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August 2005

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Subject:
From:
Lee parker <[log in to unmask]>
Reply To:
(Leadfree Electronics Assembly Forum)
Date:
Thu, 4 Aug 2005 09:44:25 -0400
Content-Type:
text/plain
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text/plain (107 lines)
Davy

At the Richmond facility, we vacuum packed (shrink wrapped) silver boards in lots of 20-100 boards at the time. As long as the boards were in these packages there was no reported evidence of deteriation.

Best regards
Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
  ----- Original Message ----- 
  From: Davy, Gordon<mailto:[log in to unmask]> 
  To: [log in to unmask]<mailto:[log in to unmask]> 
  Sent: Thursday, August 04, 2005 9:24 AM
  Subject: [LF] Protection by immersion silver


  I was interested to see Graeme Stewart's observation yesterday, under
  the heading of press-fit terminations, that an immersion silver finish
  on board lands remains solderable (which is what is implied by the term
  "shelf life") for at least a year. It prompted me to offer these
  comments. I hope that they will be of interest to some subscribers. 

  With proper storage, an immersion silver finish should remain solderable
  indefinitely. The purpose of a finish is to preserve the solderability
  of the basis metal beneath. To do this, it must 

  *    be thick enough that any remaining pores are not so numerous as to
  allow a significant amount of corrosion at their base, and

  *    not be affected significantly by chemical reactions.

  By way of comparison, for tin and solder, the concern for chemical
  reaction is less at the surface (where it very rapidly forms a
  protective oxide only a few atomic layers deep) than at the interface
  with the copper land (where it reacts during storage to form
  intermetallic compounds). The surface can't be ignored, however. Dave
  Hillman and Morgan Tench have shown that it is possible that the first
  oxide to form (SnO, tin II, the monoxide) can convert to tin IV (SnO2,
  the dioxide), which is harder for fluxes to remove. Maybe one of them
  can comment on the conditions that allow this to happen, since I can't
  recall. 

  I do know that it is possible for solder to preserve solderability
  indefinitely. (I suspect that this may be true for a properly deposited
  tin plating as well, but I don't know.) There have been many reports of
  solder-coated component terminations remaining solderable for decades in
  uncontrolled storage (i.e., no protection from air). For a tin-based
  finish, controlling the storage atmosphere is neither necessary nor
  sufficient to preserve solderability. If tin plating is too thin, not
  only will it probably be porous, but the reaction with copper will
  consume it, at a rate unrelated to the atmosphere it's in, and the
  protection will be lost. The only effective measure in reducing the rate
  would be to refrigerate the boards. (For tin plating there is of course
  the additional concern that it can grow whiskers; if any tin remains
  uncovered after soldering, whiskers may grow during service.)

  For silver, the concern for chemical reaction is at the surface. Here
  the concern is not reaction with oxygen or water vapor in the air, but
  with hydrogen sulfide. This is an air pollutant present in lesser or
  greater amounts everywhere (ppb range), and it reacts with silver to
  form tarnish. Tarnish, silver sulfide, is quite stable. It is unaffected
  by flux, which is not formulated to attack it, although I suppose such a
  flux could be created (organic chemists seem to be able to do anything).
  It also decreases the thickness of the remaining metal. In contrast,
  silver oxide is not very stable and does not become thick enough to be a
  concern. Also, unlike tin, silver won't react with the copper below. So
  an immersion silver finish simply needs to be thick enough to be largely
  pore-free, and be protected from exposure to hydrogen sulfide during
  storage. 

  It seems to me that for a board with immersion silver finish, an
  ordinary plastic bag that has been tied shut would provide adequate
  protection from tarnishing, and hence preserve solderability, not only
  for a year but indefinitely. To decrease the risk of tarnishing even
  further, there are anti-tarnish materials that scavenge hydrogen sulfide
  in the air. Marketed to protect silverware at home, they could be put
  into the bag with the boards. Does anyone have any contrary evidence?

   

  Gordon Davy 

  Baltimore, MD 

  [log in to unmask]<mailto:[log in to unmask]>

  410-993-7399 

   


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