This presentation is by John Coonrod from the Advanced Circuit Materials Division at Rogers Corporation. It addresses how to determine the dielectric constant (Dk) in circuit materials for printed circuit boards and electronics.

1. Title page
Advanced Circuit Materials Division
Design Dk

2. Page
• What is meant by Design Dk, Specification Dk, Process Dk
• Publishing the Design Dk
• Supporting Documents
• Test methods
• Other considerations
• Design Dk Test Method Defined
• Quick overview of supporting documents
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3. Page
• The Terms:
• Design Dk – The Dk value that is suggested for circuit design and modeling
• Specification Dk – The target Dk value for a material according to a standard
test method. This number can be different than the Design Dk.
• Process Dk – The Dk target used in the process of making the laminate and
typically the same as the Specification Dk. This number can be
different than the Design Dk.
• Real Dk or the Actual Dk – Not good terms to use. There are too many variables
to account for within the many different test methods, circuit designs,
circuit performance, materials, calculation methods, etc.
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4. Page
• Publishing the Design Dk
• Design Dk is currently published in:
• Product Selector Guide
• Slide Rule published in November issue of Microwave Journal
• MWI-2010 Impedance Calculator
• Datasheets for RO4000® products
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5. Page
• Publishing the Design Dk
• Design Dk values for Rogers High Frequency Laminates
shown to the right
• These are the same values in MWI-2010 impedance
calculator
• These are the same values for the published Slide Rule,
with the exception of RT/duroid® 6010.2LM is 10.72 and
ULTRALAM® 3850 is 2.90 for the Slide Rule. The
numbers given here are believed to be more accurate.
• The numbers listed here have been submitted for the
next publication of datasheets for all materials.
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6. Page
• Publishing the Design Dk
• The Design Dk has nothing to do with a material change
• This is an effort to give the circuit designers better information regarding Dk
• There are many different test methods and this is an effort to give clarification
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7. Page
• Publishing the Design Dk
• The major EM (ElectroMagnetic) software companies are being notified and
given a package of information for the Design Dk of all of our products
• Targeted EM software companies:
Ansoft (HFSS) Agilent (ADS, EM Pro) PulseEM
CST Microwave Office Polar
Sonnet Spectra PedaSoft
EM Works Genesys
MicroWorks MSpice
Eagleware IE3D
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8. Page
• Supporting Documents
• Datasheets and Product Selector Guide will give reference to these papers:
“General Design Dielectric Constant Values for Rogers Corporation’s High Frequency Circuit Materials” – this is
very basic information of Design Dk with all of the Design Dk numbers for our materials
“General Information of Dielectric Constants for Circuit Design using Rogers High Frequency Materials” – this
paper has much more detail about the different test methods, circuit design and material interactions. Has much
more Dk information from other test methods and Design Dk for different material thicknesses
“Comprehensive Information for Dielectric Constant of Rogers High Frequency Circuit Materials ” – the same
format as the previous, however it has very technical electrical engineering details for the various subjects.
“Design Dielectric Constant for RO4003TM and RO4350TM High Frequency Circuit Substrates” – Rogers Technical
Report No. 6006
“General Information of Dielectric Constant for RT/duroid® 6010.2LM and RO3010TM High Frequency Circuit
Materials” – A supplement report to the General Information addressing some other considerations for these two
materials.
All available for download from http://www.rogerscorp.com/acm
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9. Page
• Test Methods
• IPC has 13 different test methods to
determine Dk
• ASTM also has Dk test methods
• Many OEM’s have their own test methods
• Each test method has their own pros and
cons
• The few "accurate" test methods are very
engineering intense
• There are many other Dk tests:
• microstrip ring resonators, edge
resonators, cavity resonators,
waveguide, filters, several classes of
stripline, coplanar, cavity resonators, etc.
