In a message dated 12/14/98 7:35:23 AM Pacific Standard Time, [log in to unmask]
writes:

<< From:   [log in to unmask] [SMTP:[log in to unmask]]
 Sent:   Friday, December 11, 1998 6:23 PM
 To:     [log in to unmask]
 Subject:        [TN] Warped boards at PCI connector?

 To technet
 I have this board that is 18 by 14 though hole and SMT. After reflow the
 board looks good not warped. Once assembled and sent to wave the board warps,
 only around the PCI connectors. It warps so bad I don't have any lead
 protrusion. I have tried to run this board many ways fast slow and minimal
preheat. I
 still have this problem. Has anyone ran into this problem before? Is this
common
 around PCI connectors? I recall this happening once before many years ago,
 I resolved it by tacking down the edge and middle of the connectors then
 masking. Is this the only fix? Any other suggestions

 Thanks in advance
 Angie >>

Hi Angie,

     Sounds like that there's something a lot different about the board in
close around your PCI connectors as far as copper content in the inner layers
compared to the rest of the board, are there heavy power or ground planes
everywhere else except where the connectors are? Hold the board up to the
light and look through it. Another thing that could be adding to things is
possibly a very tight fit of the plastic guide pins that usually are on the
bottom of the connector...at least on the connectors I've worked with before
had them.(I have't had to wave any PCI connectors in a while).

     The reason I say it might be the hole diameter for the pins on the
connector, is that I ran into something like this before at another place I
worked at. We got one lot of boards from one of our customers that had the
hole diameters swapped for the two plastic pins on the connector (usually one
pin was larger than the other so you couldn't put them in backwards), how that
happened I don't know. But what we wound-up doing was having to clip the pins
off on the connector. We had been doing the same thing as what you talked
about...tack-soldering a few pins on the ends and center before wave because
of the warping. But found with the pins gone, the warping problem was
eliminated, which caused me to realize what had been causing the problem. The
CTE of the connector material was a lot less than the board, and the pins were
constraining the board when it needed to move as it expanded. We asked our
customer to increase the hole diameter a tad, and we didn't have to tack the
connectors down from then on.


The pins are there to take the force of insertion and removal of cards instead
of the solder joints bearing the brunt of it, so be aware of that. We had to
clip them so we could use the connectors, otherwise the customer would have
had to scrap the fabs. But you might want to try an experiment on a solder
sample fab of the ones you're running and some connectors by doing the same
thing...clipping the pins and not tack-soldering the connector. Then you can
ask for bigger holes on the fabs too.

Hope this helps in some way...

-Steve Gregory-

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