Hi Peter! There is nothing wrong with vacuum sealing the pwbs provided you
don't trap a contamination/tarnishing source inside the bag! One school of
thought in the industry is that immersion silver surface finish should be
in vacuum sealed bags for storage and my experience is that vacuum sealing
the storage bag is not necessary provided the storage/shipping environment
is reasonably controlled.  Vacuum sealing does provide an added benefit in
that handling abuse is minimized (i.e. the pwbs are not moving around).
Many of the TechNet pwb fabrication contributors use vacuum sealing as
standard procedure. I am a big proponent of the use of silver saver paper -
I think it really improves the shelf life robustness of an immersion silver
finish. Also, the IPC is working on a PWB Handling/Storage Guidelines
document which should be helpful for the industry.

Dave



             "Peter Wong"
             <[log in to unmask]
             com>                                                       To
                                       "(Leadfree Electronics Assembly
             09/07/2006 04:33          Forum)" <[log in to unmask]>,
             PM                        <[log in to unmask]>
                                                                        cc

                                                                   Subject
                                       RE: [LF] Immersion Silver and
                                       tarnish










Hi Dave,

Why do you specifically mention an "unsealed poly bag"?  I'm in the
process of updating our board handling/storage procedures wrt immersion
silver pwbs.

My process so far is to
1) wrap each board (or "brick" of boards) in silver saver paper (no tape
or elastic bands)
2) insert boards into esd shielding bags without desiccant and vacuum
seal shut
3) during kitting, boards remain packaged as stored and unwrapped from
silver saver paper during production as needed.

Do you see anything wrong with this process?

Thanks
Peter

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Thursday, September 07, 2006 2:17 PM
To: [log in to unmask]
Subject: Re: [LF] Immersion Silver and tarnish

Hi Genny - I recommend avoiding any type of cardboard whenever possible
to
avoid the sulfur contamination issue altogether. However, if you put an
immersion silver pwb wrapped in silver saver paper in an unsealed poly
bag
which then sits in a cardboard box,  you should not have any sulfur
contamination issues in most typical cases.  Immersion silver can have
some
level  of tarnish which will not impact solderability - Don Cullen of
MacDermid presented a paper at an IPC Fall meeting documenting an
approximate level of acceptable tarnish. I will jump up on my soapbox
and
recommend that you never use a "pearl pink eraser" to "clean" a
solderable
surface. The use of a pearl pink eraser is an old metallurgist tool of
determining whether a corrosion/tarnish attack is a surface phenomena or
a
subsurface phenomena (one of my mentor's taught me the trick many moons
ago
when dealing with sheet metal corrosion) - unfortunately some misguided
soul long ago considered this process to be a "cleaning" process and now
we
stuck with an incorrect idea. You would be amazed at the residue that
can
potentially be left on a "treated" pad after a pearl pink eraser
treatment.

Dave Hillman
Rockwell Collins
[log in to unmask]




             Genny Gibbard
             <Genny.Gibbard@VC
             OM.COM>
To
             Sent by: Leadfree         [log in to unmask]
             <[log in to unmask]
cc
             >

Subject
                                       [LF] Immersion Silver and tarnish
             09/07/2006 02:58
             PM


             Please respond to
                "(Leadfree
                Electronics
             Assembly Forum)"
             <[log in to unmask]
             >; Please respond
                    to
               Genny Gibbard
             <Genny.Gibbard@VC
                  OM.COM>






Are cardboard ESD boxes a source of sulfur that could tarnish Immersion
Silver PCBs?
If you have tarnish on a PCB, would you ever try to remove it with an
eraser?

Just wondering...

Genny

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