There is NO Perfect Test
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10. Page
• Test Methods
• Our standard QA test (Specification, Process Dk) we use:
• IPC-TM-650 2.5.5.5c, X-Band Clamped Stripline Resonator Test
• QA sometimes uses for internal reasons or customer requests:
• IPC-TM-650 2.5.5.6, Full Sheet Resonance Test, FSR
• SPDR (Split Post Dielectric Resonator)
• Generally we use this internally for material characterization
• Microstrip Differential Phase Length Method
• This method is what we use to determine Design Dk
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11. Page
• Test Methods
• Clamped Stripline Resonator Test
• Laminate to be tested has the copper etched off
• Laminate samples are clamped together with
resonator card in between
• The outside metal clamps act as the ground
planes for the stripline
Top view of resonator card
Stripline resonator
Stripline resonator
Side view of resonator card clamped into test fixture
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12. Page
• Test Methods
• Clamped Stripline Resonator Test
Gap coupling area
• Excellent Test method for testing raw material
• Fast, very repeatable, good resolution to variability
Top view of resonator card
• Has potential to have entrapped air
• Okay for our material test, but real circuits do not have entrapped air
• In the gap area of the resonator, the test can be sensitive to X-Y plane Dk properties
• Okay for our material test, but some circuits will not be sensitive to the X-Y Dk
In general there are two concerns for the
stripline test that may not apply to other tests:
1. Entrapped air
2. Anisotropic effects
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13. Page
• Summary of concerns for Stripline test (entrapped air) • Test Methods
• Air has a Dk = 1
• Entrapped air will lower the reported Dk value
• Two ways to have more entrapped air which alter the Dk value more
1. Sample has more surface roughness than normal
• The sample was a laminate and the copper was etched off
• The mirror image of the copper surface is now the surface of the sample
• A rough copper will give more surface area
• High profile ED copper will have more surface area, than smooth rolled
2. Sample does not conform well to the circuit image of the resonator
• A rigid substrate will not conform well to the resonator circuit
• A soft PTFE will conform well and has less entrapped air
Worse case scenario for more entrapped air is a rigid
substrate which uses rough copper – RO4350BTM laminate
Best case scenario for least amount of entrapped air is a soft
substrate with smooth copper – RT/duroid® 5880 with rolled copper
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14. Page
• Test Methods
• Summary of concerns for Stripline test (anisotropy)
• If the Dk is the same for all three axes, the material is considered isotropic
• Many laminates are anisotropic
• Dk in the Z-axis (thickness axis) is different than the X-Y plane of the material
• Typically the stripline test will determine the Dk of the Z-axis of the material
• The resonator circuit excites E-fields in the Z-axis of the material
• However in the gap area of the resonator there are strong E-Fields
• These fields can be altered by the X-Y plane Dk of the material in the gap
• If the material has a high degree of anisotropy, the overall Dk can be altered
• Most low Dk materials have little anisotropy and very little effect on the overall Dk
• Mid Dk materials (Dk = 5 or 6) have moderate anisotropy
• High Dk laminates generally have higher anisotropy
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15. Page
• Test Methods
• Full Sheet Resonance Test, FSR
Panel under test
is used as a
parallel plate
waveguide
Network Analyzer
inserts a signal
into the laminate
and evaluates at
what frequency
there are
resonant peaks.
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16. Page
• Test Methods
• Full Sheet Resonance Test, FSR
A long wavelength = low frequency
FSR uses a standing wave for the resonant peak
The wavelength for the FSR peak is established
by the physical size of the panel under test.
A 18”x12” panel will have a long wavelength, so
it is a low frequency test.
Generally the test is below 1 GHz.
Summary:
• FSR does not have the issue of entrapped air, as does stripline.
• FSR is not sensitive to anisotropic effects.
• FSR will evaluate the Z-axis Dk of the material only.
• The low frequency test of FSR can have an issue with some
higher frequency material dependencies.
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17. Page
• Test Methods
• SPDR, Split Post Dielectric Resonator
Summary for SPDR:
• Fast, easy, minimal chance of operator error.
• Determines Dk at a specific frequency (resonant peak).
• Determines the Dk value of the material in the X-Y plane only.
• Reported Dk value is only as good as the thickness
measurement; this is a concern.
