Evaluation of Anisotropic Conductive Film for a Flex to Silicon Interconnection

A commercial anisotropic conductive film (ACF) was used to form electrical interconnections between the aluminum metallization on a silicon wafer and tin plated copper pads on a polyimide flexible circuit. Some samples were subjected to temperature/humidity aging at 85degC and 85% RH for 300h, and others were subjected to thermal cycling from 0degC to 125degC for 1500 cycles. The electrical resistance of the bonds was monitored periodically during testing by taking the samples out of the chamber and measuring the resistance as a function of the electrical current. A model incorporating the a-spots on the individual conducting particles in the ACF was used to extract the constriction resistance and film resistance associated with the bond. In the model, the constriction resistance is computed from the measured resistance change versus electrical current, and the film resistance is derived from the constriction resistance and a fit to the data evaluated at zero current. This analysis predicted an upper limit on the a-spot radius for a typical sample of 6.1nm. Typically, after an initial rapid resistance increase with aging, which was attributed to a loss of electrical contact area, the resistance became stable for the remainder of the stress testing