Zhangkun and SI-list colleagues - I assume that "full-connected" means a direct tie to the plane, while "orthogonal-connected" means a plane tie using a "spoked" or "thermal" connection. The rest of this note is based on that presumption. This is a question that has concerned me for some time. I believe that most people would assume, as you did, that the direct tie presents lower resistance and inductance. However, if you consider skin effects, I think that the spoked connection offers better characteristics. This is because the small plane openings that are caused by the spokes create surface-conductive connections between the top and bottom surfaces of the plane. The skin effect in conductors means that the top and bottom surfaces of a power or ground plane are effectively insulated from each other for 100+ MHz signals. Such signals cannot transit through the bulk copper of the plane, but must find a surface-based path. The most likely such path is around the edges of nearby anti-pad openings in the plane or around the edges of spoked via openings. If you use directly tied vias (no spokes), currents must find their way to the nearest anti-pad in order to transit between the top and bottom surfaces of the plane. This increased path length translates into increased resistance and inductance. To visualize the problem, imagine a decoupling capacitor on the top surface of a board, tied by vias to two planes that are separated by a thin dielectric. The inter-plane capacitance is effectively limited to the interior surfaces of the two planes (because of skin effect), and we would like our decoupler to tie to those interior surfaces with as little inductance as possible. If we use directly tied vias, then the via that connects to the topmost plane has no surface-based path to the interior (i.e. the bottom) surface of that plane, except through nearby plane openings. The length of this path is unpredictable because it is dependent on how far away the nearest anti-pad happens to be; in any event, the increased current path length means that the mounted inductance of the decoupling capacitor is higher than it would be if we used spoked vias. I don't know if this effect is actually causing the results you are seeing in your simulation, but I believe it is real, and that plane connections from vias should use at least two spokes. I think that the only time a direct connection is better is when heavy DC or low frequency switching currents are being carried. In those cases, the full peripheral via contact to the plane should produce a slightly lower resistance. Regards, Tom Woodward Woodward McCoach, Inc Zhangkun wrote: > > Hi all: > > In PCB, which method to connect vias to power ground plane do you favour, > full-connected or orthogonal-connected? I think full-connected will be better > for less resistance and less inductance. Recently I have done some simulation > by means of Ansoft. The result surprised me. The inductance and resistance of > orthogonal-connected mode is less than that of full-connected mode. > > If anyone are interesting of my simulation result, please inform me. I will > send it to him. Si-list does not permit to add attachment. > > Best Regards > Zhangkun > 2002.12.19 > > ------------------------------------------------------------------ > 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 > ------------------------------------------------------------------ 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