Re: [SI-LIST] : Different oz copper for grounds

Pat Zabinski (zabinski.patrick@mayo.edu)
Tue, 21 Sep 1999 11:10:15 -0500

Chris,

I'll avoid the thermal issues and such, and I'll try to speak to
the question I think you're after: i.e., what's the impact of
2 oz copper. The answer: several.

* If I'm reading into your cross section correctly, you're only
placing 2 oz on L2 (not on L11). This will create an unbalanced
stack up, which most board vendors try to avoid due to warping.

* The thicker the copper, the wider the design rules you must
follow. So, make sure you use very wide spacing rules (like
10 or 15 mils) on this layer.

* In most boards, the layers are laminated in pairs, such that
L2 (the 2 oz) layer will be on a core with L3. There
are some double-sided laminates with 2 oz on one size and 1 oz
on the other, but again, board vendors try to avoid these
due to the imbalance (i.e., warpage).

* The 2 oz (i.e., 2.8 mil) copper forces you to use a thick
core dielectric layer. i.e., the dielectric must be thicker
than 2.8 mil to avoid breaking during handling.

* For true CMOS, it's best to have a threshhold voltage
at 0.5 * VDD. To maintain this, you really want good and
balanced power and ground delivery. If you're using the 2 oz
layer for ground, then you'll want to make sure your VDD
supply has the same equivalent copper weight.

* If you're not electricall-connecting the L2 and simply
leaving it floating, I recommend you re-consider. Floating
planes present several potential problems (EMI/EMC,
supply noise, etc.), and I do what I can to avoid them.

I'm sure there are a few others, but this is what I could
come with for now. All that said, I have used 2 oz copper
on some boards (one right now has 250 Amps @ 2.5V, and I
need the copper weight to get the current to the devices).
However, if you can avoid it, I recommend you do.

Pat

> > Hi,
> >
> > This should be a quick question. We are doing a 12 layer board and it
> > has a thermal layer on layer 2 (it's really just a plane with no
> > electrical connection to anything). To keep the symmetry of the
> > stackup, we have to add a third ground plane as layer 11. We were going
> > to use 1oz copper for everything, but now the thermal guys are
> > considering 2oz copper for the thermal plane. Will there be any
> > negative effect if the ground plane is increased to 2oz copper while the
> > other two ground planes (and all of the other signals) are still 1oz?
> >
> > I assume there won't be problem, but it never hurts to ask.
> >
> > Thanks,
> >
> > Chris

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