This depends on the base material system.  FR4 tends to have a broader
range when different glass to resin ratios are evaluated.  30% is a
higher deviation than I have seen - 10% I would say for FR4 is about
what deviations you can expect from a glassy build to a resin rich build
- 3.8 range to 4.6.  A material like RO4000 has a tighter variation as
the system is delta in permittivity is less across the plane, but part
of this also is the refined glass systems used in the lower loss
materials.   

Much is also dependant on how you are measuring the loss - neat material
measurements will return more accurate results than estimations made
from a completed stripline.      

Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED 
           76 Technology Drive - POB 1890 
              Brattleboro, Vermont 05302
                Voice - 802.257.4571 ext 21
                    Fax - 802.257.0011
                       <http://www.vtcircuits.com/> 
                           
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Smith, Richard
Sent: Friday, September 15, 2006 11:46 AM
To: [log in to unmask]
Subject: [TN] Laminate Material DK

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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