Subject:                              Time:  8:09 AM
  OFFICE MEMO         RE>BALANCE Cu Construction            Date:  1/26/96

James, the reply from Dave Hoover at Hadco Tech Center Two, was right on.  

A few additional comments may help you out.

The propensity to Bow and Twist are generally designed into the printed board,
and it's assembly.  In the IPC's "Acceptability of Printed Boards Wordshops"
I've been doing, we cover the complete Design / Manufacturing / Acceptance
requirements in IPC's D-275, RB-276 and the A-600, and address the "cause &
effect of bow and twist.

The key is to maintain symetry via uniform dielectric spacing, conductor
thicknesses, conductor pattern orientation, and conductor density from the
center-line of the printed board.  This becomes a significantly greater
concern the greater the distance from the center-line plane of the printed
board.

If I interpret you design correctly, you have lower conductor pattern density
(and conductor width) for the "control" circuitry on one side of the assembly,
and greater conductor width/density on the other side of the assembly.  You
also have a layer of high-mass 70 micrometer [2 oz/sqft] Cu one layer into the
printed board on each side for electrical isolation and signal integrity.  

In addition to what Dave pointed out from the manufacture of the printed
board, the great differences in the mass and possibly the orientation of the
outer layers can be "a killer" in assembly.  The differences in thermal mass
and capacity between the opposite sides of the assembly cause one-side to
expand/contract faster/slow during soldering operations, in particular surface
mount.  Heating/cooling at the same rate will not solve the problem because
your printed board assembly is like a "bi-metallic strip" that is used in some
thermostats.  The printed board and it's assembly should be re-designed to
minimize this bi-metallic strip effect.  If you're really stuct with the
design, then consider the use of tempory/permanent mechanical restraints to
minimize bow/twist.  This is not a free lunch, sometimes it doesn't work and
it imparts some thermal stress in the assembly that may relieve itself at a
later time due to thermal shock, cycling and use.

James -- if you would like to discuss this in more detail, we can do it
off-net.

Ralph Hersey
Lawrence Livermore National Laboratory
PHN: 510.422.7430
FAX:  510.424.6886
e-mail:  [log in to unmask] (work) or home [log in to unmask]

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Subject:  BALANCE Cu Construction
From:  [log in to unmask] (James Patton)

I have a Drive circuit; 2 main partitions. One is a control circuit, the other
is an output stage that is driving power. The current in the drive stage 
is substantial and on several channels. The current on the control partition
is low.

Consequently, I have the top half of the card with relatively low Cu density,
and the other 'loaded' to cover my power. I need to control 'warp and twist'
on this PWB and am concerned about the Cu distribution. I am not certain about
how this is done with a strucure like this. What is the mechanism 
for warping a card?  I don't neccessarily wan't to load up the layers with
Cu to balance the low density regions because of X-Y CTE concerns.
I cannot superimpose the two partitions on N/S and F/S because of
'edgy' 28V switching on the output stage - they must stay apart for signal 
integrity. It is a 10-lyr. with lyrs 2 and 9 being 2oz. full coverage.