Mechanisms underlying the unstable contact resistance of conductive adhesives

Conductive Adhesives have been successfully applied to hybrid circuits and die-attach applications. More recently they have become the exclusive bonding techniques for flex interconnections to LCD's and driver IC's (chip on glass). Additional applications for bonding surface mount devices as well as flip chip components on rigid and flexible printed circuits are being explored. In surface mount applications, surface mount conductive adhesives (SMCAs) are an environmentally friendly alternative to Sn/Pb metal solder. In addition, SMCAs offer many other advantages over Sn/Pb solder in surface mount applications, including high pitch capability, mild processing temperature, and fewer processing steps. However, one critical drawback of the current SMCA technology is its unstable contact resistance with non-noble metal finished components. The contact resistance of the assembly usually shows substantial increase over time particularly under high temperature (85 degrees C) and high humidity (85%RH) conditions (so called 85/85 conditions). Several possible mechanisms including metal oxidation and electrochemical corrosion have been suggested in the literature. However, prior work has not confirmed the exact mechanism responsible for unstable contact resistance. The focus of this study was to identify the underlying mechanism that results in the unstable contact resistance phenomenon of the current SMCAs. A contact resistance test device consisting of metal wire segments and conductive adhesive dots was developed for this study. Based on the results of this systematic study, electrochemical corrosion has been identified as the underlying mechanism responsible for the observed unstable contact resistance phenomenon of conductive adhesives.