Carol Spiers has asked about how to specify thickness of immersion silver, and about exposure of this finish to air. The IPC has a draft specification for immersion silver, IPC-4553, that addresses minimum thickness of silver for two kinds of deposit, "thin" and "thick", at http://members.ipc.org/committee/drafts/4-14_d_4553%20(10-20-04).pdf. 

According to that document, a typical thickness for "thick" deposit is 0.3 micrometers, which is roughly a thousand atomic layers. The committee that has been working on this spec has actually measured thicknesses several times this great. There's something funny going on here, because as Appendix 1 describes the immersion process, deposition can only occur until the basis metal layer below is covered. How can the copper land below continue to give up electrons to ions in solution when it is covered by thousands of layers of silver atoms? I've not found a satisfying answer to this question.

Some people are concerned that allowing too much silver on a land would make the resulting connection made with tin-silver-copper solder brittle due to too much silver, but since tin-silver intermetallic compound is not brittle, there doesn't seem to be much evidence to support the notion. 

As for exposure to air, silver reacts with hydrogen sulfide in the air to form silver sulfide, a black deposit known as tarnish. Tarnish cannot be expected to be removed by solder flux (since flux isn't designed for that purpose), but it's easy to prevent tarnish: store the boards in an airtight plastic bag that won't allow hydrogen sulfide to penetrate. Include some anti-tarnish paper for good measure. Silver will remain shiny indefinitely if it is protected from hydrogen sulfide.

Gordon Davy 
Baltimore, MD 
[log in to unmask]
410-993-7399 

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