Moisture induced interface weakening in ACF package

One interesting degradation mechanism in flip–chip interconnections using anisotropic conductive film (ACF) is delamination between the adhesive and the sandwiched material. To understand the delamination process, an attempt was made to quantitatively measure the interfacial fracture toughness of both silicon/ACF and flame retardant 4 (FR4)/ACF sandwiched specimens exposed to various humidity conditions using a 4-point bending test. For moisture absorption, specimens were stored inside a humidity chamber (85 °C, 85% RH) until they were fully saturated, and a pressure cooker test (PCT) was additionally performed to expose the specimen to harsher environmental conditions. Delamination occurrence after moisture absorption was checked by means of a scanning acoustic microscope (SAM). Based on the experimental results, we have determined the energy release rate of each interface under different humidity conditions and have located the weaker region where crack initiation would be likely to occur under service conditions in interconnections using ACF. Evidence of crack propagation along the interface was checked by cross section optical images and scanning electron microscopy (SEM). By comparing the results of the specimens under different humidity conditions, interface weakening by moisture absorption was observed and quantitatively analyzed. Interestingly, we found that ACF can show severe degradation of adhesion strength caused by moisture absorption without any delamination occurring.