Evaluation of self-assembled monolayer treatment for wire bonding with ENEPIG surface finish
Extended service life plastic ball grid array (PBGA) packaging with Au ball wire bonding and die-side electroless nickel electroless palladium immersion gold (ENEPIG) surface finish is in development. Post-microfabrication ENEPIG plating is proposed for the extended service life PBGA application to reduce the risk of brittle failure of wire bonds with Au-Al intermetallics over decades of exposure to thermal extremes in the field. An experimental study was undertaken to measure wire bond strength in PBGA samples with ENEPIG plating and varying die and substrate surface preparation. Liquid-phase immersion surface treatment with alkanethiol-based self-assembled monolayers (SAMs) was investigated as an alternative to argon plasma cleaning. Wire bond pull and shear force measurements per MIL-STD-883 were performed on four groups of PBGA samples with prescribed combinations of ENEPIG plating, plasma cleaning and SAM surface treatment. Results reveal that average wire bond pull and shear force in ENEPIG-plated samples exceed six grams force in all cases and are consistent with that of non-plated samples for constant bonding parameters. Surface treatment yields a measurable increase in wire bond strength relative to untreated samples, and ENEPIG-plated samples that receive plasma cleaning exhibit 23% greater average wire bond pull force relative to plated samples treated with SAMs. Wire bond strength was lower in samples where greater ENEPIG surface roughness was observed, consistent with data from the literature. The outcome supports the importance of surface cleaning for robust wire bonds, and suggests that in the context of strength, ENEPIG plating is compatible with the legacy Au ball-on-Al pad thermosonic wire bonding process.