James,
I'm right there with you. I've always had pretty good results with the tool
that I'm using, but don't think I've ever had a request for 50ohm
differential impedance and felt a little uncomfortable blindly accepting the
numbers. This is a microstrip design with two traces that maintain a
consistent distance from each other over a GND plane.
It's a very simple layout and is not a dynamic application. I can specify
1/2 oz Cu, but that resulted in a wider trace width and was going to present
a few routing problems.
This request is from the flex manufacturer. They apparently do not have any
real design capability (a CAM guy for the panelization) or the ability to do
these calculations themselves.
Thanks for the help !
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of James
Jackson
Sent: Monday, June 25, 2007 10:47 AM
To: [log in to unmask]
Subject: Re: [DC] Impedance calculations
Okay Scott,
I'll express my ignorance on this issue... I'm sure someone will chime in
with a more informed reply.
I am thinking that you are looking at calculating the differential impedance
-
which looks to be 50 Ohms - between two signals. Correct?
Using one of the 'free' (but usually accurate - and in agreement with the
other 'free' calculators that I use) Impedance Calculators that I use
(AppCAD), I can choose 'microstrip' for determining Impedance, however this
places a calculation between one trace and a Plane (usually Ground).
If I choose the 'Coplanar Waveguide' option, this will calculate the
Differential Impedance usung two traces - side by side (which is similar to
what you are doing). Without a plane under the traces, I get about 66 Ohms -
which is not too far off - and that is without a 'groundplane' as the bottom
layer. WITH a 'groundplane', I get an Impedance of about 1/2 that, or about
36 Ohms.
I think that by your using a Trace Thickness of 1.4 mils on a flex-circuit
with a thinkness of 2.0 mils is kinda marginal... it's been a while since
I've done a flex design, though.
The other poster is correct. You should probably contact the fabricator and
ask them for assistance on what you are trying to do. From memory, the flex-
circuit fabrication house that I used was very helpful to me when I was
designing my boards, and saved me from making too many mistakes in my
design.
Regards,
James Jackson
Oztronics
Quoting Scott Riley <[log in to unmask]>:
> I've got a design that needs to have 50ohm differential pair on a two
> layer
> flex design (microstrip). I need to come up with the material stack up for
> this application. I came up with line width/space and material thickness
> using a calculator that I purchased.
>
>
>
> Dielectric constant = 3.5
>
> Substrate thickness = 0.002"
>
> Cu weight 1oz (0.0014")
>
> LW = 0.007"
>
> Space = 0.004"
>
> Calculated differential impedance = 56.7ohm
>
> * This calculator also takes into account the cover film/solder mask
>
>
>
> However, I ran the same numbers through a different calculator (a freebie)
> and there is a warning that the results are not valid unless the trace
> width
> is less than twice the dielectric thickness. The resulting impedance with
> this tool is 39.1 ohm.
>
>
>
> I get no errors or warning from the "store bought" calculator, but before
> I
> proceed I want to be confident that the numbers I'm getting are indeed
> valid. I'm assuming that all the calculators use the same general
> formulas,
> but get significantly different results using these two. I also asked
> another designer to try the numbers in his tool (Polar I believe), and he
> gets completely different results.
>
>
>
> Question: Is the "trace width must be less than 2x the material thickness"
> a
> valid rule for all calculators, or is this something that might be unique
> to
> this specific tool ?
>
>
>
>
>
>
>
>
>
> Scott D. Riley
> Technical Design Service
> 607-658-9322 - Office
> 607-785-1696 - FAX
> <http://www.techdesignservice.com> www.techdesignservice.com
>
>
>
>
>
>
>
>
>
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