smaher
Automotive
- May 20, 2002
- 4
I currently have a problem with too much silver plating on a PCB. The Pb/Sn solder appears to be disolving the silver into solution and raising the liquidus point of solder. The result being poor infil 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 liquidus temperature to 300-400 deg 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.
On a "good" board the silver is barely visible.
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?
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 liquidus temperature to 300-400 deg 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.
On a "good" board the silver is barely visible.
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?
