Development of high performance surface mount conductive adhesives

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 limitations. Two critical ones are the unstable contact resistance on non-noble metals and the inferior impact performance. The ultimate goal of our study has been to develop conductive adhesives with stable contact resistance and superior impact performance. Our previous study indicated that electrochemical (galvanic) corrosion was the predominant mechanism for the unstable contact resistance during elevated temperature and humidity aging. In this study, the effects of several additives (oxygen scavengers and corrosion inhibitors) on contact resistance stability during elevated temperature and humidity aging have been investigated. Effective additives have been identified based on the evaluation results. The adsorption of the additives on Sn/Pb solder surface are studied and correlated to their effectiveness to prevent the electrochemical corrosion. In addition, several rubber-modified epoxy resins and epoxide-terminated polyurethane resins have been introduced into the adhesive formulations to determine their effects on impact strength. Tan δ of each formulation was measured using a dynamic mechanical analyzer (DMA) and impact strength was evaluated using drop test. Resins that improve the impact strength of adhesives have been identified. Finally, high performance conductive adhesives have been formulated by combining the identified resins with the effective additives. It is found that (1) the oxygen scavengers tested can delay the resistance increase of the adhesives; (2) the effectiveness of the best performing corrosion inhibitor is probably due to its strong adsorption to the metal surfaces; (3) some rubber modified epoxy resins and epoxide-terminated polyurethane resins provide the superior impact performance of conductive adhesives; and (4) conductive adhesives with both stable contact resistance and superior impact performance can be developed based on the above findings