Study on isotropic electrically conductive adhesive for medical device applications

In implantable medical device applications, quality and reliability are critical aspects of the product. There are some unique mechanical and electrical challenges to the silver filled isotropic electrically conductive adhesive (ECA) used in these devices. The mechanical challenge is the result of the very complicated system design which involves a wide variety of integrated circuits directly bonded and over-molded onto the double-sided printed wiring board (PCB). Only a very small percentage of ECAs are capable of meeting the mechanical strength requirements for this application. These mechanically acceptable ECAs must also be thoroughly studied to ensure that a strict resistance stability requirement can be met. It is imperative to fully understand the fundamentals of the ECA material chemistry and performance relationship before it can be used in medical applications. A series of studied results will be presented in this paper. The system design challenge on ECA formulation is presented. The numerical and experimental study results on PCB design and its impact on bond pad-adhesive delamination are reported. The electrical performance of epoxy-based and acrylate-based conductive adhesives was comparatively studied. The device level critical electrical performance of conductive adhesives was found not to be correlated to bulk resistivity of cured ECA, but significantly correlated to the interfacial resistance between the cured adhesive and bond pad. Unfolding the atomic level phenomenon occurring at this interface during adhesive curing and the device burn-in process is one of the main focuses of this paper. The fundamental atomic and molecular level mechanism of the assembly process and the material chemistry impact on resistance stability will be discussed. The mechanism includes how the conductive path was formed on the adhesive and gold pad interface during adhesive cure and how the interface evolves during the burn-in process. Also discussed are ingredients in the adhesive formulation that may impact the resistance instability, and the solution space for stable resistance.