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December 2003

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Subject:
From:
"Crain, Bob" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 17 Dec 2003 11:24:09 -0500
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Being in aerospace, we have faced this issue, both successfully and
unsuccessfully. I remember pulling 5 year old parts off the shelf for a
program only to see the welds at the body encased in a white oxidation (on
that one, we redesigned and re-qualified the product, ouch).

The main issues are oxidation, tin migration and left over acids from
manufacture on the die itself. I'm no expert on tin migration, but my
understanding is that in tin-lead alloys, even at room temperature, the tin
will migrate until at some point, the parts are no longer solderable. We do
periodic testing on our 30 year life time buy parts. If they look marginal,
we'll have to dip tin them all.

Since you are probably dealing with commercial parts, I recommend delidding
a representative sample at an appropriate interval (no longer than 1 yr) to
see if they are deteriorating inside. If they are, you get to figure out
some way to save them or redesign and requalify.

Let's move on to oxidation and storage. Humidity indicators, paper labels,
plastic carriers, most anything can be a source of contamination that
migrates to the leads and accelerates loss of solderability. Here's what we
did for 30 year storage:

1. Pretin the parts.
2. Clean them really really well including plastic carriers for DIPS etc -
use clean latex gloves for all handling of parts and carriers - final rinse
in DI water followed by grain ethyl alcohol O-E-760 (a FED spec for
Everclear).
3. Bake out at 150 F for 1 hour min (carriers air dry 1 hour in a clean
place)
4. Unit pack in a non-amine based ESD shielded bag - pull all air out and
purge w/ nitrogen and heat seal.
5. Slip that bag into a moisture vapor barrier bag - TYVEK/foil/anti-static
poly non-amine based bag w/ a clay based desiccant in TYVEK  pkg and a
humidity indicator card - pull all air out and purge w/ nitrogen and heat
seal.
6. Put the vapor barrier bag into a pink poly non-amine based bag with
humidity indicator card and heat seal.
7. Finally, all of this goes into 55 gallon steel drums with say 80 units of
clay desiccant in TYVEK bags and with humidity indicators installed in the
lids.
8. All storage must be temperature controlled. We use 70 F, colder may be
better to retard chemical processes internal to the ICs.

It seems like overkill 7 times over, but if you don't do enough, the heart
break comes when it is too late to do a good enough job.

Just remember this, to keep moisture out, metal or glass containers are
nearly perfect and all plastics work poorly in comparison - even with the
aluminum foil, the moisture barrier bags allow moisture to penetrate.

Contact me offline for specific brands that we determined to be acceptable.
Good luck.

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