Leadframes' AuAg plating thickness influences to stitch bonding of palladium coated copper wires

Abstract

Palladium coated copper (PCC) wire bonding on roughen preplated leadframe is a preferred electrical interconnect in the semiconductor package due to its robustness package performances. PCC wire is replacing bare copper wires to eliminate the copper oxidation problem. Whereas, roughen treated preplated leadframe is used to eliminate post-plating of leadframe as well as to enhance the adhesion of leadframe to the epoxy mold compounds. However, role of the plating thickness of roughen preplated leadframe on the PCC wire has not yet been fully understood. Thus, this paper investigates stitch bonding strength of PCC wire on the AuAg/Pd/Ni/Cu preplated leadframe with different AuAg plating thickness. Stitch pull test results on PCC wire bonding on different preplated leadframe showed higher stitch bonding strength for samples with higher AuAg plating thickness. Optical micrographs showed mechanical failure at bond heel of test sample, due to higher tensile stress during the pull test. Thicker AuAg layers may provide greater cushioning effect against wire deformation at bond heel during stitch bonding process. This in turns increased the stitch bonding strength of the PCC wire bond. STEM micrograph and EDX line-scan profile in all samples showed formation of interdiffusion zone at the PCC wire/ leadframe bonding interface. At the distinct bonding interface, interdiffusion zone extended from Pd layer of PCC wire to Pd layer of leadframe, where Pd species from both sides were intermixed. Whereas at blurred bonding interface, interdiffusion zone was extended further to Pd/Ni interface region. Formation of interdiffusion zone may strengthened wire bond between the PCC wire and the leadframe, thus prevented bond delamination during the stitch pull strength test.