Hi Jack,
The best fab notes we've seen will specify that the traces on layer(s)
#, with a width of W and edge to edge spacing of X shall have a
differential impedance of Zdiff.
In the design, it is important to insure that the differential traces
show up as unique aperture sizes in the Gerber data. This is
especially important if there are single-ended lines on the same layer
that are designed at the same width. Simply assigning these traces a
different aperture station is not good enough. Many CAM softwares
will, as a matter of course, combine like aperture sizes into one
aperture, making it impossible for the fabricator to adjust the lines
independently.
For Zdiff impedance modeling, I've been using Engineering Solutions
"Full House Plus" software (714-721-0668). Very reasonable in price.
I find it to be as accurate as 2d and 3d field solvers after
calibrating it to some empirical data. This should not be overlooked,
with any impedance model. Many models are available, but I've yet to
see one that is accurate across the full range impedance values and
board constructions.
Hope this helps...Andy
______________________________ Reply Separator _________________________________
Subject: Re: [TN] Specifying DIFFERENTIAL Impedance
Author: Jack Olson <[log in to unmask]> at smtplink-hadco
Date: 04/08/99 9:16 AM
Can someone else please try to answer this question, too (nothing against
you, Raymond)
But differential pairs are a tricky area for us as well. Most people are
familiar with impedance control for single traces, but some fab shops don't
have decent software for pairs, and try to adjust trace widths incorrectly.
We are tempted to say "no modifications to artwork allowed" but I hate to do
that...
p.s. can anyone recommend the best impedance calculator (for the price) FOR
PAIRS?
We don't need some $20,000 package, just something decently reliable.
Jack
-----Original Message-----
From: Raymond Aguilar [mailto:[log in to unmask]]
Sent: Wednesday, April 07, 1999 8:26 PM
Subject: Re: Specifying Differential Impedance
Aloha Joe ,
On design where I have to be concern with impedance I
typically specify the trace width with a tolerance
and the layer it is on and flag note it on the fabrication
drawing as a impedance controlled item.
Aloha Ray
"Hurst, Joe" wrote:
> Hi
>
> I have some very important differential pairs on a
PWB that must
> have a particular differential impedance.
>
> The problem is: how can I specify this on the
drawing sent to the
> fab shop?
>
> Our usual method would be to call for a particular
single-ended
> impedance, which seems to be beating around the bush.
It work to a
> point; but I think it leaves too many variables that could
change the
> differential impedance.
>
> Any thoughts on this?
>
> Joe Hurst
> PWB Designer
> Computing Devices Canada
> A General Dynamics Company
> [log in to unmask]
> 613-596-7816
>
>
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