Joint Morphologies and Failure Mechanisms of Anisotropic Conductive Films (ACFs) During a Power Handling Capability Test for Flex-On-Board Applications

In this paper, joint morphologies and failure mechanisms of anisotropic conductive films (ACFs) during a power handling capability test for flex-on-board applications were investigated in terms of four conductive particles: gold-coated polymer ball, Ni metal ball, SnBi58, and Sn-3Ag-0.5Cu particles. It was found that all the polymer ball and Ni ball joints could not handle the ACFs' joint temperature of 200 °C presumably due to polymer resin expansion and physical contacts loss, and their corresponding contact resistance also became open circuit after the power test. In contrast, the failure mechanism of SnBi58 solder joint was that when temperature was over 138 °C and Sn element was quickly consumed by Sn-Ni intermetallic compounds resulting in brittle Bi-rich phase remaining, more severe crack occurred along the Bi-rich phase of SnBi58 joints. Sn-3Ag-0.5Cu solder ACF joints due to a higher solder melting point were the best among the four conductive particles in terms of the little crack formation in its joint morphology and the morphology and the remaining stable electrical resistance even after the highest power test to 300 °C, when FR4 printed circuit board burned. Beyond the FR-4 decomposition at 300 °C, ACF joints were burned due to epoxy resin decomposition from 330 °C.