Plasma Surface Modification and Impact on MSL Performance for Flip Chip Packaging

The effectiveness of "under-chip" plasma cleaning and its impact on moisture sensitivity level (MSL) performance for flip chip packaging was investigated in this study. A customized test vehicle was designed for surface analysis on the flip chip silicon nitride (SiN) passivation and substrate solder mask surfaces. Optimization of the plasma conditions was achieved using contact angle measurement, adhesion testing, and verified with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis. The impact of plasma cleaning on MSL performance was assessed using a molded flip chip in package (FCIP) test vehicle. It was found that significant adhesion enhancement of SiN passivation to underfill can be achieved by a synergistic combination of the following plasma surface modifications: removal of organic contaminants, formation of certain functional groups (C=0, O-C-O and 0-C=0), and an increase in effective surface area. In summary, this study has demonstrated the benefits of an optimized "under-chip" plasma cleaning process for MSL performance improvement of a molded FCIP. Findings from this extensive plasma characterization also enable a better understanding on how plasma surface modification can contribute towards packaging of more robust products.