Adhesion and Reliability of Anisotropic Conductive Films (ACFs) Joints on Organic Solderability Preservatives (OSPs) Metal Surface Finish

The effect of final metal finishes of Cu electrodes on the adhesion and reliability of anisotropic conductive film (ACF) joints was investigated. Two different metal surface finishes, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs) coated on Cu, were selected in this study. These are commonly used as final metal finish materials in the printed circuit board (PCB) industry. However, the effect of OSPs coated on Cu on the adhesion and reliability of ACF joints has not been studied yet. Therefore, the adhesion and reliability of ACF/OSP joints were investigated. The results of ACF/OSP joints were compared with those of ACF/ENIG joints to evaluate the feasibility of ACF/OSP joints in real packaging applications. For electrical continuity and reliability tests, patterned flexible substrates and FR4 rigid substrates were prepared. A flexible substrate was bonded to two types of organic FR4 substrates with different metal surface finishes, ENIG and OSP, using ACFs. According to the result of contact resistance measurement, the initial contact resistances of the ACF/OSP joints, approximately 15 mOmega, were almost the same as those of the ACF/ENIG joints. For the reliability test, a pressure cooker test (PCT) was performed in order to verify the stability of the ACF/OSP joints in high temperature and humid environments. According to the results of PCT, ACF/OSP joints showed better reliability compared to ACF/ENIG joints. The adhesion strength of ACF/OSP joints was higher than that of ACF/bare Cu and ACF/ENIG joints. The fracture site of the ACF/bare Cu and ACF/ENIG joints was the ACF/metal interface, while that of ACF/OSP joints was inside the ACF. TEM and FT-IR analyses showed that the OSP coating layer on the Cu electrodes remained after ACF bonding, and OSP layer acted as an adhesion promoter to ACFs.