| Anisotropic conductive adhesive (ACA) assembly is emerging as one of the most flexible and cost effective packaging interconnect methods in the microelectronics industry. One of the major impediments to the full realization of the fine pitch (<200 mum) capabilities of this assembly method is accurate prediction of the electrical contact resistance. This work presents a detailed overview of the prediction of electrical contact resistance in ACA assemblies, presents two new models to improve the prediction, a single particle model and a multiple particle model, and provides guidelines for robust design in ACA assembly.; The review shows that large discrepancies exist among current models and between contact resistance values experimentally measured and what these models predict. It is found that important issues generally not considered in current models include elastic recovery, residual stresses, and realistic multiple particle models. A single particle model is then proposed to investigate how the contact resistance changes as the bonding force is removed at the end of the assembly process. It is found that the contact resistance increases due to the elastic recovery of the conductive particles and tracks, and that the increase of resistance depends highly on the modulus of elasticity of the cured resin and on the adhesive strength. The results of the single particle model are then extended to a multiple particle model to include in the study other parameters such as the number of particles and their spatial distribution. It is shown that the spatial distribution of the particles plays an important role in the rate at which the resistance decreases when either the number of particles or the magnitude of the initial bonding force is increased. A uniform distribution of particles is preferred for a more predictable, stable and lower value of contact resistance. Finally, the characterization of the bonding force, the number of particles, the modulus of elasticity of the cured resin, and the adhesion strength obtained from the two models proposed, are used to create design diagrams which provide guidelines to assemble, in a robust manner, ACA interconnects free of delamination. |
