Understanding the Contact Resistance in an ACF Bonding

Abstract

Anisotropic Conductive Film (ACF) bonding, the technique of choice for display interconnects, allows very fine pitch interconnect at a moderate process temperature, typically at the expense of a non-negligible interconnect resistance. Modern ACFs have conducting particles in a thin layer of highviscosity adhesive. This allows even finer pitch but comes at the risk of high interconnect resistance due to trapped adhesive between pad and conductive particle. Conductive particles with spikes are designed to penetrate such a trapped adhesive layer. In this paper, we compare spiky conductive particles with the traditional, spherical, ones. We measure interconnect resistance of individual particles in a nano-indentation test setup, as well as the interconnect resistance of ACF bonding using the two different particles, in daisy-chain measurements. Interconnect resistance of individual spiky particles is more than an order of magnitude higher than the one for spherical particles. However, when mixed in an adhesive and used for ACF bonding, the interconnect resistance of ACF with spiky particles is somewhat lower than that of the ACF with spherical particles. The interconnect resistance of ACF with spherical particles is two orders of magnitude higher than that of the individual particles, whereas the interconnect resistance of ACF with spiky particles is comparable to the individual-particle resistance. We conclude that trapped adhesive is a major concern for fine-pitch ACF bonding, and that the use of spiky conductive particles has the potential to overcome these challenges.