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SEPTEMBER 2020 I SMT007 MAGAZINE 91 From the barrel fill images, it is very clear that Flux A containing solvent-activator combi- nation 3 has the worst performance among the fluxes. Flux B and C with combination 5, and Flux D with combination 6, showed excellent barrel filling. Wave soldering performance of these fluxes corroborates the results obtained in wetting balance experiments. Additionally, Flux E having solvent SOL3 and activator com- bination 5, and Flux F with combination 11, demonstrate soldering performance equivalent to Flux B, C, and D. For Fluxes E and F, sec- ondary solvent helps the flux to remove the thermally treated OSP, thus assisting in the solder wetting process. In many Pb-free capa- ble fluxes, secondary solvents help to dissolve OSP and also preserve the activity for solder- ing at higher temperatures. Conclusion The study showed when OSP-coated cop- per pads undergo thermal excursions, two major changes take place. Copper oxide film at organic metal interface thickens due to the formation of Cu 2 O and CuO. In addition, the organic layer becomes compact as substituted benzimidazole molecules rearrange them to form elongated fiber-like structures via inter- molecular attraction. This organic layer of pre- reflowed OSP can be dissolved by some sol- vents or solutions of organic acids in these solvents. Flux activators can react with the oxides and remove them at elevated tempera- tures. Different activators, found to react selec- tively with one of the oxides at different tem- perature ranges, indicate the necessity of a multi-activator package for soldering flux. The wetting balance method was found to be very useful in screening activators that may effectively remove pre-reflowed OSP, but the results may sometime lead to false failure, especially where a secondary solvent is used in addition to IPA to make the activator solutions. Thus, for fluxes with a secondary solvent, it is advisable to compare the flux activity using wave or selective soldering processes. Results obtained from both wetting balance and wave soldering experiments, however, confirm the effectiveness of fluxes with multi activator packages achieving superior wetting on ther- mally treated OSP. SMT007 Ansuman Das is an R&D manager at MacDermid Alpha Electronics Solutions, Bangalore, in India, working on new product development in the area of soldering fluxes and pastes. He joined MacDermid Alpha in 2004 and has 25 years of experience in chemical and materials research. plasma. This process most often occurs around sharp, conductive objects, such as cell towers and wing tips. The MIT team wondered about a sharp, ungrounded object, such as an airplane wing. They fabricated a simple wing structure out of wood and wrapped the wing in foil. The team subjected the wing to increasingly higher veloc- ities of wind, up to 50 meters per second, and varied the amount of voltage that they applied to the wire. They found that the strength of the corona dis- charge and its resulting brightness decreased as the wind increased—a surprising and opposite effect from grounded structures. (Source: MIT News) At the height of a thunderstorm, the tips of cell towers— and other tall, electrically conductive structures—can emit a flash of blue light, known as a corona discharge, or St. Elmos' fire. Scientists found that corona discharge strengthens in windy conditions, glowing more brightly as the wind further electrifies the air. Now, aerospace engineers at MIT have found that wind has an opposite effect on ungrounded objects, such as airplanes. In a storm cloud, friction can build up, creating an electric field that can reach all the way to the ground. If that field is strong enough, it can turn neutral air into a How Airplanes Counteract St. Elmo's Fire During Thunderstorms

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