The effects of the degree of cure of anisotropic conductive films (ACFs) on the contraction stress build-up of ACFs and ACF joints stability for chip-on-flex (COF) applications

In this paper, the effects of the degree of cure of an anisotropic conductive film (ACF) on the material properties and the contraction stress build-up of the ACF and ACF joints stability were investigated. The degrees of cure of the ACF as a function of bonding times were quantitatively obtained by a dynamic DSC study and an attenuated total reflectance/Fourier transform infrared (ATR/FT-IR) analysis. According to the results, the thickness expansion rate of the ACF as a function of temperature decreased and the storage modulus increased as the degree of cure increased. In addition, the contraction stress of partially cured ACF with the degree of cure below 40% was much smaller than that of fully cured ACF. The ACF contact resistances decreased and the ACF peel adhesion strengths increased as the degree of cure of the ACF increased. In particular, poor electrical contact was observed when the degree of cure of ACFs was below 40 %. The ultimate tensile strengths (UTSs) of the ACF increased as the degree of cure increased, and they were closely related to the ACF peel adhesion strengths. Furthermore, the ACF joints with the degree of cure below 40% were more unstable than those with the degree of cure over 90% during 85 degC and 85% relative humidity test (85 degC/85% RH).