Let IPC-4761 be your 'guide'...

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette
Sent: Monday, November 06, 2006 2:06 PM
To: [log in to unmask]
Subject: [TN] Chemistry in Vias

How long would it take for flux from a HASL finish process trapped in a
via to cause a defect?  The PCB is .050 thick with 8 mil vias.  Regular
FR4 material for SnPb solder process.  8 mil vias, 4 layer, 1/2 oz all
layers with 1 oz plate up on outer layers.  the board is fabricated to
IPC Class 2.  The board is all SMT components on the top side only.  We
had been tenting both sides of the board with primary Solder mask, but
have been having problems with shorts due to in complete coverage of the
via.  We tried a Post HASL plug process, but have had problems with
bumps under a 64 pin QFN package.  The reason for the problem is that
sometimes the vias are filled with HASL other times open, which makes
the caps inconsistent causing assembly issues.  So we are considering
going back to tenting the bottom and pre-HASL capping the vias that are
causing shorts.  Only the vias under the QFN need to be fully covered.
The PCB Quality Engineer at our CM tells us that tenting both sides
prior to HASL would not be a big concern, but since 35 vias are in test
points on the bottom, both sides on these vias can't be covered before
HASL and covering only one side can create a situation where flux from
the HASL process which contains corrosive stuff is trapped in the via by
the HASL.  The product has a shelf life of about 2 years with 3V applied
(if it stays on the shelf for that long) and only needs to operate for 3
days.  Do I really need to be concerned about this?  When the PCBs go
through a reflow oven with the HASL side of these vias facing down,
wouldn't this reflow the HASL and allow gravity to work in our favor?

The other option is to do 100% non-conductive fill and re-planerization
after plating, but this a more expensive process.  Also, changing board
finishes would make the post finish cap process more predictable, but
that is not an option right now.

Jim Verrette
Electrical Engineer

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