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December 1998

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Subject:
From:
Gary Camac <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 14 Dec 1998 14:28:07 -0600
Content-Type:
text/plain
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text/plain (166 lines)
Angie
    Several years ago we had a PWA warp after wave soldering.  This assembly had a high pin count MTA
connector on the edge of the assembly.  We determined the difference in expansion coefficient between the
connector's header material and the PWB, over the length of the connector was significant.  As the
materials would expand during the wave soldering process, the pins would move off center in their holes.
Typically center pins looked centered, pins on the right were shifted to the right and pins on the left
were shifted to the left.   The solder joints, after solidifying, would lock the pins in this off center
position.  As the assembly cooled the bow appeared.  Our cure was to install several smaller pin count
connector, so the expansion mismatch was not that great on any one connector

Gary Camac

Angie Marques wrote:

> Steve,
> The first board we ran we left the pins on the connectors, the second
> board I removed the pins and the results were the same. I have also
> viewed the gerber and found that the board has a lot of copper in other
> areas but not around the connector. I also made a selective wave
> fixture, unfortunately they did not isolate each connector and we got
> the same results. My next fixture will have a cross bar between each
> connector (x8) this should keep the board nice and flat. It sure points
> back to the design of the board, I was hoping someone out there that is
> familiar with designs could tell me where the customer went wrong and
> how they could avoid this in the future.
>
> Angie
>
> > -----Original Message-----
> > From: Stephen R. Gregory [SMTP:[log in to unmask]]
> > Sent: Monday, December 14, 1998 9:21 AM
> > To:   [log in to unmask]
> > Subject:      Re: [TN] Warped boards at PCI connector?
> >
> > 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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