# RE: [SI-LIST] : parasitic value for via

Date: Tue Sep 19 2000 - 14:22:05 PDT

Hello All,

It sounds like John is correct. On a board of of thickness T the
unwrapped via will have an total area of T*D1*pi. The second
cylinder is not really a "cylinder."

If you have (for instance) 4 layers of GND. Then the structure
looks like this:

via
\ ___
gnd lyr --- || --- |
|| |
gnd lyr --- || ---
|| T
gnd lyr --- || ---
|| |
gnd lyr --- || --- _|_

Zooming in on one ground layer:

Gnd to Via
Clearance
\
-----+ +--------+ +-----
GND | | via | | GND foil with thickness 't' (i.e. 1 oz.)
-----+ | cylndr | +-----
| |
| |
| |

So, if you unwrap the via cylinder you end up with a perfect
rectangle. And if you unwrap the structures of ground you end
up with slots of GND separated by (D2-D1)/2 from the via.

Next, looking at the side-view it will look something like this:

+--------------------------+
| | (side-view of unwrapped via cyl)
+--------------------------+

+--+ +--+ +--+ +--+
| | | | | | | |
~ ~ ~ ~ ~ ~ ~ ~

You will be able to find capacitance contributions between the
via and each ground slot. But, you also get small contributions
from the neighboring gnd slots:

+----------------------/
| VIA / (side view of unwrapped via cyl)
+------------------/
| | |
_|_ _|_ _|_
___ ___ ___ <-- these represent the symbol for a cap.
| | |
| / \ |
+--+/ \+--+
| | | |
|G | |G |
|N | |N |
|D | |D |
~ ~ ~ ~

So, thinking of the ground slots as a solid (second) cylinder,
in my opinion is a good approximation. Otherwise, for
a more exact evaluation, you will need to use a 3-D field solver.

Regards,

-- Sal Aguinaga Jr.
-- 3Com - Carrier Business Networks
-- HW EE

JNH <[email protected]> on 09/19/2000 07:16:26 AM

Please respond to JNH <[email protected]>

Sent by: JNH <[email protected]>

To: Pierre-Luc Cantin <[email protected]>
Subject: RE: [SI-LIST] : parasitic value for via

Dear Cantin,

It is very good of your explanation. However, I have one question. Look
at
the profile of a via, we know that
a via is a cylinder physical structure. I cannot see the second
cylinder
wrapped the first one, connected to ground.

The Capacitance of via seems to be formed by the first cylinder and the
cross section of plane of ground.
D2-D1 is the clearance hole between the cylinder and the plane.

Best Regards,

John Lin
SI Specialist, ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: [email protected]
Tel: 886+3+3979000 ext. 5183

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