Quick Overview of Organic Solderability Preservatives (OSPs)
Are all OSPs the same? And how does an OSP actually protect the copper base?
OSP is a process for printed circuit board (PCB) surface treatment. The OSP film itself, is an organic film as opposed to a metallic coating, such as immersion silver, nickel and gold immersion. Most people have thought over the years that OSPs are weak coating in terms of protecting the copper surface from oxidation, but, thanks to significant research and improvement in developing new reaction products, today’s OSPs are a significant improvement over last 10 years of OSPs.
In this schematic, we show how the new coating has a higher ability to cross link, and then improve and minimize the effects from humidity and from oxidation. You can see a with higher crosslinking density on the right side of the graphic.
What is a thin OSP coating?
In various specifications, you’ll see two thickness ranges for OSP. One we refer to as thin, which is typically between .15 and .3 microns, and the other is thick between .3 and .6 microns. Again, it is up to you as a fabricator and as an OEM, who maybe specifying OSP for your boards, to check with the supplier and the fabricator using these processes as to whether these are truly super solid coatings that minimize the effect of oxidation and humidity. The technology says thin and thick both work as long as the key is preventing oxidation penetration to the copper surface.
Regardless of what OSP you pick, whether it’s the thin versus thick, there’s several things you should ask. Number one, you’ve got to control the thickness. There are several areas in process control that are critical. The first is to maintain the pH in the required range. pH is important because that allows the coating to form uniformly and in the proper thickness. With most of OSPs, if the pH is too low the coating would be very thin, if it’s too high it could be excessively thick, and that’s not good either, so again, keep the pH in the required range depending on the process you’re using. Certainly, the time in the solution affects the growth of the OSP film. The good news is most OSPs easily form to the proper thickness within 45 seconds and 60 seconds, so it’s a pretty simple process. Of course, like all cleaning and coating processes, temperature is important as well as the concentration of the coating itself. Again, OSPs are based on some type of imidazole coating.
You also must keep the pH in control to maintain the process while keeping proper ventilation. One thing you can do to control the pH is to include some type of acidic ingredient. All OSPs need this acidic ingredient, whether it’s acid or formic acid, to keep the organic material in the solution. If that acidic ingredient is not being controlled properly or it’s evaporating, that will also cause changes in the pH, which will then affect the coating.
OSP is an organic, and temporary, coating. In other words, during the process of assembly or soldering, when the fabricator either goes through wave solder or printing solder paste for reflow, the fluxes both in the solder paste and in the flux during and once it becomes activated, will remove and dissolve the OSP film. As the OSP film dissolves, it allows the solder to spread and make complete contact with the copper surface. This is different than immersion gold over nickel, where the gold actually dissolves in the solder. In this case, the OSP just completely dissolves and basically is sublimed into the atmosphere and no longer stays with the solder joint that has been formed.
Proper surface preparation is critical to any process, and OSPs are no different. You’ll hear us talk about surface prep prior to electroplating and surface prep prior to photoresists. It’s critical to have a clean copper surface, free of oxides and completely properly microetched so that the OSP coating can form uniformly. You could do everything else correctly – proper pH control, proper time and temperature – but if your surface is not clean, you will have non-uniform coating.
