Design Guidelines for Anisotropic Conductive Adhesive Assemblies in Microelectronics Packaging

The multiple parameters that are involved in the design of anisotropic conductive adhesive (ACA) assemblies, and the overlapping influences that they have on the final electrical contact resistance, represent a difficult challenge for the designers. The most conflicting parameters include initial bonding force F, number of particles N, adhesion strength G_A, and modulus of elasticity E of the cured resin. It is well known that as the bonding force increases, the contact resistance decreases. However, when the bonding force reaches a certain maximum value, the contact between conductive particle and conductive track is disrupted due to delamination of the cured resin during the elastic recovery. The authors have shown in previous studies that the delamination is caused by high residual stresses and that it depends largely on the adhesion strength of the assembly and on the modulus of elasticity of the cured resin. Additionally, the authors have provided a methodology to quantify the maximum threshold value of the bonding force for different numbers of particles trapped between mating conductive tracks. In this paper, the relationships between contact resistance R and each one of these conflicting parameters are systematically combined to create design diagrams that give regions of robust design. It is found, for example, that for particles of radius 5μm, E = 0.9 GPa, G_A = 50 J/m², and 2 ≤ N ≤ 5, the required bonding force to achieve contact resistance within the range 7 < R < 40 mω, is 2.5 < F < 20mN.