I have had ENIG boards from a reputable supplier that have soldered well long after 6 months. As you questioned, moisture absorption of the substrate, if left open, could lead to other problems depending on normal local R.H.  With regard to ImSn, sealed or not, your're going to see solderability problems at 6 months or before from Cu-Sn intermetallics.  As discussed here and elsewhere, a Ni barrier layer over the copper could mitigate that but I'm not sure how widely available it is or what new problems it might create.

George
State of Franklin

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting
Sent: Friday, April 28, 2006 9:03 AM
To: [log in to unmask]
Subject: [TN] Shelf life of circuit board plating


Good Friday Morning All,
 
This topic may have been previously discussed, but let me try to phrase
my "problem" completely so the answer can be simpler.
 
I have a board vendor (Fab house) stating that for my ENIG boards the
plating only has a 6 month shelf life if the package is opened.  Did I
miss something here?  I thought Gold doesn't tarnish.
 
If this is true, then Immersion Tin, Immersion Silver and ENIG all have
storage issues.
 
OK, so I open a sealed package and take out half of the boards in the
package.  I may not need the rest of the boards for many months.  So how
do I protect the unused boards?  Should I vacuum seal them?  Add a
desiccant?  If I add a desiccant, are there different desiccants which
might damage the boards?
 
I'm sure I'll have more questions right after I send this.
 
Phil

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