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January 1999

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Subject:
From:
KELLY M SCHRIVER <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 8 Jan 1999 16:19:00 -0500
Content-Type:
text/plain
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text/plain (46 lines)
Hey Brian - sounds like you're having a fun day.

In general, the ability of a multilayer board to warp (or not warp) is
governed by distance that the copper planes are separated from a theoretical
center line in the laminate structure, and the mass of the copper.  The two
principle factors in this situation are: 1] the distribution of metal
resulting from the design itself; and 2] how well the board fabricator is able
to balance or equalize spacings of the metal in the laminate structure.

Copper has a TCE of 17.6 PPM/degC.  Glass in the fabric bundles has a TCE of
about 7-8 PPM/degC.  The resin in the laminate is the wild card.   Finished,
unclad laminates will have X-Y TCE's ranging from 6 PPM to 18 PPM below their
Tg.  Interestingly their X-Y TCE dimishes slightly above Tg.  At any rate,
these expansion differentials produce the force necessary to generate warp and
twist.

There are a number of techniques to minimize warp and twist during design: 1]
keep the copper as thin as practical;  2] try to keep equal metal distribution
on layers which will be equidistant from that theoretical centerline; 3] break
up large plane areas by cross hatching, or with thin separation lines on the
artwork, etc.

Most board fabricators are well aware of the need to maintain flatness in
their products, but it has to be supported by a design which will allow the
fabricators work to succeed.  In many cases, if you can pattern the physical
warp and twist, the board designer and board fabricator can help eliminate the
problem for the next build.

I would also add, it is possible to cool a board in a laminating press,
yielding an acceptably flat product, only to have it curl up like a potato
chip when it goes thru reflow or wave solder.

Good luck - Kelly

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