Any environment that contains sulfur, like hydrogen sulfide or sulfur
dioxide should preclude the use of IAg. The sulfur will react with the
silver and "grow" sulfides which can be electrically conductive
resulting is shorts between vias. This can occur even when the
concentrations are within OSHA limits for human exposure.
Silver sulfides can be semiconductors or conductors depending on the
chemical composition. They are black in color instead of silver, any
exposed silver will be affected.
Mark Woolley
Failure Analyst
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette
Sent: Tuesday, August 26, 2008 8:25 AM
To: [log in to unmask]
Subject: [TN]
What characteristics of the end use environment would cause IAg to not
be an appropriate finish and why?
Jim Verrette
Senior Electrical Engineer
------------------------------
Date: Mon, 25 Aug 2008 08:24:35 -0500
From: "Stadem, Richard D." <[log in to unmask]>
Subject: Re: ENIG White paper
Dave,
You write as if you are not at all familiar with IAg, which is totally
understandable. However, IAg has been around in full production mode for
many years now, and is rapidly becoming the most-used finish in the
market. Do not confuse it with immersion tin finish, which has little or
no shelf life.
IAg-finished circuit boards have a recommended shelf or storage life of
about 1 year. However, if packaged properly in a hermetically sealed bag
with silver saver paper liners, and stored away from sulfates, it is
possible to use boards up to 5 years old or even longer with no issues
whatsoever. All you need to do is call out the packaging on the
fabrication drawing per IPC 4553.
I have soldered IAg boards that sat around exposed to the elements for
more than 4 years in previous lives, and they soldered up just fine, no
change in the DPMO.
Some of the bare boards we use here at GD are 34" by 40", 48 layers,
1/2" thick, cost is about $48,000 each. Their finish is IAg. =20
I have had a lot of experience with IAg over the past 10 years or so,
having seen at least 150,000 IAg boards assembled and soldered with
every type of component you can think of, and I have never experienced
any issues with them. I have experienced many, many issues with ENIG,
however, and that is the finish that would keep me up at night waiting
for the bomb to go off. There are many reliability issues associated
with ENIG such as black pad, brittle nickel, etc., etc, but just think
about the fact that you are soldering to nickel, which has a dissolution
rate about 50 times slower than copper.
Should you ever have a solderability issue with IAg, you can have the
fabricator rework the finish. If you have an issue with ENIG, the only
thing you can do is rework your scrap account.
However, you need to take the end-use environment into consideration
when you select a circuit board finish. ENIG (unfortunately) may be your
best choice. I would fully qualify just one assembly at a time, and I
would fully qualify it. For Space use, I would not suggest IAg.
-----Original Message-----
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