimental warpage. The bi-material beams were exposed to a humid environment and their warpage was observed to change during the moisture absorption. Warpage measurements together with a viscoelastic FE analysis allow for the calculation of two important material properties, namely, cure shrinkage and the Coefficient of Hygroscopic Swelling (CHS). Fracture toughness of the EMC/Cu interface as a function of exposure time to moisture was measured. End-Notched Flexure (ENF) tests at various intervals after moisture loading were performed. The Virtual Crack Closure Technique (VCCT) was applied to measure the adhesion strength in terms of interfacial fracture toughness by considering the sample manufacturing and sorption history.