Reliability improvement of solder anisotropic conductive film (ACF) joints by controlling ACF polymer resin properties

As the use of ACFs has increased in electronic packaging applications, demands for highly reliable ACFs has been also increased. For this demand, solder ACFs which use solder particles as conductive particles of ACFs has been previously introduced combined with an ultrasonic bonding method. In the solder ACFs bonding, solders are melted simultaneously with polymer resin curing reaction and the solder particles between two metal electrodes form various solder joint morphologies depending on the ACF resin curing characteristics. Furthermore, the solder joint morphologies between two metal electrodes can affect the ACF joint reliability at high temperature and high humidity reliability conditions. Therefore, the study on the morphology control of solder ACF joints and understanding the solder crack initiation and propagation during the reliability tests are needed for the highly reliable solder ACFs bonding. In this study, to understand the formation of solder morphology during solder ACFs bonding process, in-situ ACF resin viscosity was investigated. In-situ ACF resin viscosity can be predicted by the fitted equation at the specific temperatures. Resin viscosity at specific temperatures increased as degree of cure of ACF resin increased with the S-shaped growth curve, which was fitted with the Boltzmann equation. From the fitted equation, the ACF resin viscosity can be predicted at solder melting point (MP). Using this prediction, the effects of resin properties on solder morphologies were investigated. Slow curing acrylic ACFs showed concave shaped solder joints with larger spreading of solder particles due to the lower viscosity of resin surrounding solder particles. The degree of cure of slow curing acrylic ACFs at the solder MP was lower compared with that of conventional acrylic ACFs. In terms of resin viscosity, lower viscosity epoxy resin showed concave shaped solder joints because of lower viscosity at the solder MP compared with that of conventional acrylic resin which showed convex shaped solder joints. Solder joints bonded with slow curing acrylic ACFs and low viscosity epoxy ACFs showed higher reliability than that of solder joints with conventional acrylic ACFs, because solder joints with slow curing ACFs and lower viscosity epoxy ACFs formed concave shaped solder joints resulting in lower stress concentration due to larger curvatures of stress concentration regions than that of conventional acrylic ACFs.