Hi Scott,
There are LOTS of different impedance equations and they will all give you different answers!!! In general, they are best for traces between .004 and .015 in width and between 35 and 90 ohms (single ended). I agree with the recommendation to get your fabricator to solve this on a 2D Field solver (Polar is the most common). That is the most accurate way I know of to get the information you need.
However, I would strongly recommend that you NOT use the calculator in your software without being very sure of its accuracy and that it matches what the fabricator can produce. Many of the EDA software calculators are based on equations that are very, very wrong and that can get you in a world of trouble.
Lastly, I think some clarification may be needed on your 50 ohm differential. The odd mode impedance of differential lines is less than that of a single ended line. The differential impedance of the pair is twice the odd mode impedance. I ran your numbers on the APPCAD RF calculator and the odd mode impedance is @35 ohms on that calculator which means the differential impedance would be about 70 ohm. I am not sure that calculator (or any Calculator) is accurate to those low numbers, but again, I would suggest you talk to your fabricator. Let him know everything you are trying to accomplish and he can help you.
Good luck,
Susy Webb
>
> From: Donald Kyle <[log in to unmask]>
> Date: 2007/06/25 Mon AM 07:35:07 CDT
> To: [log in to unmask]
> Subject: Re: [DC] Impedance calculations
>
> Scott, freebie calculators are not a good idea if you want to use
> them to build a stack-up that is accurate. They should be uses only
> for a ballpark idea. They can be off sometimes more then 30%.
>
> If you need a stack-up for a differential pair at 50 ohms then you
> should be using a 2d field solver like one from Polar, or one that is
> built into your EDA software. Keep in mind that a 2d field solver is
> more accurate then a set of equations setup for certain conditions,
> conditions that may not exist in your design. Also, EDA software
> packages don't necessarily have a 2d field solver built in.
>
> In the end, you should use what your PWB manufacturer suggests. They
> will use a 2d field solver to calculate the impedance based on the
> parameters you give them and then suggest a layer stack-up to achieve
> that impedance.
>
> Hope this helps
> Donald Kyle
>
>
>
> At 06:52 AM 6/25/2007, you wrote:
> >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
> ---------------------------------------------------------------------------------
>
---------------------------------------------------------------------------------
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
---------------------------------------------------------------------------------
|
