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84 SMT007 MAGAZINE I SEPTEMBER 2020 Article by Ansuman Das MACDERMID ALPHA ELECTRONICS SOLUTIONS Introduction Organic solderability preservative (OSP) is an ultra-thin organic final finish coating for copper pads on PCBs, which provides protec- tion from oxidation under a wide range of con- ditions. Several advantages—such as lower cost, easier processing, and excellent coplanar- ity compared to metallic finishes—has made OSP an attractive choice to PCB fabricators and assemblers. Solder joints formed on OSP have a negligible tendency of electromigration, as they do not contain any metallic impurity coming from other finishes. OSP finish is being used on over 60% of all PCBs made today, and it has proliferated into high-reliability markets like automotive, which have traditionally used metallized finishes. However, OSP coatings suffer from a few draw- backs as well. After exposure to one or more thermal excursions, OSP coatings can become more challenging to solder, especially during wave and selective soldering applications. For example, a thick multi-layered double-sided PCB coated with OSP that has been exposed to one or more SMT solder paste reflow cycles may experience reduced solder wetting or hole fill by the assembler when wave soldered. This poses a huge challenge to all the assemblers, especially for Pb-free processes using SAC alloys with higher operating temperatures. Solutions suggested by engineers and researchers to mitigate poor soldering perfor- mance on pre-reflowed OSP were limited to PCB and component design improvements or optimizing soldering process parameters. Some groups suggested using more active fluxes, but these fluxes tend to leave corrosive residues and compromise the electrical perfor- mance of the assembly. Keeping this in mind, we have tried to understand the chemical changes OSP coatings undergo during thermal excursions and identify elements of soldering flux, which will interact with such thermally exposed OSP to convert them into a clean cop- per surface, which will result in better wetting performance. Characterization of OSP Coating OSP coating is achieved by dipping specially treated copper surface into a solution contain- ing active OSP chemicals. During this process, Can All Liquid Fluxes Work Well on A Reflowed OSP Pad Finish?

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