OSP Handling

Since you�re asking the question about proper handling of OSP board, there may still be time to talk you out of using that material. * Cleaning of mis-printed board could cause non-wetting problems. This tends to remove a fraction of OSP coating. 0.35um thickness is indeed thin. * Training of assembly people for the proper handling of OSP is important. We have seen that copper boards can be affected by poor handling. Touching solderable areas is poor practice. * If you have a batch process, beware. Once the OSP boards go through the first reflow process, you don't want them sitting around too long. You might have problems if you run side A on Monday, side B on Tuesday, and wave them on Wednesday. Especially if you have additional processes, like a few water cleanings. * If you use alcohol to clean misprints from screen print or the SMT adhesive process, your WIP shelf life is reduced, and you might run into problems. Each reflow might remove 20%, each wash might remove 25%, and alcohol could remove 75% of the OSP. * While OSP provides nice contrast to the solder paste for post screen print visual, however you can loose contrast from between your masking and the copper colored fiducials. So you might have occasional lighting issues with your placement equipment fiducial cameras, since the fiducials may have a changing range of copper coloration over time. * If you use a bed of nails tester, contact will differ. So, you may need new probe pins. * If this is a mixed technology board, OSP typically does not wet (accept solder) as well as some of the other finishes, and many folks have a tough transition over to no clean wave solder process with OSP products. Top side solder is a bit more difficult to achieve. So you may need to invest in a nice spray fluxer if you don't already have one. * One issue that OSP can not resolve that HASL can, is confirming the inherent solderability of the copper. We have experienced busts where very thin layers of solder mask residue or incomplete tin stripping have caused nice, clean looking copper surface to be unsolderable. With OSP, you don't realize the problem until after many expensive components have been placed. While it is possible to scrape �lacquer� from the pads that don't wet and rework, the question remains about all the other areas on the board being suspect. So, we end up scrapping this product rather than repairing. * An even more prevalent issue is if the product has via in pads, then if the via hole is not sufficiently rinsed and thoroughly dried, the OSP protection on the surface of the pad is degraded over time and can cause soldering issues. Again, this is not apparent until after assembly and is not something we would repair. It doesn't take too many issues such as this to make OSP less desirable. * If there are problems soldering to OSP check the quality of the OSP coating, too thick and there'll be soldering problems, too thin and there'll be too much copper oxidation, hence the variability in results. OSP coatings are very simple chemistry that are relatively easily soldered to, however although relatively simple chemistry they are not necessarily easy to lay down, careful process control is required to get the correct thickness of coating. Get it wrong and the above circumstances can occur. * OSP coatings the solder wet only where paste is applied, without further spreading over the pad. So, you end-up with copper ghosts on pads that are ugly and stupid looking, even though they are �legal�.

reply »