Epoxy Molding Compound Development for Improved MSL1 Delamination Resistance in Plastic Encapsulated Clip Bond Power Package

Abstract

Package delamination after reflow is a critical failure attribute for power electronic devices. Delamination reduces the path for removing the heat generated by the device and severely affects its functionality. Major factors affecting delamination in clip bonded devices include the moisture absorption of epoxy molding compound (EMC) and induced thermomechanical stresses in the package. In this study, four advanced formulation EMCs developed to improve delamination resistance of a clip bond power package were evaluated. These EMCs were formulated to the have low moisture absorption as well as low modulus. This was achieved using multi-aromatic resins, low moisture absorption hardeners and increased filler content (>80wt%). To determine the moisture absorption characteristic of the materials, these were subjected to moisture sensitivity level 1 (MSL1) conditions and 3x reflow at 260°C. Preconditioning was performed to simulate the stress that the package would experience during PCB assembly. The amount of moisture absorbed and moisture diffusion coefficient of the EMC after saturation were determined using weight gain experiments. Thermal analysis was done to determine coefficients of thermal expansion, glass transition temperature and elastic modulus of the EMCs. The stress induced by the EMC on die and leadframe interfaces were calculated based on the material property characterization results. Amount of delamination before and after reflow simulation was measured and analyzed using Scanning Acoustic Microscopy. Overall results showed that low moisture absorbing (<0.3wt%) molding compounds performed best. Lower stress index at reflow process conditions were also obtained with lower CTE mismatch to Si and Cu, low Tg (<150°C) and low modulus at high temperature (<0.74 GPa). Zero delamination was achieved on Si die interface while the delamination on Cu leadframe interface after MSL1 preconditioning significantly improved with low stress EMC properties. Excellent combination of mold compound material properties enabled a plastic encapsulated power clip bond package to achieve improved MSL1 delamination resistance towards more robust power electronics packaging.