Conductive adhesives for solder replacement in electronics packaging

Electrically conductive adhesives (ECAs) are an environmentally friendly alternative to tin/lead (Sn/Pb) solders for surface mount applications. Compared to Sn/Pb solders, conductive adhesive technology offers numerous advantages. However, this new technology still has many limitations. Two critical limitations are unstable contact resistance on non-noble metals and poor impact performance. Our previous study indicated that galvanic corrosion was the dominant mechanism for the unstable contact resistance during elevated temperature and humidity aging. The ultimate goal of this study is to formulate conductive adhesives with stable contact resistance and desirable impact performance. In this study, the effects of several additives (oxygen scavengers and corrosion inhibitors) on contact resistance stability during elevated temperature and humidity aging are studied. Effective additives are identified based on this study and their adsorption of on Sn/Pb surfaces are also studied and correlated to their effectiveness to prevent corrosion. Then, several rubber-modified epoxy resins and epoxide-terminated polyurethane resins are introduced in adhesive formulations to determine their effects on impact strength. Tan δ of each formulation is measured using a dynamic mechanical analyzer (DMA) and impact strength is evaluated using drop test. Resins which can improve impact strength of ECAs are identified. Finally, high performance conductive adhesives are formulated by combining one of the identified resins and the effective additives. From this study, it is found that (1) the oxygen scavengers tested can delay resistance shift; (2) one of the corrosion inhibitors studied is very effective in stabilizing the contact resistance probably due to its strong adsorption to the metal surfaces; (3) some rubber-modified epoxy resins and epoxide-terminated polyurethane resins provide the conductive adhesives with superior impact performance; and (4) conductive adhesives with stable contact resistance and desirable impact performance are developed