Process and performance modelling for individual ACA conductor particles

Flip-chip assembly using anisotropic conductive adhesives (ACAs) has been successfully applied to the achievement of fine pitch electrical interconnections for certain niche applications, particularly flat panel displays. ACAs are an adhesive polymer containing a low volume fraction of conductive particles, which are typically comprised of a polymer core coated with nickel and a thin layer of gold. The properties of these particles are critical to the long term stability of ACA interconnections. However, only limited work has been reported on characterisation of individual particles, and more research is required to gain a complete understanding of their behaviour throughout the range of temperatures and stresses they may experience both during the assembly process and in service. The work presented in this paper utilises finite element analysis (FEA) to compare purely elastic models with models including the viscoelastic behaviour of the particle core. The FEA results show that the viscoelastic characteristics of the polymer core have a significant effect on the stress distribution, deformation and fracture behaviour of the particles. The interface between the Ni coating and polymer particle core is identified as the area where the highest stresses can be potentially induced, thus this area is most vulnerable to crack initiation at high loading rates.