We used J-STD-003A wetting balance coupons and they were exposed to EIA-364-65A T-65,IIA mixed flowing gas for time periods ranging from 0 to 5 years. The coupons were both non-processed, for shelf life study, and processed for field exposure. Processed meant the coupons were part of a board which had been passed up an assembly line in combinations of surface mount only and surface mount and wave. The process variables included single and double pass through SM and coupon up or down through the wave. The intention was to expose the coupons to the various chemistries and see if they interacted with the surface and if this then had an impact when exposed to a polluted atmosphere.
The intention wasn't to age the surface but to understand what happens to it when exposed to a polluted atmosphere. I had found plenty of references to ageing using various conditions and was happy to use those results. However I couldn't find anything, at the time, about how it interacted with Sulphur and Chlorine airborne pollutants. Remember the Imm Ag PCB finishes are co-deposited with organics so we are not looking at pure silver systems.
Graeme Stewart,
Agilent Technologies
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of David R1 Nelson
Sent: 04 August 2005 19:01
To: [log in to unmask]
Subject: Re: [LF] Protection by immersion silver
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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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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