Hi Dave! The quest for an "equivalent storage factor" is something that the
JSTD-002/003 committees have been engaged in for a good 15 years (kinda
like the quest for the Grail)! The committee (and industry) focused on the
use of steam as a conditioning methodology that would produce "accelerated
aging" and give the industry/committee the ability to construct an storage
factor. The IPC-TR-462 and addendum reports document the committees efforts
in this area. However, a number of industry reports/data sets have been
produced that demonstrates that the oxide species and structure produced by
"steam conditioning" is not what is produced in a typical storage
condition. And "steam conditioning" actually produces oxides
species/structure on copper that do not form in typical storage conditions.
The Alternative Final Finishes committee, chaired by Dennis Friz, is
finishing up a report on an investigation of the impact of time,
temperature and humidity level as potential conditioning methodology
alternative to steam conditioning. There has also been some interesting
work in the area of mixed flowing gas and "equivalent storage factors" by
Sandia Labs. The JSTD-002/003 is finishing up their Pbfree solderability
test parameter work - once that is complete the conditioning methodology
topic is next on the task list.
Dave Hillman
JSTD-002/003 Chair
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David R1 Nelson
<DNelson@RAYTHEON
.COM> To
Sent by: Leadfree [log in to unmask]
<[log in to unmask] cc
>
Subject
Re: [LF] Protection by immersion
08/04/2005 01:01 silver
PM
Please respond to
"(Leadfree
Electronics
Assembly Forum)"
<[log in to unmask]
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to
David R1 Nelson
<DNelson@RAYTHEON
.COM>
This has provided me with the opportunity for data gathering. I apologize
to the forum if this takes it off on a tangent as it's not purely a
Pb-Free issue but I just have to ask.
From reading the attached, I assume testing was done and somehow
correlated to "equivalent" storage time. I'm presently attempting to
develop a test for PCB surface finishes and am a bit stuck on how to
accelerate aging and be able to predict that equivalent time, haven't been
very successful in locating reports which deal with the subject.
Currently, we are planning a humidity bake to simulate accelerated aging
as steam aging, based on research we have found, isn't appropriate for Im
Ag platings. Can anyone pass on some info on how specific aging test
correlate to storage conditions and time? Don't know if it's germane but
our present test candidates include ENIG, HASL, IM Ag and Pd/Au.
Graeme Stewart <[log in to unmask]>
Sent by: Leadfree <[log in to unmask]>
08/04/2005 09:37 AM
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"(Leadfree Electronics Assembly Forum)" <[log in to unmask]>; Please respond
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Subject
Re: [LF] Protection by immersion silver
We did accelerated life testing in "clean air" and mixed flowing gas to
simulate an light industrial polluted atmosphere. In a non-polluted
atmosphere the Imm Ag passed solderability tests after 1 year equivalent
and may give protection beyond that but we didn't test for greater than 1
year equivalent. In a polluted atmosphere it failed before six months
equivalent exposure.
It appears that the problem isn't so much the Sulphur as this forms a self
limiting layer (similar to the oxide layer in Al) and the fluxes appeared
to deal with thin layers. The main problem was chloride which formed a
friable layer which was not self limiting as it kept breaking off and
exposing fresh silver.
We recommended storage in sealed bags or containers. No desiccants or
rubber bands to be used as they may contain sulphur and any paper used to
be sulphur free. Boards to be used within 1 week of opening or to be
resealed.
I don't think it would be possible to prevent any tin layer, no matter how
well plated, from reacting to form an non-wetting intermetallic after
about 6 months.
Graeme Stewart,
Agilent Technologies
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Davy, Gordon
Sent: 04 August 2005 14:25
To: [log in to unmask]
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]
410-993-7399
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