Indium Corporation - Mailchimp

Surface Finishes 2: Soldering to Copper and Soldering to OSP

Soldering to copper has been around since the beginning of soldering itself. Copper wiring and copper pipes are where soldering began, and copper is still the most common wire conductor, and soldering to it is still easy.
 
Copper for printed wiring boards (PWB's) have been around just as long, dating back to the first "computers", basically room sized calculators. But, since copper oxidizes, one way of protecting it is coating it. The coating is called Organic Solderability Preservative, or OSP. OSP copper is different from the other surface finishes because it is the only surface finish that covers the solderable surface and is eliminated during soldering, rather than consumed. And since copper is usually the base metal that we are soldering to anyway, why pay for the extra metal, such as tin, silver or nickel/gold, if just a "plastic" coating will work. This is especially true since OSP copper does not require any special reflow profiling or needs, such as a high peak temperature, or long time above liquids (TAL).
 
Like all surface finishes, OSP copper has some issues that we must look out for. First is the fact that since it is a non-metallic coating, any in-circuit testing must be done on a solder joint, as the test probes cannot pierce the coating to get to the copper underneath. One way around this is to apply solder paste to the test probe pads, and allow the solder to wet through the OSP.
 
Another potential issue is that since OSP is eliminated during the soldering process, multiple reflows, such as for 2nd side soldering or a final step of selective wave soldering, tend to break down the OSP surface and allowing the copper oxidize. Typically, the copper is pretty well oxidized once the board has been sent through the reflow oven twice, and then sent to the wave machine for selective soldering, requiring the use of a strong flux to remove the copper's oxidation.