[SI-LIST] Dk value to use
- From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 29 May 2009 16:42:18 +0100
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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