We have a lot of experience w/ this type failure.  I'm working on the
assumption that there's a thermal via farm in that center gnd slug area.
A couple recommendations ff the cuff...
    * Stay away from conductive via fill material whenever possible when
vias are tightly packed in "thermal via farms."   Conductive fill is almost
never needed -- adds very little actual thermal/conductive benefit. We've
modeled it and test it.  For my money, conductive fill is almost always a
waste of time and a real pain in the patootie from reliability standpoint.
The biggest benefit is that it's easier to plate over and get good copper
adhesion and that usually only matters on probe sites. Esp
evil/unpredictable in this application is CB100 (I've had lots of dialogue
w/ Dupont over the years on this type failure).
    * Use a non-conductive fill that's more closely matched in CTE (e.g.San
eh) and don't overplate the fill.
    * Arguably the most significant factor:  don't plate over the thermal
vias.  All overplating does is give the fill something to push against and
force the whole plated area up during thermal excursions. It's like
screwing a cylinder head on tightly and then pushing all the pistons up
against it (expanding fill at temp).  Pop goes the weasel!  Overplating
doesn't add any grounding or thermal benefit. The solder joint in the gnd
slug will still be fine w/ the dot pattern that emerges.
    * As a work around/mod: We've eliminated a couple of these problems on
finished product by just etching or laser ablating a dot of copper off over
the fill.

there's my $.02.  Free and worth every penny.  ;^)

cheers,
dw

At 07:29 AM 10/19/2005, Steve Gregory wrote:
>Hi Wee Mei!
>
>I've got your pictures posted. Go to:
>
>http://www.stevezeva.homestead.com/files/U2_top.JPG
>http://www.stevezeva.homestead.com/files/U2_bottom.JPG
>http://www.stevezeva.homestead.com/files/U2_delamination.JPG
>
>Are the vias filled? Are you seeing this delamination on very many
>PCB's? Are you seeing it only on the bottom of the PCB?
>
>Kind regards,
>
>-Steve Gregory-
>Senior Process Engineer
>LaBarge Incorporated
>Tulsa, Oklahoma
>(918) 459-2285
>(918) 459-2350 FAX
>
>
>
>|---------+---------------------------->
>|         |           Wee Mei          |
>|         |           <[log in to unmask]
>|         |           SG>              |
>|         |           Sent by: TechNet |
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>  >--------------------------------------------------------------------------------------------------------------|
>   |
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>   |       To:       [log in to unmask]@SMTP@Exchange
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>   |       cc:       (bcc: Stephen R
> Gregory/LABARGE)                                                             |
>   |       Subject:  [TN] Delamination at Thermal Via
> area?                                                       |
>
>  >--------------------------------------------------------------------------------------------------------------|
>
>
>
>Hello,
>
>Just asked Steve to download 3 pictures on the defects. Kindly take a look
>at them and would appreciate some feedback on the possible root cause.
>
>Steve : Thanks for the downloading.
>
>Regards,
>Wee Mei
>
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