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From:
"Kane, Joseph E (US SSA)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kane, Joseph E (US SSA)
Date:
Thu, 15 Jul 2010 11:09:42 -0400
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I'm not aware of any standards for long term storage,
but we've stored some boards for many years, so here's
some opinion.

Tin/lead finish doesn't oxidize very readily, so I'm not
sure about the value of storing in nitrogen, unless you're
trying to keep the boards dry.  But if you want to keep
them dry, you can do as well with dry-pack, i.e. a heat-sealed
moisture barrier bag with desiccant and humidity indicator card.

According to J-STD-033, this kind of dry-pack system is good
for at least 12 months at <40C/90%RH.  In practice, it's good
for quite a bit longer than that, but if you want some 
insurance, double-bag them (seal the first bag inside a second).

The biggest problem with long-term storage will be intermetallic
growth, which depends on time and temperature.  This often shows
up first with solderability problems at the knee of the
plated-through hole ("weak knee").  But if you have good tin/lead
thickness, it shouldn't be an issue.  If you want to slow down
this reaction, about the only thing you can try is to keep the 
boards cool, but this won't help much, and introduces other 
complications.

-Joe

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Thursday, July 15, 2010 5:28 AM
To: [log in to unmask]
Subject: [TN] PCB storage

Hello Technetters,

we have to store High reliability SnPb assemblies for some years. They will be stored under N2 cabinets and temperature 22°C±2°C.
Are there any standards about the storage times or technical articles on the subject ?
Thanks.

CANTAGALLO Luigi
Technology Engineer
[log in to unmask]

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