Highly Reliable Solder ACFs FOB (Flex-on-Board) Interconnection Using Ultrasonic Bonding

Abstract

In this study, in order to improve the reliability of ACF interconnections, solder ACF joints were investigatedinterms of solder joint morphology and solder wetting areas, and evaluated the electrical properties of Flex-on-Board(FOB) interconncections. Solder ACF joints with the ultrasonic bonding method showed excellent solder wetting by brokensolder oxide layers on solder surfaces compared with solder joints with remaining solder oxide layer bonded by the conventionalthermo-compression (TC) bonding method. When higher target temperature was used, Sn58Bi solder jointsshowed concave shape due to lower degree of cure of resin at solder MP by higher heating rate. ACFs with epoxy resinsand SAC305 solders showed lower degree of resin cure at solder MP due to the slow curing rate resulting in concaveshaped solder joints. In terms of solder wetting area, solder ACFs with 25-32 μm diameters and 30-40 wt% showed highestwetted solder areas. Solder ACF joints with the concave shape and the highest wetting area showed lower contact resistancesand higher reliability in PCT results than conventional ACF joints. These results indicate that solder morphologiesand wetting areas of solder ACF joints can be controlled by adjustment of bonding conditions and material propertiesof solder and polymer resin to improve reliability of ACF joints.