An analytical characterization and reliability testing of an adhesion enhancing Zn-Cr leadframe coating for popcorn prevention

Abstract

A novel Zn-Cr (A2) leadframe coating has been developed and demonstrated to be an effective solution in eliminating popcorn cracking. In this investigation, the A2-coating structure and molding compound (MC)/Cu leadframe (LF) interface was analyzed by Transmission Electron Microscopy (TEM) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The A2-coating was found to be a thin, partly crystalline layer comprising a continuous interfacial layer and a network of whiskers. The TOF-SIMS and TEM results have identified Zn silicate compound, ZnO and Cr oxide(s) as possible phases present in the coating. The effect of temperature cycling, pressure cooker testing, moisture pre-conditioning, and overheating on the A2 adhesion and degradation mechanism was investigated using a lead pull test. The adhesion data showed that the A2-enhanced MC/Cu LF adhesion is resistant to thermomechanical stress and moisture degradation. The wetting of the molding compound and mechanical interlocking offered by the whiskers are believed to be key factors of adhesion enhancement. Degradation of the A2-enhanced MC/Cu LF adhesion was observed at 300/spl deg/C after 100 mins exposure through Cu oxide formation by Cu outward diffusion. This was verified by Auger Electron Spectroscopy (AES) analysis operated in the depth profiling mode. Lead pull test specimens were mechanical cleaved to have access to the MC/Cu LF interface for Rutherford Backscattering Spectroscopy (RBS) analysis to identify the locus of failure for various samples.