FW: [SI-LIST] : RE: 2.5 GHz in FR4

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From: Javin Olson ([email protected])
Date: Mon Apr 30 2001 - 06:14:35 PDT


Your comment about making the via act like a transmission line is correct.
There are 2 ways that I know of to remove the stub caused by a via:

Blind vias - via stub is completely removed, but fabrication cost is
Counterboring - basically this method drills out the rest of the via, and it
adds about 25% to fabrication

Both methods obviously hurt the testability access to the board. The other
thing you can do is remove all non-functional pads within the via to reduce
the overall capacitance of the via.

I have one question about stripline and referencing in general: Is there a
benefit to having your high-speed diff-pair signal layers sandwiched between
2 GND layers? Are power layers generally a noisier reference?

Any thoughts on my last 2 questions would be appreciated...

Javin Olson
Plexus Technology Group
Design Engineer

  -----Original Message-----
  From: [email protected]
[mailto:[email protected]]On Behalf Of [email protected]
  Sent: Monday, April 30, 2001 5:56 AM
  To: [email protected]; [email protected];
[email protected]
  Subject: Re: [SI-LIST] : RE: 2.5 GHz in FR4

  After reading all of the replies to the 2.5GHz questiion, I can see the
  for using stripline to keep phaseshift constant on long runs. In trying
  make the stubs a part of the transmission line, wouldn't it be advisable
  utilise most of the via as part of the line by using the stripline located
  the opposite side of a card from the IC?
  Consider the 8 layer stackup shown here for clarification.

  ------- Sig and IC layer
  ------------- Plane
  ------- Sig
  ------------- Plane
  ------------- Plane
  ------- Sig 2.5 GHz stripline
  ------------- Plane
  ------ Sig

  The stub resulting from the the unused portion of the via is reduced in
  length. The trick would be to make the vias look like the transmission

  Another thought--It seems like an imbedded microstrip line would possess
  almost the same qualities of a stripline in terms of dielectric affecting
  phaseshift. Can a card be made so that the vias would go only from the IC
  layer to an imbedded microstrip layer directly below? This would get rid
  any stubs. What do you think?

  Richard Ellison

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