[SI-LIST] Dk value to use

Hi

what Dk value to use for PCB trace impedance calculation?

I thought, I would use the measured Dk values recalculated to a suitable 
high frequency. 2GHz, because the Polar software uses the same frequency for 
skin-effect-based RL calculation. These values are different for each 
thickness-option or glass-style for each material. For example Isola 
supplies this data, because they did VNA measurements. Then I compensate Dk 
@ 1MHz (or Dk @ 1GHz) to Dk @ 2GHz based on some equations about Dk/Df 
frequency dependence (check this: 
http://www.buenos.extra.hu/iromanyok/E_r_frequency_compensation.xls ).

We wanted to use a new PCB manufacturer, who actually told me that they did 
extensive evaluations, and the result was that they have to use Dk=3.5 for 
each material, each glass-style, each signal. As a reason, they were talking 
about thinkness tolerances, match-with-measurement, tolerances in resin 
content, copper-embedding, and "other" parameters. They say if I use Dk from 
Isola, then it will be "wrong", and will not match with their TDR 
measurements.

Who has right?
I think if a variable has a distribution (not a fixed number), it still has 
a mean value, so its worth using its mean value. the mean value is the 
supplied Dk data.

Istvan Nagy
Concurrent Technologies Plc, UK


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