Properties of conductive adhesives based on anhydride-cured epoxy systems

Recently, isotropic conductive adhesives (ICAs) have been identified as a potential alternative for lead-containing solders in surface mount technology (SMT) applications. However, current commercial ICAs have some reliability issues that seriously limit their universal acceptance in electronics packaging areas. One of the critical reliability issues is that contact resistance between the ICAs and non-noble metals increases dramatically particularly during elevated temperature and humidity aging. The objective of this study is to investigate the contact resistance behaviors of a class of conductive adhesives, which are based on anhydride-cured epoxy systems, in elevated temperature and humidity environments. Cure profiles, moisture pickup, and shifts of contact resistance of the ICAs on different non-noble metals, such as tin (Sn), tin/lead (Sn/Pb), and copper (Cu), during aging are investigated. It is found that (1) this class of ICAs shows low moisture absorption; (2) the contact resistance of the ICAs on Sn and Sn/Pb decreases first and then increases slowly during 85 degrees C/85% relative humidity (RH) aging; and (3) the contact resistance of the ICAs on Cu remains almost constant after the initial decrease during aging. In addition, the reasons for the contact resistance decrease are studied. Experimental results indicate that the initial contact resistance decrease is probably due to the removal of the insulating organic lubricant layer on silver (Ag) flakes or/and the removal of metal oxide layer on the non-noble metal surfaces by the acid, which is formed after the moisture reacts with the anhydride. From this study, it can be concluded that ICAs based on anhydride cured epoxy systems are promising formulations for surface mount technology applications.