Effects of reflow process on the reliability of flip chip on flex interconnections using anisotropic conductive adhesives

In this paper, the effects of the solder reflow process on the reliability of anisotropic conductive film (ACF) interconnections for flip chip on flex (FCOF) applications are investigated. Experiments as well as computer modeling methods have been used. In the experiments, it was found that the contact resistance of ACF joints increased after the subsequent reflow process, and the magnitude of this increase was strongly correlated to the peak temperature of the reflow profile. Nearly 40% of the joints were opened (i.e. lifted away from the pad) after the reflow process with 260 /spl deg/C peak temperature while no opening was observed when the peak temperature was 210 /spl deg/C. It is believed that the CTE mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. It was also found that the ACF joints after the reflow process with 210 /spl deg/C peak temperature showed a high ability to resist water absorption under steady state 85 /spl deg/C/85%RH conditions, probably because the curing degree of the ACF was improved during the reflow process. To give a good understanding, a 3D model of an ACF joint structure was built and finite element analysis was used to predict the stress distribution in the conductive particles, adhesive matrix and metal pads during the reflow process.