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
>
>
>
>
>
>
>
>
> ----------------------------------------------------------------------------
-----
> DesignerCouncil Mail List provided as a free service by IPC using LISTSERV
> 1.8d
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
> To temporarily stop/(restart) delivery of DesignerCouncil send: SET
> DesignerCouncil NOMAIL/(MAIL)
> Search previous postings at: www.ipc.org > On-Line Resources & Databases >
> E-mail Archives
> Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
> for additional information, or contact Keach Sasamori at [log in to unmask] or
> 847-615-7100 ext.2815
> ----------------------------------------------------------------------------
-----
>
>
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------
|
