Effects of dimple and metal coating on interfacial adhesion in plastic packages

Abstract

This paper reports on the effects of dimple and metallic coating of the Cu-alloy lead frame on interfacial adhesion with an epoxy-molding compound. Round dimples of varying number are introduced on one side of the lead frame by chemical etching. The plating materials studied include bare-Cu alloy and microetched Cu, Ag, Ni, Pd/Ni, and Au/Ni coatings. The surface characteristics, such as wettability, surface roughness, and element compositions, were evaluated based on several characterization tools, which, in turn, are correlated with adhesion performance. The dimples enhanced the interfacial-bond strengths through improved mechanical interlocking of the molding compound, depending on the type of coating. The improvement was much more significant for the coatings with inherently weak interfacial adhesion (e.g., microetched Cu and Ni coating) than those with inherently strong adhesion characteristics (e.g., Au and Pd coatings). The wettability of the metal surface represented by the surface energy or interfacial energy played a dominant role in the resulting interfacial adhesion. Elemental analysis of the fracture surface indicates that the silicon content had roughly a linear relationship with the interfacial-bond strengths for different coatings. The surface roughness was insensitive to the interfacial-adhesion performance. The silicon content measured from the lead-frame fracture surface was shown to directly correlate to the interfacial-bond strength. Higher silicon content was a reflection of larger surface-area coverage by the molding compound associated with cohesive failure.