AW: 2 layer PCI and meeting spec

[Bernd Riemann <BRiemann@Pinnaclesys.com>]

Although all your observations are correct and two layer designs do work in most PCs the problem is named impedance. Here chapter 4.4.3.3 states that the unloaded characteristic impedance of the shared PCI signal traces on the expansion card shall to be in the 60 to 100 Ohm range. The geometry of a PCI board with its typical 1.6mm thickness requires at least .75mm (30mil) of trace width to reach 100 ohms. Our typical PCI chips have 12 mil wide pads...

Routing of a SMT board is definitely impossible with these geometries therefore I guess there is definitely no two layer design out there not violating chapter 4.4.3.3.

 

By the way 4.4.3 is the chapter Physical Requirements.

 

Bernd Riemann

Manager Hardware Engineering

Pinnacle Systems GmbH

Germany

www.pinnaclesys.com

-----Ursprüngliche Nachricht-----
Von: Rob Kellogg [mailto:RKellogg@NetOctave.com]
Gesendet: Mittwoch, 27. März 2002 13:10
An: 'Daniel DeConinck'; pci-sig@znyx.com
Betreff: RE: 2 layer PCI and meeting spec

Very good observations. I do not know of any specification which speaks to the number of required layers, and I looked. The closest thing to a layer specification is  4.3.6.1 "Routing and Layout recommendations for Four-Layer Motherboards." And these are just recommendations not requirements.

-rgk

-----Original Message-----
From: Daniel DeConinck [mailto:daniel.deconinck@sympatico.ca]
Sent: Tuesday, March 26, 2002 4:51 PM
To: pci-sig@znyx.com
Subject: 2 layer PCI and meeting spec

Hello,

 

Observation re 2 layer PCI:

 

I can name two common devices that are sold in the tens if not hundreds of millions of units and that are 2 layer PCI boards. These would be ethernet cards and sound cards. These cards sell for around $20. They need to be made dirt cheap and they need to work on a very very high percentage of motherboards to avoid a bankruptcy inducing support nightmare. And they must work in conjunction with whatever else is plugged into the host PCI bus.

 

So my conclusion is, that in the real world you can often ( and many do ) get away with it. 

 

I am not qualified to answer the question from an engineering perspective but I would like to hear what others have to say.

 

Sincerely
Daniel DeConinck
www.PixelSmart.com
TEL: 416-248-4473