New stepped process and materials for chip bonding technology on non-rigid and flexible substrates

A reliable interconnection of electrodes to the flexible polymer substrate with anisotropic conductive film (ACF) of which the conductive particles were similar in elasticity to the substrates was accomplished. The contact resistance value was maintained even while the junction was stressed under sudden changes in temperature and pressure. We found that the conduction failure is caused by the action of a complex mechanism on the changes of a joint structure. The major driving factor seems likely to be defects in the transparent electrodes due to the thermal strain of the substrate and penetration of conductive particles into the substrate. The materials of substrate and conductive particle are polycarbonate (PC) with a gas barrier layer and poly-divinylbenzene (DVB) whose hardness is similar to that of the polymer substrate, respectively. In this paper, several experiments were performed with an emphasis on the properties of polymer substrates. Furthermore, the electrical and mechanical performance of the joint structure was studied by evaluating contact resistance and scanning electron microscope (SEM) observations. Moreover, for the experiments, we fabricated test IC with Au bumps through photolithography and electro-plating process. Conductive particle with elasticity similar to that of the polymer substrates did little damage to the transparent electrodes on the substrates, and low temperature and pressure under a stepped process did not bring about their deformation either. Consequently, we adopted the results on the fabrication of plastic based flat panel display module and thus could obtain a highly reliable interconnect with a low contact resistance (20-25 Ω).