Epoxy flux for lead-free soldering

A novel liquid halide-free epoxy flux, PK-002, is reported for SnAgCu soldering, where the thermally cured flux residue is designed to provide an interference-free service performance, in addition to the low cost advantage of a no-clean process. PK-002 exhibits a slightly lower flux activity than conventional fluxes. However, soldering wetting for flip chip assembly process is outstanding. Presumably this is due to the lower flux viscosity of PK-002 which allows a higher flux volume to be picked up at flip chip flux dipping step. At ribbon connector assembly, the peel strength of PK-002 is the highest, attributable to the combined effect of solder bond strength and epoxy adhesion. The flux residue of PK-002 is virtually fully cured when processed through a simulated hot-bar soldering cycle, as evidenced by DSC and solvent cleaning test. BGA solder bumping and BGA assembly are accomplished satisfactorily. The voiding content in the solder bumps and BGA joints made with PK-002 is lower than all conventional fluxes tested, mainly attributable to the negligible outgassing rate of PK-002 at soldering temperature.