Re: gEDA-user: General ground/power plane design questions

[Wojciech Kazubski <wk@ire.pw.edu.pl>]

> Signal layers should be on the outside, because the signal propagation
> speed (I am not sure, wether this is the correct term for
> Ausbreitungsgeschwindigkeit) on the outer layers is higher. Tis
> probably does not matter on low speed designs but is yet another rule
> of thumb on layer ordering.
The dielectric surrounding a line on outer layer is partially air, which has 
lower dielectric constant (1) than glass and epoxy (some 4.5 to 5.5) so the 
effective dielectric constant is a bit smaller. The propagation speed is 
inverse proportion to the sqare root of dielectric constant and is greater.

Another (practical) reason for making signal layers outside is the possibility 
of modification. If the prototype board has to be modified it is much easier 
to cut the track on outer layer and solder some wires to make new connection.
Error on inner track usually means new board to make.

Wojciech Kazubski