Conductivity assessment of conductive polymer bonding for flip chip applications

Numerous experimental studies of electrically conductive polymers have shown that bonding systems between flip chip and substrate with conductive polymers have a certain amount of contact resistance between contact pad and conductive polymer and among the fillers, in addition to the bulk resistance of the fillers themselves. The contact resistance of a conductive polymer bonding system and its variation in a life cycle are obviously two important parameters that impact the electrical behavior of electrical components and devices, if flip chip bonding is involved in their packaging. There are two kinds of conductive polymers, isotropically conductive polymer and anisotropically conductive polymer, which have been intensively studied for application to flip chip packaging. Conductive polymers generally consist of conductive filler and adhesive polymer matrix. The electrical conductivity depends not only on filler material, filler ratio, and operating environment, but also on filler packing structure, voltage drop across the contact spot and mechanical stress due to the mismatch of coefficients of thermal expansion, which results in deformation of the chip and substrate. In this paper, we conducted an analysis, based on fundamental electrical contact physics, material kinetics and particle packing structure, to deepen our understanding of the correlation between the conductivity of the polymer bonding system and key variables such as filler packing configuration, stress, voltage drop and operating environment.