A highly reliable flip chip joint consisting of an anisotropic conductive film

The application of anisotropic conductive films (ACF) to form interconnections between chips and substrates is one potential variation of flip chip technology. Its appeal comes from the possibilities of forming very fine pitch interconnections that are not feasible with solder, avoiding the use of lead, and achieving simpler, lower cost processing. However, since the interconnection mechanism of ACF joints is complicated and has not been explained quantitatively, this technology has not been applied widely. In this study, we evaluate the interconnection reliability of ACF joints in terms of a reliability criterion that maintains the compressive contact force stably at the contact face. We have clarified the influence of the material properties of the ACF and the build-up substrate on the interconnection reliability of ACF joints during temperature cycling. The key factor that dominates the interconnection reliability is the amount of plastic strain of the chip bump and the substrate pad at the contact face. Therefore, it is very important to control the plastic deformation of the chip bump and Cu pad by optimizing the mechanical properties of the ACF and the build-up material. On the other hand, we found that the interconnection reliability during moisture resistance testing may deteriorate due to stress relaxation at the contact face, which occurs at temperature near the glass transition temperature of the commonly used build-up material.