Effect of bump height on the reliability of ACA flip chip joining with FR4 rigid and polyimide flexible substrate

Flip chip joining using anisotropically-conductive adhesive (ACA) has become a very attractive technique for electronics packaging. However, many factors can influence the reliability of the ACA flip chip joint. Bump height is one of these factors. In this work, the reliability of hip chip joining with different bump heights was studied using a temperature cycling test. Both the assemblies on FR4 rigid and polyimide flexible substrates were examined. In the temperature cycling test, the assemblies on FR4 substrate show that reliability decreases with increasing bump height. On the other hand, with the flexible substrate, the electrical performance of the ACA flip chips does not change. The mechanisms for the different reliability behaviors of the ACA joints on the two kinds of substrate were discussed. The results show that the different mechanisms are attributed to the different stiffness of the substrate. Furthermore, given the thickness of the adhesive layer, an excessively high bump height can easily prompt the formation of porosity during assembly.