Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding

In this paper, in order to improve the reliability of anisotropic conductive film (ACF) interconnections, solder ACF joints were investigated in terms of solder joint morphology. ACFs are film-type interconnection adhesive materials that consist of polymer adhesive resins and randomly dispersed conductive particles. Recently, in order to obtain high reliability of ACF joints, solder ACFs that use solder particles as conductive particles of ACFs have been introduced combined with an ultrasonic bonding method. However, further researches are needed in the area of crack initiation and the propagation of solder ACF joints at high temperature and high humidity conditions due to the stress generated by hygroscopic expansion of ACF resin. Therefore, in order to solve the crack initiation of solder ACF joints, solder ACF joint morphology should be controlled and optimized to minimize the solder crack-related reliability problems. In this paper, solder ACF joint reliability was investigated depending on the solder morphologies of ACF joints bonded with various bonding pressures and temperatures. According to the results, as bonding pressure increased from 2 to 6 MPa, aspect ratio (joint diameter/joint gap) increased due to the increased joint area and decreased joint gap. In the pressure cooker test (PCT) reliability tests, as solder aspect ratio increased, electrical resistances were more stable after 60 h of the tests due to higher joint strength. In the target bonding temperature profile of 250 °C, solder joints showed a concave shape. However, joints bonded at in the target bonding temperature profile of 200 °C showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder Melting Point (MP) in the target bonding temperature profile of 250 °C compared with that in the target bonding temperature profile of 200°, because ACF temperature reached to solder MP in shorter time in the target b- nding temperature profile of 250 °C. Concave-shaped solder joints showed higher PCT reliability than the convex-shaped solder joints due to the lower stress concentration. These results indicate that solder ACF joint morphology was a significantly important factor for highly reliable solder ACF joints in high temperature and high humidity reliability conditions.