The effects of self-fluxing additives in solder anisotropic conductive films (ACFs) on solder wettability and joint reliability of flex-on-board (FOB) assemblies

In this study, self-fluxing additives were added in solder anisotropic conductive films (ACFs) in order to eliminate solder oxide resulting in an excellent solder wetting on metal pads during ACF bonding. Since the solder oxide causes poor wettability of solder particles, solder oxide was chemically removed by self-fluxing additives in solder ACFs. The test boards were 25 μm-thick polyimide based FPCs and 1 mm-thick FR-4 organic rigid PCBs which have 400 μm pitch Cu patterns with electroless nickel and immersion gold (ENIG) surface finish. Newly formulated solder ACFs were acrylic based adhesives film which can be fully cured above 150°C containing self-fluxing additives. The film contained 25 μm diameter Sn58Bi particles which has 138°C melting point and 8 μm diameter Ni particles as a spacer to maintain the gap between metal electrodes. According to the experimental results, the addition of self-fluxing additive caused significant improvement of solder wettability and reliability of solder ACF joints. Therefore, using a flux function added solder ACFs can be used for various applications such as FOB and FOF assemblies, and can provide an alternative interconnection method for high power and fine pitch assemblies for many other applications.