Reliability improvement methods of solder anisotropic conductive film (ACF) joints using morphology control of solder ACF joints

As the use of ACFs increased in various areas of electronic packaging such as semiconductors and flexible devices, demands for highly reliable ACFs have been also increased. For this demand, solder ACFs which use solder particles as conductive particles were introduced. In the solder ACF bonding, the solder ACF joint morphology should be controlled because the failures of solder ACF joints by crack propagation have close relationship with morphology of solder ACF joints. In this study, in order to improve the reliability of solder ACF joints in electronic packaging, the morphologies of solder ACF joints were controlled and the reliability depending on the morphologies was investigated. For the investigation of ACF joint reliability, unbiased autoclave tests were performed at 121°C, 2atm, and 100% relative humidity. According to the results, as bonding pressure increased from 2 MPa to 6 MPa, aspect ratio (Joint area/joint gap) increased by increased joint area and decreased joint gap. In unbiased autoclave tests, some of solder ACF joints with bonding pressure of 2MPa showed electrical open failures after 60 hours because tensile stress was applied to solders due to polymer resin expansion by water absorption. On the other hand, solder ACF joints with bonding pressure of 6 MPa showed no open failures for 60 hours due to higher tensile strength by higher aspect ratio compared with those with bonding pressure of 2 MPa. Solder ACF joints with bonding temperature of 250°C showed hourglass shape by large spreading of solders and 5 times higher radius of curvature of stress concentration region than that of solder joints with bonding temperature of 200°C which showed barrel shape. Solder ACF joints with bonding temperature of 250°C showed higher reliability than those with bonding temperature of 200°C due to smaller amount of concentrated stress by hourglass shape. In terms of ACF resin materials, despite of bonding temperature of 2- 0°C, low curing rate acrylate ACFs showed hourglass shape and higher reliability than that of solder joints with conventional acrylate ACFs. The reason of that was low curing rate acrylate ACFs showed lower degree of cure of resin around solder at solder melting point than that of conventional ACFs. These results indicate that solder ACF joint morphology can be controlled by adjusting bonding conditions and ACF materials. Furthermore, the morphologies of solder ACF joints can be significantly important factors for highly reliable ACF joints in high temperature and high humidity.