Effects of tacky flux formulation on solder ball attach to a Cu-OSP surface finish in air and N2

Selected physical properties of four water-soluble tacky fluxes were measured, including viscosity, acid number, weight loss during heating, pH and rheological properties. The capability of the fluxes to perform ball attach in air and N₂ using an in-house test board with Cu-OSP surface finish was studied in a Malcom reflow simulator using freshly manufactured and highly oxidized SAC305 solder spheres. Flux performance was also rated from observations of fluxing action in a reflow simulator and the ability to realign deliberately misplaced solder balls and avoid the occurrence of exposed regions of the copper bond pad that have a `crescent´ shape. Two of the four fluxes exhibited good ability to realign misplaced solder balls and one flux exhibited outstanding ease of cleaning. Contrary to common perception, the best performing flux in terms of reactivity and activity with tin oxides did not have the highest acid number.