Effect of Void in the Anisotropic Conductive Assembly

The design of the ACF is used for very fine pitch application in the microelectronic industry such as flip chip technology. During the bonding process, bumps on the chip and pads on the substrate has been first aligned and then heat and pressure have been applied so as to apply thermal energy to the ACF for curing and to make a plastic and permanent deformation of the conductive particles. Consequently, a permanent mechanical contact can be formed between the bumps and the pads allowing the flow of electric field. In this study, the FC assembly using ACF was assumed to be a visco-elastic material subjected to various void sizes, which were entrapped inside the ACF. A finite element method is used to analyze the stresses in the FC assembly. The result found that the smaller the void size, the larger the stress concentration. Also, the von mises stress is larger at the upper position than the lower position. It implies that the four corners of a void is seen to be a preferable site for crack initiation, propagation and possibly failure to occur. Moreover, temperature profile of the reflow and the location of the void would also affect the stress level of the ACF.