[SI-LIST] Re: [Bulk] Re: PCB Impedance Failure

Kai

I think you are missing the point.  The board was designed to have 50 Ohm SE
traces and at the same time 100 Ohm differential traces.  What was received
was 59 Ohm SE traces and 86 Ohm differential traces.  When the impedance of
the SE traces increases the differential impedance should also increase.
The differential impedance is 2*SE - coupling impedance.  If we assume the
coupling did not change significantly since the original was no coupling the
as fabricated board's impedance should have been 118 Ohms.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Kai Keskinen
Sent: Wednesday, January 11, 2006 2:47 PM
To: Aubrey_Sparkman@xxxxxxxx; a.ingraham@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [Bulk] Re: PCB Impedance Failure


Aubrey:

That is really easy. Just closely couple the diff pairs and you can go from
60 Ohms single ended to 90 Ohms differential with very little effort. It is
not really what you want to do if you have complicated routing through pin
fields since you then get a huge impedance hit if you have to separate the
pairs. Just use any field solver that handles diff pairs and try it. Set up
a 60 Ohm single ended line. Use the same cross-section and add the second
line and move them closer together. You will get a lot lower than 2x single
ended impedance.

Cheers,

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
Aubrey_Sparkman@xxxxxxxx
Sent: Wednesday, January 11, 2006 1:45 PM
To: a.ingraham@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [Bulk] [SI-LIST] Re: PCB Impedance Failure


I'm a little puzzled about something else.  Did you really get the diff
impedance to be on the low side (100->86) on the same board / layer
where the single ended impedance is on the high side (50->59)?  Please
explain how that was done!


Aubrey Sparkman=20
Enterprise Engineering Signal Integrity Team
Dell, Inc.=20
Aubrey_Sparkman@xxxxxxxx=20
(512) 723-3592

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Ingraham
Sent: Wednesday, January 11, 2006 11:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PCB Impedance Failure

Clayton,

I was a little puzzled about two things you stated.

One, that the specified differential impedance was exactly twice the
specified single trace impedance.  That requires no coupling between
traces of a differential pair (if they are routed on the same layers
with the same trace widths).  Is that what you really intended?  Since
you got 86 ohms diff. and 59 ohms s.e., your differential pairs must not
have been routed with nearly enough isolation.

The other is about the error on the test coupon.  Can you trust any
measurements on those coupons?

Did your company actually specify 50 +/- 10%, or was that only a
recommendation?  If it was in the spec, then I wouldn't even bother
asking about the risk; I'd send the boards back to the board vendor and
tell them to make new ones that satisfy what you paid them to do.  The
fact that they were made wrong, is reason enough to be concerned.

Regarding the question of the impact on SI, that depends entirely on the
application, on how long the traces are (vs. risetimes and/or frequency
content), on the types of signals, on the chips used, and if and how
lines are terminated.  Sometimes it makes little difference whether the
actual impedances are even close to the target.  Sometimes it makes all
the difference.  59 ohms isn't that far from 50 ohms, but if you were
using leading edge technologies and pushing all the margins (which you
probably aren't, since you didn't use SI tools), it could indeed be
significant.

Regards,
Andy Ingraham

> A board we just had fabricated failed the impedance test coupon.  59=20
> ohm vs 50+/- 10%.  They could not measure the diffferential pairs=20
> because of an error on the coupon, but calculated 86 ohm impedance vs=20
> 100 +/- 10%


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