In a 4 layer PCB, the stack-up is signal(layer-1), ground(layer-2), power(layer-3) and signal(layer-4). For a crowded routing, some signals need to be routed through vias from layer-1 to layer-4, that means signals will change reference plane. If we make the reference plane in layer-3 also be ground, there will be no reference-plane issue. But it needs two vias for each signal to transition between layer-1 and layer-4. In another way, not using vias, signal path (trace length) will be routed longer to avoid crossing. The longer trace length will result in more attenuation and increasing loop inductance. There is a trade-off between the two approach, which way is preferable? The frequency will be 400MHz And the over routed length is about 2 ~ 3 inch. Thanks Ethan Tsai ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu