Gordon
You raise a very interesting question that needs to be addressed, if for
no other reason technical understanding. As you mentioned, an immersion
coating can at most be only a few atomic layers thick. The so called
thick coatings go well beyond this. Consequently, there are mechanisms
involved other than immersion plating which is an exchange process.
Since a current is apparently not involved only a catalytic mechanism
remains.
Since the plating technique, to large measure determines the structure
and some of the properties of the coating, it would be constructive if
the chemical suppliers would inform us of some of the details.
Best regards
Lee
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Davy, Gordon
Sent: Tuesday, December 21, 2004 11:52 AM
To: [log in to unmask]
Subject: Re: [LF] Silver emersion shelf life.
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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