Development of conductive adhesives for solder replacement

With the phasing out of lead-bearing solders, electrically conductive adhes ives (ECAs) have been identified as an environmentally friendly alternative to tin/lead (Sn/Pb) solders in electronics packaging applications. Compare d to Sn/Pb solders, conductive adhesive technology offers numerous advantag es. However, this new technology still has reliability limitations. Two cri tical limitations are unstable contact resistance on non-noble metals and p oor impact performance. Our previous study proved that galvanic corrosion i s the dominant mechanism for the unstable contact resistance during elevate d temperature and humidity aging. The ultimate goal of this study is to dev elop conductive adhesives with stable contact resistance and desirable impa ct performance. In this study, effects of purity of the resins and moisture absorption on contact resistance are investigated. Several different addit ives (oxygen scavengers and corrosion inhibitors) on contact resistance sta bility during elevated temperature and humidity aging are studied, and effe ctive additives are identified based on this study. Then, several rubber-mo dified epoxy resins and two synthesized epoxide-terminated polyurethane res ins are introduced into ECA formulations to determine their effects on impa ct strength. The loss factor, tan delta, of each formulation is measured us ing a dynamic mechanical analyzer (DMA) and impact strength is evaluated us ing the National Center for Manufacturing Science (NCMS) standard drop test procedure. Finally, high performance conductive adhesives are formulated b y combining the modified resins and the effective additives. It is found th at 1) purity of the resins and moisture absorption of the formulation affec t the contact resistance stability of an EGA; 2) the oxygen scavengers and corrosion inhibitors can delay contact resistance shift; 3) one of the corr osion inhibitors is very effective in stabilizing the contact resistance; 4 ) some rubber-modified epoxy resins and the epoxide-terminated polyurethane resins can provide the conductive adhesives with superior impact performan ce; and 5) conductive adhesives with stable contact resistance and desirabl e impact performance are developed.