Reliability of conductive adhesives as a Pb-free alternative in flip-chip applications

The temperature-humidity reliability of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects is investigated by measuring the interconnect resistance during temperature-humidity testing (THT) at 85°C and 85% relative humidity. The four-point probe method was used to measure the interconnect resistance of the adhesive joints constructed with Au bumps on Si chips and Cu pads on flexible printed circuits (FPCs). The interconnect resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The interconnect resistances of both interconnects decreased with increasing bonding force, and subsequently converged to about 10 mΩ and 1 mΩ at a bonding force of 70 N and 80 N, for the ACF and NCF joints, respectively. During the THT, two different conduction behaviors were observed: increased interconnect resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.