PCB Design (was Computer repair shop)

[Jim Kuzdrall]

On Monday 05 May 2008 19:27, VirginSnow at vfemail.net wrote:

> > Power *should* be on the internal layers.  That's where it belongs,
> > for several engineering and manufacturability reasons.  Call ideas
> > dim-witted
>
> Care to enlighten us?  I guess PCB design is on-topic enough for this
> list. (/me ducks).  The only reason I can think of to run power on
> internal layers is that if the designer's loose screw shorts out
> something on the surface of the board, the sparks will only be 1/2
> the size.  You sound like you've designed you share of PCBs.  What
> other reasons are there?

    I've designed a number of high performance digital and RF boards.  
Two functions of the buried ground plane have not been mentioned.

    If a signal has to go any distance over 10mm it is wise and often 
necessary to send it via a PC board transmission line known as a strip 
line.  The trace width, spacing to ground plane, and board dielectric 
constant determine the impedance.

    These lines have a constant impedance for all frequencies present in 
the signal.  Stated another way, all frequencies travel at the same 
speed in such lines.  You don't want the high and low frequency 
components of your square pulse arriving at different times.  That 
creates jagged leading and trailing edges that can look like multiple 
clock pulses where there should be only one.

    Another benefit of a ground plane is in preventing crosstalk between 
2 parallel traces.  The coupling capacitance between such traces 
reduces by an order of magnitude or more with a ground plane beneath.  
This allows the designer to pack the lines closer together without 
degrading the signals.

    By the way, a "ground plane" can be any low impedance surface.  The 
+3.3V sheet works as well as the 0V (ground) sheet.

    During circuit development, the engineer will bring test points out 
from the buried traces.  Since these stubs degrade the signals, they 
are removed for production.

    I don't think you will find many 4-layer motherboards around.  They 
go up to 8 or 12 layers to route all the lines from the big 
ball-grid-array packages.

Jim Kuzdrall