Low peel strength investigation in direct gold System in Package module

A System in Package (SiP) module development including Surface-Mount Technology (SMT) and Flip Chip (FC) for the passive components and controllers placement on direct Au substrate. In addition, the Laterally Diffused Metal-Oxide-Semiconductors (LDMOS) and GaN dies are also placed by die attach process. An investigation was conducted to analyze the root cause of the low peel strength on this Au stitch bond to Au pad structure. In this investigation, a low peel strength mechanism of Au wire bonding on direct Au substrate was demonstrated. The interdiffusion behavior of Cu and Au was analyzed by a Transmission Electron Microscope (TEM) with simulated heat treatment condition. In order to validate the root cause, the fracture mode was investigated and its corresponding pad surface analysis was performed by Auger Electron Spectroscopy (AES) analysis on the most surface through a step-by-step method from substrate incoming until wire bond completed. The results indicate that the low peel strength was caused by the poor adhesion between Au wire and Au pad, fracture occurred at wire and pad interface. With a deep dive analysis by AES in comparison with pure and processed Au pad surface, the results indicate the pad was contaminated by Cu migrated from base and its oxide caused degradation of adhesion of Au wire to Au pad.