Conduction mechanisms in anisotropic conducting adhesive assembly

This paper explores experimentally and through analytical and computational models, the mechanisms of conduction in flip chip interconnects using anisotropic conducting adhesives. A large number of assemblies were constructed with geometries in the 200-500 μm range, and wide variations in joint resistance were found to occur both within the same assembly and between assemblies under the same experimental conditions. In order to explain the origin of these unsatisfactory connections, a series of tests to measure the contact resistance linearity of both high and low resistance joints were made. The results from these measurements show that a large number of low resistance joints are ohmic, while most joints of relatively high resistance show resistive heating. However, in some of the initially high resistance joints there is an initial ohmic behaviour which is followed by a breakdown of a dielectric or insulating film, resulting in lower resistance. In addition to linearity measurements, computational models of metallic conduction in solid and polymer core particles were constructed to help understand the conduction mechanism. These models, which are based on the finite element method, represent typical conductor particles trapped between appropriate substrate and component metallisation. The model results show that the contact area required to explain high resistances is small and that the likelihood of obtaining a high resistance through such a small area of metal-to-metal contact is small, thus giving a strong indication of the presence of high resistivity films at the joint contact surfaces