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

fabrizio zanella (fzanella@fishbowl02.lss.emc.com)
Tue, 9 Mar 99 13:59:09 -0500

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
-------------
Original Text
From: "Daniel Roganti" <droganti@lucent.com>, on 3/9/99 2:57 AM:
To: smtp@Eng@EMCHOP1[<si-list@silab.Eng.Sun.COM>]

SUBJECT too long. Original SUBJECT is
Q:Via Capacitance....Re: [SI-LIST] : Pass-through vias as BGA lands (?)

---------------------- Original Message Follows ----------------------
All,

Would somebody have some info on what the average Via capacitance would
be ?
I recall using a average of 0.5pf per Via.

But now I'm dealing with a 16 Layer PCB and the Vias are 26mils.
Also, I would expect Blind Vias would have less capacitance than a
Thru-Hole Via.

I also recall about the Traces having an average Capacitance of
1.5pf/inch.
Would there be any current statistics on this ?

My margin on the Bus Loading is at a minimum.
We're using the MPC8260(Motorola) and the timing is gautanteed at only
50pf if we're expected to run at 66Mhz with 0 Waitstates.

thanks,
Daniel

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