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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