Contraction stress build-up of anisotropic conductive films (ACFs) for flip-chip interconnection: Effect of thermal and mechanical properties of ACFs

The important mechanical mechanism for the electrical conduction of anisotropic conductive films (ACFs) is the joint clamping force after the curing and cooling processes of ACFs. In this study, the mechanism of shrinkage and contraction stress and the relationship between these mechanisms and the thermomechanical properties of ACFs were investigated in detail. Both thickness shrinkages and modulus changes of four kinds of ACFs with different thermomechanical. properties were experimentally investigated with thermomechanical and dynamic mechanical analysis. Based on the incremental approach to linear elasticity, contraction stresses of ACFs developed along the thickness direction were estimated. Contraction stresses in ACFs were found to be significantly developed by the cooling process from the glass-transition temperature to room temperature. Moreover, electrical characteristics of ACF contact during the cooling process indicate that the electrical conduction of ACF joint is robustly maintained by substantial contraction stress below T-g. The increasing rate of contraction stresses below T-g was strongly dependent on both thermal expansion coefficient (CTE) and elastic modulus (E) of ACFs. A linear relationship between the experimental increasing rate and E x CTE reveals that the build-up behavior of contraction stress is closely correlated with the ACF material properties: thermal expansion coefficient, glassy modulus, and T-g. (C) 2004 Wiley Periodicals, Inc.