2 mils Au wire interchip wedge bond cratering study

Abstract

Au wire thermosonic wedge bonding is applied for die to die interconnect on accelerometer device. With the fragile bond pad structure of MEMS device, bond pad cratering or bond pad metal peel is a fatal wedge bond defect during assembly packaging and reliability failure during field application. Typical interchip wedge bond failure mode and the cause of failures are studied through analyzing wedge bonding mechanism, packaging & tooling design and bond pad metallization. Comprehensive wedge bond process characterization was carried out to improve wedge bond quality and reliability. Critical wedge bond responses (wedge pull, wedge shear, Cratering) were studied during wedge bond process characterization. Post wedge bond cratering has good correlation with the wedge tool in both position and shape. Si nodule underneath the bond pad metal generates higher local stress to the bond pad during wedge bonding. Thinner bond pad metal could not act as a cushion to mitigate the wedge bond stress to the poly silicon layer beneath the bond pad. The initial post wedge bond cratering / bond pad damage will get deteriorated during following assembly reflow process as well as SMT process, eventually the weakest interface layer beneath the bond pad will be separated, reflected as the bond pad metal peel. Approaches to enhance interchip wedge bond integrity were explored and significant improvements were seen.