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Self limiting electroless silver plating thickness
Q. I currently have a problem with too much silver plating on a PCB. The Pb/Sn solder appears to be dissolving the silver into solution and raising the liquids point of solder. The result being poor infill of the solder joint. I believe the problem to be silver plating that is too thick. We have measured the silver content at the front of the flow to be approximately 20%. This ups the liquids temperature to 300-400 °C and explains the poor flow of solder.
Now to the question :)
I thought I understood that an immersion or electroless silver plating process was self limiting due to the substitution reaction which takes place between the copper of the board and the silver in solution. This limiting the plating thickness to approximately 0.15 microns as exposed copper is required for the plating process to work. By microsection and SEM we have measured the plating on a "bad" board as being 2 microns and having a very rough surface. On a "good" board the silver thickness is barely visible and looks much smoother under SEM.
Could someone please tell me if I am right in my thinking? Has anyone had a similar experience or could anyone explain why the plating is getting so thick?Simon Maher
A. Dear Mr. Maher,
It's strange that you find the silver thickness to 2 microns, it's not the dissolved silver you find to be so thick. However we have just developed a electroless silver plating process that is autocatalytic, so we build layers of 1-1.5 microns on the copper. We have managed to stop it from dissolving by passivation of the silver. With enough thick silver layer the problems of intermetallic layers is prevented so it's strange that you have 2 microns thick with that problem.Peter Karlsson
- Gothenburg, Sweden
April 9, 2009
A. Dear Mr. Maher,
The enhanced thickness of the Ag plating is related to it's porosity. Any changes in plating parameters, such as increased bath temperature, increased cation concentration, etc., will make the deposit less sheet-like and more tree-like allowing diffusive transport of Ag and Cu ions to longer distances. The more porous the deposit, the greater the thickness in a self-limiting manner. Yes, I believe your thinking to be correct.
- Sun Valley, California