To me, coplaner waveguide means having ground plane on both sides of a
single-ended trace (or both sides of a diff pair) as well a ground plane
below it on the adjacent layer.
Differential impedance works out to be twice the impedance of a
single-ended trace if the differential lines are far enough apart. As
you bring them together, the impedance goes down. I usually put them
far enough apart that pair separation is not too critical to their
impedance.
I use an online calculator to work out my stack up. I create a
spreadsheet showing the dielectric thicknesses and the widths and
separations of all controlled impedance traces. I give that to the
boardhouse as a starting point. The boardhouse responds with their
recommended stackup and thicknesses. They supposedly know which
thicknesses to use to hit a target impedance, and they certainly know
which laminate thicknesses are easiest for them to use.
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
James Jackson
Sent: Monday, June 25, 2007 9:47 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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