[SI-LIST] Capacitance and excess propagation time of via
- From: "Sogo Hsu" <sogo.hsu@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 23 Mar 2004 11:48:35 -0000
Hi, SI Gurus,
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=A1=A6s 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 consideration
as below.
The test vehicle is a 4-layer PCB.
Case I.
(Top) =3D=3D=3D=3D=3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D
|| ||
|| ||
|| ||
(Bottom)
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Case II
(Top)=3D=3D=3D=3D=3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D
|| || || ||
|| || || ||
|| || || ||
(Bottom)=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
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
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