Re: Q:Via Capacitance....Re: [SI-LIST] : Pass-through vias a...

David Instone (dinstone@uk.xyratex.com)
Wed, 10 Mar 1999 10:44:42 -0500

The following is quoted for microstrip and stripline capacitance in Nat
Semis AN-905,

microstrip pF/cm = (33.36 x SQRT(0.475er + 0.67))/Zo

stripline pF/cm = (33.36 x SQRT(er))/Zo

where er is the dielectric constant of the pcb material and Zo is the
characteristic impedance of the trace. (SQRT = square root)

regards

Dave Instone
Test Systems, MP24/22
Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
Tel +44 (0)1705 486363 ext 3071
or +44 (0)1705 443071 (direct line)
Fax +44 (0)1705 499315
http://www.xyratex.com

_______________________________________________________

fabrizio zanella wrote:
>
> Dan, the via capacitance depends on the pad, hole diameter, ground
> clearance and thickness of the board. The 0.5pf via you mention is
> probably for a 0.063" thick board?
> For an 0.093" thick board, 26 mil vias you can expect around 1pf via
> capacitance. For a backplane which is .225" thick with 26 mil vias the
> capacitance will be around 2pf. The best way to characterize it is with an
> LCR meter. For the trace capacitance, it depends on the distance to ground
> planes, 90 ohm surface microstrips will be around 1.5pF, 50ohm stripline
> traces will be around 3pf.
> Hope this helps,
>
> Fabrizio Zanella
> EMC, Hardware Engineering
> fzanella@emc.com
> 508-435-2075, x4645

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