[SI-LIST] Capacitance and excess propagation time of via
- From: "Sogo Hsu" <sogo.hsu@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 24 Mar 2004 00:43:36 -0000
Hi, Gurus,
Sorry to my ambiguity ASCII code in the previous message. I try to
send the mail again.
The capacitance of via was usually estimated by the formula as below
C =3D 1.41 epsilon_r * T * D1 / (D2 =A1VD1).
The definition of T is the total thickness of PCB by refering Dr.
Johnson's black magic bible. It seems like that the formula implied
the capacitance is formed by plate capacitor where the area is
constructed by the conducted via body and space is (D2- D1)/2. Is it
reasonable for a multi-layer PCB structure? In my opinion, the only
effective thickness is the total thickness of pwr/gnd plane which
the via passing through. Therefore, I think the estimation of
capacitance is over-estimate. The full wave analysis results, such
as HFSS, also presented the characteristic of via is inductive
dominantly. That is said, via did not have such large capacitance
inherently.
Besides, we also ! tried to estimate the excess propagation time
when the signal traveling through via. Two cases are under
onsideration as below. The test vehicle is a 4-layer PCB.
Case I
(TOP) ==== ======
|| ||
|| ||
===============================================
(Bottom)
Case II
(Top)
==== =============== ======
|| || || ||
|| || || ||
================= ==================
(Bottom)
Case I consists of two vias for layer change and Case II has four
vias. The total trace length is same to each other. The propagation
time was measured by TDT. The excess propagation time caused by via
can be obtained by subtracting with the propagation delay of
microstrip in equal length. The excess propagation time per unit via
for Case II is obviously faster in a factor of 60% than Case I as
well. Does any one meet this phenomenon before? It is suspect that
is the result of via coupling. I hope any expert can give me a
reasonable explanation on this phenomenon. Thank you in advance.
Sogo Hsu, Ph. D.
Foxconn Electronic
------------------------------------------------------------------
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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl
List technical documents are available at:
http://www.si-list.org
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
Other related posts: