A novel high performance adhesion enhancing Zn-Cr leadframe coating for popcorn prevention

A novel adhesion enhancing Zn-Cr (A2) leadframe coating has been reported to be a highly effective solution for popcorn prevention in plastic surface mount packages. In this paper, we report our recent understanding of the adhesion and degradation mechanisms of such a coating system. TEM and TOF-SIMS were employed to study the structure of A2-coating and the molding compound/leadframe (MC/LF) interface. The A2-coating was found to be a partially thin, crystalline layer comprised of a continuous interfacial layer and a network of whiskers. Zn silicate compound, ZnO and Cr oxide(s) were identified as possible phases present in the A2-coating. The adhesion enhancing and degradation mechanisms of the A2-coating were investigated by lead pull test to study the influence of temperature cycling, pressure cooker testing, moisture preconditioning, and leadframe oxidation. The adhesion data showed that the A2-enhanced MC/Cu LF interface is not susceptible to thermomechanical stress and moisture degradation. The wetting of the molding compound coupled with mechanical interlocking mechanism offered by the whiskers are believed to be key contributing factors of adhesion enhancement. Degradation of the A2-enhanced MC/Cu LF adhesion was observed after 100 min exposure at 300/spl deg/C by Cu oxide formation from Cu outward diffusion through the A2-layer and was validated by AES analysis operated in the depth profiling mode. Lead pull test specimens were cleaved to have access to the MC/Cu LF interface for RBS analysis to identify the locus of failure.