Simulation Study of bump metallization on the stress and strain distributions of ACF interconnections for flip-chip-on-flex (COF) applications

Anisotropic conductive films (ACFs) are composed of an adhesive polymer matrix and fine conductive fillers using metallic particle or metal-coated polymer balls. Flip chip interconnection using ACF is being widely used in the electronic packaging. In this paper, two-dimensional nonlinear finite element analysis model was built to simulate effects of bumps´ metallization on the stress and strain distributions of ACF interconnections for flip-chip-on-flex (COF) applications. The simulation results show that the bump metallization has big influence on the stress and strain. Under the thermal mechanical simulation, the stress is mainly relied on the Young´s modulus of the bump while the strain is mainly depended on the CTE of the bump. In some kinds of bump/pad metallization, such as gold, nickel and copper, the copper has smaller stress, only bigger than gold, and minimum strain. If both chip bump and substrate bump fabricated by same copper, the galvanic corrosion, which occurs between two different metals and the main cause attributed to the shift of the contact resistance of ACF, does not exist. Regarding the cost and reliability synchronously, the copper may be the better substitute of gold as bump material in flip chip using ACF