Oxidized Silver? What do you do???
I've written before about the shelf life of immersion silver PCBs. In that post, I alluded to the possibility of carefully cleaning an oxidized immersion silver pcb but I didn't say how to do it. Sometimes an oxidized board can be cleaned. Sometimes it can't.
Their are really only two industry recommended practices:
- Send the boards back to the fab house to have them re-plated
- Get new boards if re-plating is too expensive
That being said, you can sometimes use a large rubber eraser to clean them. It's easy to mess the boards up though so it won't always work. I would not do that in a production environment or with an expensive board. And it won't do anything for plated through holes for thru-hole parts. For that, you'll need to go back to your board house.
Here's what one of our engineers said about the process:
"The problem with immersion finishes, silver in this case, is that the oxidation or tarnish is the remaining oxide/sulfide/sulfate/chloride compound (depending on what's in the area to react with) conversion of original silver content. The immersion coating is so thin to begin with, cleaning away the tarnish would basically leave you with little to no silver and the undercoat (usually nickel) exposed. Also, the type of tarnish/contamination is a big factor. The sulfur compounds (sulfide/sulfate) are worse - i.e. cause more solderability problems, then the oxide/chloride compounds. I'm not a chemist, so don't ask why.... :) Of course, the only way to find out what particular type of contamination is most prevalent on a give board is to have it analyzed.... Probably cheaper to get new boards!"
I've heard about some studies at Sandia National Labs regarding the life and solderability of tarnished silver boards. I'll try to look that up and if I can find it, I'll post some notes about it.
Swiftly fly the years
One season following another
Laden with happiness and tears