Effect of the plasma cleaning process on plastic ball grid array package assembly reliability

Interface delamination is recognized as one of the major failures of microelectronics packaging. It can result from various factors, including stresses from mismatch of adherent materials, hygrothermal stress from the release of vapor pressure of moisture during soldering reflow process, and interface material adhesion strength. The failure mechanisms are associated with cyclic loads, temperature and moisture condition as well as interface adhesion strength degradation. This paper focuses on the evaluation of plasma cleaning on PBGA assembly, including resistance to interface delamination. Two different plasma systems, powered by radio frequency (RF) and microwave (MW) energy, are studied. The optimized plasma cleaning process parameters are obtained by surface contact angle measurements. The plasma cleaning results are also verified by scanning electron microscopy (SEM) as well as physical pull and shear tests. The test vehicles are 27×27 mm 292-ball PBGAs. The results from encapsulation peel tests, die and encapsulant pull tests, bonding wire pull tests and C-Mode SAM (C-SAM) examination are presented. It is clear that an optimal plasma cleaning process can be achieved with different plasma systems. The experimental results also demonstrate that plasma cleaning has little effect on wire bonding process and die attach pull strength for given substrates and assembly materials. In all the cases, optimal plasma cleaning steps improve PBGA resistance against interface delaminations for cases where plasma cleaning is carried out before encapsulation process. Moreover, different plasma cleaning techniques would affect the assembly productivity, investment and yield. This paper demonstrates that the optimized plasma cleaning process would enhance PBGA package qualification level and improve the process yields and productivity.