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
>
>
>
>
>
>
>
>
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