Effects of metal surface finish on the anisotropic conductive adhesives (ACAs) joints

Abstract

This work was performed to investigate the effect of final surface finish of Cu electrodes on the adhesion and reliability of anisotropic conductive films (ACFs) joints. Two different materials, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs), were selected because these materials has been most commonly used as the final surface finish materials in printed circuit board (PCB) industries. However, the effect of OSPs on the adhesion and reliability of ACF joints has not been studied.Therefore, 1) investigation of the adhesion of ACF/OSP joints, and 2) evaluation of the reliability of ACF/OSP joints were performed in this study. Then, the results of ACA/OSP joints were compared to those of ACF/ENIG to confirm the feasibility and reliability of ACF/OSP joints.For the electrical continuity test and reliability tests, patterned flexible substrate and FR4 rigid substrates were prepared. A flexible substrate was bonded to two types of organic substrates with different metal surface finishes, ENIG and OSP, by using ACFs. According to the result of contact resistance measurement, the initial contact resistances of OSP/ACF joints were almost the same as those of the ENIG/ACF joints about 15 m . Four reliability tests, 85 o C/85% RH test and thermal cycling (TC) test, high temperature storage test (HTST) and pressure cooker test (PCT) have been performed to verify the stability of ACF/OSP joints in high temperature and humid environments. According to the results of reliability tests, ACF/OSP joints showed better reliability than ACF/ENIG joints, particularly in the PCT test.Adhesion strengths of ACF/OSP joints were also higher than ACF/bare Cu and ACF/ENIG joints. The fractured site of ACF/bare Cu and ACF/ENIG joints was ACF/metal interface but that of ACF/OSP joints was ACF inside. TEM and FT-IR analyses showed that the OSP coating layer on Cu electrodes remained even after ACF bonding and it seemed to play as an adhesion promoter.