Effect of Flux Activators on the Solder Wettability of Solder Anisotropic Conductive Films

In this paper, a thermal acid generator was added as a flux activator in solder anisotropic conductive films (ACFs) to eliminate solder oxide resulting in stable solder wetting on metal pads. Since the native solder oxide causes poor wettability of the solder particles, solder oxide was chemically removed by the flux activator in solder ACFs. The test vehicles consisted of polyimide-based flexible printed circuits and FR-4 organic rigid printed circuit boards, which have 300-μm pitch Cu pads with electroless nickel immersion gold metal surface finish. The thickness of the solder incorporated ACFs were 50 μm and it consists of epoxy-based adhesive resin, 20 wt% of 5-15-μm diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particle, flux activator, and 5 wt% of 5-μm diameter Au-coated Ni particles, which act as a spacers to maintain a stable gap between metal pads. According to the experimental results, solder wettability improved significantly, as the amount of the flux activator increased in the solder ACFs due to solder oxide removal. As a result, the flux activator caused significant improvement of electrical properties of solder ACF joints, such as contact resistance, power handling capability, and reliability. Therefore, the flux activator added solder ACFs can provide an excellent interconnection method for high-power flex-on-board and flex-on-flex assemblies.