Conduction mechanism of anisotropic conductive adhesives (ACAs): conductor ball deformation and build-up of contraction stresses

In this paper, the effects of ACA conductor ball deformation and build-up of contraction stresses on ACA conduction is discussed. Precise ACA conductor ball deformation was measured using a nano-indentation machine. Better electrical contact resistance model of ACAs can be obtained using measured deformation behavior of ACA conductor balls. In addition, contraction stresses of ACAs during thermal curing are important factor determining the ACA contact resistance. Both ACF thickness shrinkages and modulus changes of four kinds of ACFs with different thermo-mechanical properties were experimentally investigated using thermo-mechanical and dynamic mechanical analysis. Based on an incremental approach to linear elasticity, contraction stresses of ACAs developed along the thickness direction were numerically predicted. It is found that contraction stresses of ACAs were developed during the cooling process from the glass transition temperature of ACAs to room temperature. The build-up of contraction stresses below Tg was strongly dependent on both coefficient of thermal expansion (CTE) and elastic modulus (E) of ACAs. A nanoscale deformation field of thin ACF layers was obtained to measure the contraction stresses experimentally using a phase shifting moire technique. Good agreement between the contraction stresses predicted from an incremental approach and the actual vertical stresses measured from a phase shifting moire analysis was obtained. Therefore, full temperature-evolution of contraction stress based on the incremental approach to linear elasticity is reliable and thereby can be used to predict the contraction stress behavior of polymeric ACF materials. As a summary, better understanding of ACA contact resistance can be achieved by analyzing conductor ball deformation and contraction stresses analysis.