Effects of Heating Rate on Material Properties of Anisotropic Conductive Film (ACF) and Thermal Cycling Reliability of ACF Flip Chip Assembly

In this paper, the effects of heating rate during anisotropic conductive film (ACF) curing processes on ACF material properties such as thermomechanical and rheological properties were investigated. It was found that as the heating rate increased, the coefficient of thermal expansion (CTE) of the ACF increased, and the storage modulus and glass transition temperature (T(g)) of the ACF decreased. Variation of the ACF material properties are attributed to cross-linking density, which is thought to be related with the ACF density. In addition, as the heating rate increased, the minimum viscosity of the ACF decreased and the curing onset temperature increased during the curing process. The similar phenomenon was also found in in-situ contact resistance measurement. As the heating rate increased, contact resistance establishing temperature increased and the contact resistances of the ACF flip chip assemblies decreased. The decrease in contact resistance was due to larger conductive particle deformation which leads to larger electrical contact area. The effect of the heating rate of ACFs on thermal cycling (T/C) reliability of flip chip assemblies was also investigated. As the heating rate increased, the contact resistances of the ACF flip chip assembly rapidly increased during the T/C test. The T/C reliability test result was analyzed by two terms of shear strain and conductive particle deformation. Reduced gap of joints due to reduced ACF viscosity resulted in larger shear strain. Moreover, many cracks were observed at metal-coated layers of conductive particles due to larger deformation.