Hi, Gary
I think that your concern for solderability of the remaining
hand-soldered parts on your CCAs returned from outside CEMs is valid.
If the CEMs are providing you with the assemblies inside of a
static-shielding bag, as you indicated below, there is little additional
cost to have the bag heat-sealed. No dessicant is required, and a vacuum
seal is also not required. As far as I know, nearly all ESD bags also
work as a moisture barrier bag, and as such should provide a barrier for
airborne sulphates for some length of time. I would skip the ESD label,
as you can buy the ESD bags with the label imprinted on it.
Even for the local CEM, I would require the sealed bag if there is any
possibility that CCAs from that local CEM will be retained for any
length of time before the final parts are added and soldered.
If you do the receiving inspection in your facility, simply require that
the CCA be shipped in an unsealed ESD bag, and upon completion of RI you
can seal the bags in your plant. Short term exposure to any sulphates
between shipping from CEM to your facility is probably not going to
result in any major solderability issues. Minor tarnishing does not
usually equate to a big loss of solderability for the hand-soldering
process. Major tarnishing (boards turning black) from long-term exposure
does.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gary Camac
Sent: Tuesday, February 03, 2009 12:49 PM
To: [log in to unmask]
Subject: Re: [TN] Storage of Iag PWAs
Denny,
Thank you for your input. I will elaborate.
Since the change in our business model, we will no longer be building
PWAs at our facility. Now we receive from outside suppliers, IAg
finished PWAs that will require in-house hand soldering operations. It
is the storage of these PWAs that is my concern. Some of our suppliers
are a distance from us, so their PWAs are received in shielded bags
which are packed in pink bubble wrap and placed in a cardboard box. One
of our suppliers is close, so their assemblies are typically delivered
to us in covered conductive totes.
The static shielded bags are not sealed, only folded at the top and
taped down with a yellow ESD warning label. The totes are covered
either a snap-on lid or a flat lid which just sits in a recess in the
top of the tote. None of these methods provide the gas tight seal that
would be required to protect the silver's solderability. We would
undoubtedly be charged for requiring gas tight packaging, which is
something we would like to avoid.
I had checked the Technet archive on this subject and it appears I am in
uncharted waters. I would think some assemblers would have addressed
this packaging concern with their customers. Perhaps packaging with
silver saver papers would be good. Normally the stock turns on these
assemblies are quick enough that I wouldn't be concerned about a
degradation of solderability. But, when production forecasts are
written on a magic slate, you don't know when some PWAs might become
lifetime companions.
Again, thanks for any input.
Gary Camac
Senior Manufacturing Engineer
DICKEY-john Corp.
5200 Dickey John Road
Auburn, IL 62616-0010
Telephone 217-438-2288
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
Sent: Tuesday, February 03, 2009 10:13 AM
To: [log in to unmask]
Subject: Re: [TN] Storage of Iag PWAs
Gary,?
We need a bit more information here.? You say "covered tote" and the
critical item here is a vapor barrier.? Immersion silver is tranished by
the diffusion of hydrogen sulfide gas?(or other sulfur
bearing?gases)?from the cardboard.? Air flow will not defeat
diffusion.?The gas laws work - higher pressure (and not much at that
from just fan circulation) may limit gas movement into the tote.?
However, you really need a gas-tight seal to prevent tarnish.? With a
quality vapor barrier bag, you can ship immersion silver boards in
cardboard.? If your totes are not sealed, your boards are going to
tarnish.
Air movement helps particulate in the air - smoke, fumes, etc. but does
not do much for diffusion.?
Denny Fritz
MacDermid, Inc.?
-----Original Message-----
From: Gary Camac <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, 2 Feb 2009 2:24 pm
Subject: [TN] Storage of Iag PWAs
Good day all,
I am looking for some guidelines that I fear don't exist. If you had a
PWA with an Iag finish, which is stored in a covered conductive tote,
how close could the container be stored to sulfur bearing cardboard
boxes. This could be in conditioned factory air or slightly less
conditioned warehouse air.
I see the variables as the number of sulfur emitters (boxes in this
case) in the area and how much the air is turned (lowering sulfur
concentration).
Your thoughts would be appreciated.
Gary Camac
Senior Manufacturing Engineer
DICKEY-john Corp.
5200 Dickey John Road
Auburn, IL 62616-0010
Telephone 217-438-2288
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