Characterization of anisotropic conducting adhesive used as a flex-to-card interconnection

Commercial anisotropic conducting adhesive (ACA) film was used as a solder substitute to interconnect the polyimide flexible circuit from a flat panel display to the FR4 circuit card. The electrical interconnection properties of these bonds were measured shortly after bonding and after subjecting the bonded assemblies to thermal cycling and temperature and humidity. Two bond geometries were investigated, one with an area of 0.409 mm² and the other with an area of 1.29 mm². The effect of bonding temperature and pressure on the bond thickness, number of trapped particles, and resistance was investigated. Over the range of bonding pressures, a systematic variation in the average bond thickness was found, but no variation was found in the number of particles in the bond, or the resistance. Using a technique to separate the a-spot constriction resistance from the bulk resistance, observations were made of the change in these two components of the total resistance as the specimens were aged. The change in the two types of resistance were monitored both in thermal cycling, and as a function of time for aging at 85°C and 79% relative humidity (RH). The thermal electric voltage at the a-spot contact was also monitored as the a-spot was heated with excess electrical current