On Mon, Oct 19, 2009 at 03:18:25PM +0100, Andy Fierman wrote: > So your consultant thinks it's a bad idea to have a Vcc plane because > it takes up space that you could use for additional ground planes and > that you might need to run traces ... > > ... and then urges you to run power traces where? > > In the - now empty - Vcc plane layer? > > Or in the same layer as your already (according to your consultant) > full signal layers, leaving you a whole layer to put a shiny new > ground plane into? > > > Hmmm. > > Suppose you have a 6 layer PCB with one continuous ground pane and one > continuous Vcc plane (dream on ...). It's not ideal but not everyone > wants to pay the extra for 8 layers. > > > layer 1 = signal (set up for 50R trace impedance w.r.t. ground plane) > : > small spacing > : > layer 2 = ground plane > : > small spacing > : > layer 3 = signal (set up for 50R trace impedance w.r.t. ground plane) > : > big spacing! (to minimise Xtalk between layers 3 to 4) > : > layer 4 = signal (set up for 50R trace impedance w.r.t. Vcc plane) > : > small spacing > : > layer 5 = power plane > : > small spacing (set up for 50R trace impedance w.r.t. Vcc plane) > : > layer 6 = signal
The problem is that you often have no choice with regard to layer spacing. For 6 layers, low cost prototype quantity, my fab does indeed have a large spacing in the middle, a small one between layers 2 and 3 (and 4 and 5), and an intermediate one between the outer layer and the next one. That's really messy for impedance control... > All the return currents for signals on layers 4 & 6 run over the power > plane until they get decoupled to layer 2 ground plane. Not ideal but > if the Vcc plane is properly decoupled to the ground plane with good > ceramic decouplers adjacent to all signal sources and destinations > (some of which may be dedicated and some shared Vcc decouplers anyway) > then that is OK. Indeed. Gabriel _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user