[SI-LIST] Re: PCB STACKUP

Perry Qu wrote:
> 
> snip <
> Also, what is causing warpage other than copper unbalance  in an
> assymetrical stackup ? From our experience, copper balance is an issue on
> its own especially when dealing with large boards.


        Also consider laminate imbalance.  Each sheet type (between
        layers may include multiple sheets) will have a different
        behavior.  This can be seen if one looks into the efforts
        a vendor does in precompensating the images used before a
        board is laminated.

        I have seen asymmetric stackups get adjusted by changing
        the construction between layers to become symmetrical. In
        other words the complement that lacks a copper layer gets a
        surface where that copper layer would have been.

        Addition and removal of non-functional copper is only one
        part of the bow & twist scene.  I would guess that Mr Sef-
        ton's fortunate result may have had some behind the scenes
        machinations as above in addition to the weight changes.
        This sort of situation depends a great deal on the fabri-
        cator's engineering expertise and will likely increase the
        base materials cost at least to a small extent.

        Best regards,
-- 
 
      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800
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