Re: 2 layer Versus 4 layer boards

From: Keith (krw_at_att.bizzzz)
Date: 08/08/04 

Date: Sun, 08 Aug 2004 10:45:50 -0400

On Sat, 07 Aug 2004 18:09:33 +0000, Ian Stirling wrote:

> Keith <krw@att.bizzzz> wrote:
>> On Sat, 07 Aug 2004 12:54:52 -0700, Rob wrote:
>>
>>> What are the benefits of 4 layer over a 2 layer (plain old double sided)
>>> PCB?
>>> We are reworking some old product designs and one line of thought is to go
>>> to 4 layer.
>>
>> Usually four-layer boards are laid out 2P(ower)-2S(ignal) with power
>> on the inner planes (there is an argument for them on the outer also.
>> Using planes reduces the power impedance making lower noise easier. At
>> high speeds this is a requirement (I wouldn't think of using signal wiring
>> as power distribution). Power planes also allow a controlled trace
>
> It depends how much work you'r willing to put into the design.
> I had one PCI soundcard, 2 layer, that beat the chipset manufacturers
> S/N figures for their 6 layer reference design.

That's usually not hard. I haven't seen too many reference designs that
were optimized for anything other than expediency (which is often why they
use many planes). I'd think about not using planes under analogs. As I
mentioned in another post I deleted the standard planes under the
low-level analogs on one project, to lower trace capacitance and
cross-talk. That worked well. ...though I had the normal planes under all
of the logic and controls. Trace impedance and high dI/dT aren't
usually issues on sound cards. ;-) 

-- 
  Keith
--

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