Re: [SI-LIST] : Traces Over Plane Clearances

Ron Miller (rmiller@Brocade.COM)
Fri, 06 Aug 1999 13:20:01 -0700

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Dave

You need to consider this from the viewpoint of the
ground currents which flow in the reverse direction
under the traces. When you cut the ground plane you
might just as well cut the trace.

If there are other paths which the ground current can
follow it will flow there, but the frequency response will
be severly limited.

An alternate possibility is to use differential lines, in
which case the return current can flow through the
opposite line. When these lines are over a ground plane
which has been cut, the effect is to add an inductive
discontinuity, which might be tuned out by using very
tightly coupled differential lines.

Good luck

Ron Miller

Dave Hoover wrote:

> I have a customer who has a 2000+ I/O BGA with
> tight pitch. They are using 2 track routing to
> escape the device on the PCB. At the device,
> the traces cross over the edge of the reference
> plane clearances. The microprocessor and
> clocks currently run at ~1 GHz (headed above 3 GHz).
> The distance across that clearance would be
> less than .020" in length. There is a copper
> web of around .008" between the plane
> clearances (thinking adequate signal return path).
> My question is that the customer is afraid the short
> distances without the reference plane will impact
> his electrical performance properties.
> The problem I face is currently the clearance size
> is reduced to the point that it can't be manufactured.
>
> Faced with a dilemma, so I went to the pros.
>
> David Hoover
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"> Dave

You need to consider this from the viewpoint of the
ground currents which flow in the reverse direction
under the traces.  When you cut the ground plane you
might just as well cut the trace.

If there are other paths which the ground current can
follow it will flow there, but the frequency response will
be severly limited.

An alternate possibility is to use differential lines, in
which case the return current can flow through the
opposite line.  When these lines are over a ground plane
which has been cut, the effect is to add an inductive
discontinuity, which might be tuned out by using very
tightly coupled differential lines.

Good luck

Ron Miller
 

Dave Hoover wrote:

I have a customer who has a 2000+ I/O BGA with
tight pitch. They are using 2 track routing to
escape the device on the PCB. At the device,
the traces cross over the edge of the reference
plane clearances. The microprocessor and
clocks currently run at ~1 GHz (headed above 3 GHz).
The distance across that clearance would be
less than .020" in length. There is a copper
web of around .008" between the plane
clearances (thinking adequate signal return path).
My question is that the customer is afraid the short
distances without the reference plane will impact
his electrical performance properties.
The problem I face is currently the clearance size
is reduced to the point that it can't be manufactured.

Faced with a dilemma, so I went to the pros.

David Hoover

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
  --------------8DA47FC95D0301B1F777EA2F-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****