I've done a 5-layer PCB before with no problems with warpage, and it's a
fairly big board ( about 10 x 5 in. ).  The trick is to not have much
unbalanced copper on that extra layer.  In this particular board I am
referring to, it was an existing design on a 4-layer PCB, and the customer
wanted a custom version of it with some additional app-specific circuitry.
So I added the 5th layer to accomodate the extra nets from the additional
circuitry.  That sure was a lot better option than ripping up the
already-routed 4 layers and trying to cram all of it on the same 4 layers.
All of the 5th layer copper was lightweight stuff (8 mil traces), and was
fairly low density.  The power and ground planes were left in the same inner
layers as they were on the 4-layer version.

In a nutshell, if your "odd layer" has a lot of traces (copper) on it, you
should consider adding another layer to balance it.  If it's sparse with
lightweight traces, the extra odd layer should not cause problems.

Best regards,
Ivan Baggett
Bagotronix Inc.
website:  www.bagotronix.com


----- Original Message -----
From: "Harry Selfridge" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Wednesday, July 21, 2004 1:28 PM
Subject: Re: [PEDA] Board warpage?


> It isn't an odd number of PAIRS that causes problems - it is an odd number
> of LAYERS.  There are some advanced fab techniques that can reliably
> produce boards with odd number of planes or odd number of signal layers;
> however, there are very few fabs I would trust to do it.  You can
sometimes
> get away with odd number stackups, but it will eventually bite you in the
butt.
>
> Six layers is a common balanced stackup - provided there are an even
number
> of planes, and even number of signal layers with reasonably distributed
copper.
>
> At 01:56 AM 7/21/04, you wrote:
> >SNIP
> >Some say that an odd number of layer pairs can cause problems, indeed we
> >had problems with 6 layers at first, though we now use 6 layers to great
> >success.  Problems with that were caused by bad process control, not the
> >design (though the manufacturer tried to blame design at the time!!)
> SNIP
>
>
>
>




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