Jim,

We finally had to give up on dry film around the first of the year. Since then we have been bringing our
boards in plugged on the primary side.  We require our suppliers to plug the holes with a screenable mask,
such as SR1010, prior to the application of the LPI.  This approach leaves all of the vias wide open on the
bottom.  You might ask why we don't plug both sides in this manner. The supplier we worked with on this
project indicated that when he attempted plugging both sides, air would get trapped between the top and
bottom plugs, which during curing would expand and and do a Mt. St. Helens to one of the plugs.  The
resulting bumps ,of course, would not enhance the solder/adhesive stencil printing operation.

I was concerned that plugging from one side would be a contamination trap.  We ran a test that consisted of
building several (around 35 if I remember correctly) assemblies using normal production procedures.  Some
were plugged on top and some were plugged on the bottom.  Some of the test units were conformal coated and
some were left uncoated.  All units were then exposed under power to the heat and humidity cycles as
described in IPC-TM-650, para. 5.3.5.  After the units were removed from the chamber they were allowed to
dry down to ambient conditions and were given a full functional test.  All units passed.  These units were
then examined visually and no signs of corrosion existed.  Microscopic examination of the barrels of the
vias on the uncoated assemblies showed no signs of corrosion.  SMDs were removed to allow the examination
of the vias hidden under them.  Again, no signs of corrosion were detected.

Even though our test indicated no difference between the two plugging methods, I felt that since most of
our assemblies go through in-line aqueous cleaning, there would be less chance of entrapping the cleaning
fluids with the plugs on top.  Just a margin of error thing.

Gary Camac

Jim Kittel wrote:

> >-----Original Message-----
> >From:  Kittel, H. James @ SLC
> >Sent:  Tuesday, March 23, 1999 3:35 PM
> >To:    'Send Tech Net'
> >Subject:       LPI Tented Vias for BGA's
> >
> >Gentleman,
> >We just received some PWB's with what I expected would be "tented VIA's"
> >inside the BGA footprint.  The VIA's were tented with LPI solder mask. They
> >were tented closed on the top side but open on the back.  On the back side,
> >the mask had dropped into the 12 mil diameter VIA hole instead of bridging
> >across and sealing the VIA, something I didn't expect.
> >
> >The vendor tells me that is the only way to tent with LPI and other users in
> >the industry are assembling cards with these partially closed VIA's without
> >concern or problems.
> >
> >Are you guys building cards with partially closed VIA's?  What about
> >contamination getting trapped in the VIA?  Is this a valid concern?
> >
> >Appreciate any comments.
> >
> >Jim Kittel
> >
> >
> >
> >
>
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