I've seen differences of as much as 30% between a laminate manufacturer's specified DK value and the board fabricators "effective DK" value for the same material (FR4). Often the disparity is well outside of the range specified by the material manufacturer. I understand that DK values can change due to resin/content and frequency however, a 30% difference just seems awful big. 

Here's the part where I seek help from the experts. Thanks!

Is 30% difference in manufacturer DK versus Effective DK really possible?

Assuming the same material, will the DK difference be the same from fabricator to fabricator?

Thanks Much!

Rick

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