We found essentially the same thing, perhaps this is due to the different
frequencies used. I believe that the laminate suppliers test at 1 Mhz
whereas our TDR operates at 2 GHz.

Dave Rooke
Viasystems Canada
Pointe Claire Division
= = = = =
>Jack,
>
>It has been my experience that the Laminate Suppliers tend to report Dk on the
>high Side. We have found, that when applying our Classical Impedance model to
>the Board build that the apparent Dk is lower than the Suppliers literature. We
>verified this by measuring Dielectric Spacing and Line widths and Transposing
>the model to back calculate the Dk. We nearly always found the Actual Dk lower.
>
>Within some bounds the higher the resin content the lower the Dk, This is
>because the Glass reinforcement has a Higher Dk. The composite somewhat follows
>the "Rule of Mixtures".
>
>We usually find the Epoxy Dk to be between 4.0 and 4.2.
>
>I hope this is some help.
>
>Les Connally
>[log in to unmask]
>
>>  From: "Olson, Jack C" <[log in to unmask]>, on 3/3/98 9:21 AM:
>>  WARNING: this is a very unexciting question...
>>
>>  My software does some basic signal integrity analysis,
>>  and I just noticed it defaults the dielectric constant for
>>  regular FR4 to 4.7
>>
>>  I was under the impression that most FR4 is at 4.2-4.3
>>  these days (more resin? thinner cores? can't remember...)
>>
>>  Even a brief response will be appreciated!
>>                                                                  Jack
>>
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