RE: [SI-LIST] : RF Via Advice?

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From: Bob Weber (rweber@txc.com)
Date: Mon Nov 27 2000 - 07:42:12 PST


I've asked about the use of pads on through vias to a number of people over
the past 2 years. Most say that they are not needed with current PCB
fabrication techniques (FR-4 glass & epoxy boards). I'm of the opinion that
they are needed if the clearances are too small to allow good fill by the
epoxy. Conversely, if you remove the pads, you should be able to prevent any
problems by adding some additional clearance from the via to the plane. The
PCB fabricator for your type of board should have answers on what works and
what doesn't.

I intend to investigate further. Please let us know what you find out.

Bob W.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Bradley S Henson
Sent: Monday, November 27, 2000 9:52 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : RF Via Advice?

SI and Board Experts:

I've asked our route folks to use "RF vias" for our 1Gbs fibre channel
signals in our PWBs, where we must use vias. My definition of RF vias, from
the literature, is that the pad is omitted from the signal via on power and
ground layers to reduce capacitance. I also asked for larger than normal
clearances to the planes, assuming no signals routing through the area. The
product design folks have responded by saying there could be a reliability
problem (thermal cycling induced barrel cracking) if they omit the pads
from the internal power and ground layers on my FC signal vias. I'm only
proposing a couple of RF vias in a PWB with 1000's of conventional vias.
Could these few vias really cause a problem? Anyone have any experience
with this? BTW, our boards are typically 14 layer dual offset stripline and
use 4 mil polyimide dielectric layers (fairly lossey, but useable for short
runs). I don't know the via aspect ratio...I'm trying to find out.

Thanks,
Brad Henson, Raytheon

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