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]

410-993-7399 

 


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