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18. Page
• Test Methods
• Microstrip Differential Phase Length
• Two microstrip circuits of significant different length are made with the same material
• They use the same connectors (or fixture) to test for phase angle difference
• Example:
• A 2” long microstrip circuit has a wave enter it at 0º and exits the circuit at 70º
• A 6” long microstrip circuit has a wave enter it at 0º and exits the circuit at 210º
• There is a phase angle difference realized between the two circuits (70º vs. 210º)
• There is also a physical difference in length between these two circuits (2” vs. 6”)
• Using these differences of phase angle and physical length, a simple calculation
will determine how much the circuit altered the wave propagation on the circuit. The
alteration of the wave propagation is used to determine the Dk of the circuit material
at one specific frequency. This is repeated for many frequencies.
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19. Page
• Test Methods
• Microstrip Differential Phase Length
During calibration
During Testing
0 degree IN ? Degree OUT
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20. Page
• Test Methods
• Microstrip Differential Phase Length
• Summary
• This is not a fast test method; should not be used for high volume sampling
• The test method evaluates a “real” circuit
• A microstrip transmission line is the most common microwave circuit
• This method evaluates the Dk of the material in the Z-axis primarily
• Anisotropic effects of the material are insignificant in this test
• Capable of determining Dk over a very wide range of frequencies
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21. Page
• Test Methods
• Microstrip Differential Phase Length
• Example of test results
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22. Page
• Other Considerations
• It has been found that copper roughness can affect the Dk value of a circuit
• A substrate or circuit with a rougher copper will have a higher apparent Dk as
compared to the same substrate with smooth copper
• Excellent paper available on our website explains this phenomenon
• “Effect of conductor profile on the insertion loss, propagation constant,
and dispersion in thin high frequency transmission lines”
A low frequency test method
may not be as sensitive to the
copper roughness effect due
to skin effects being less
significant
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23. Page
• Other Considerations
• Due to the copper effect, the Design Dk may be different for the same material when
considering different thicknesses
• In the case of smooth copper (rolled) on RT/duroid® 5880 5mil substrate vs. a
20mil substrate has little difference; change in Dk of approximately 0.07
• In the case of a rough copper with RO4350BTM 4mil substrate vs. a 20mil
substrate there is a significant difference; change in Dk of approximately 0.25
• Having a Design Dk for different thicknesses of a substrate with rough copper is ideal
• When considering different Design Dk for different thicknesses of a particular
laminate, the raw substrate is not changing Dk; the change in Dk is due to the
copper effects becoming more significant as the laminate is thinner.
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24. Page
• Other Considerations
• By the nature of the circuit design, a perceived Dk may be different for one circuit as
compared to another when using the exact same material.
• Example:
• A microstrip transmission line uses mostly the Z-axis Dk property of the material
• An edge coupled filter uses some of Z-axis Dk property and some of the X-Y
plane Dk properties.
• These two circuits, when using the exact same material, could report a different
Dk value due to the nature of how the circuit uses the material.
Different edge coupled
features use a varying
amount of the X-Y Dk.
The perceived Dk will
not be the same for all
edge coupled features,
when using the same
material and thickness.
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25. Page
• Other Considerations
• A potential worse case scenario would be if the E-fields (red lines) interacted with the
glass for the X-Y plane interaction
E-fields using
resin only in the
X-Y plane
E-fields using
resin and glass in
the X-Y plane
The glass will typically have a much
different Dk than the resin of the laminate
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26. Page
• Other Considerations
• Example: RT/duroid® 6010.2LM has been reported by a customer to have a Dk of
10.8 with their design, while another customer using the exact same
substrate reports a Dk of 11.3.
• The RT/duroid 6010.2LM has a high degree of anisotropy; X-Y Dk is much higher
than Z-axis Dk
• One customers’ application is a transmission line and the 10.8 Dk number seems
reasonable.
• The other customer’s application is an edge
coupled filter and 11.3 is reasonable.
• Depending on how tightly coupled an edge
coupled feature is, then the Dk will
report higher or lower.
In general….very tightly coupled features
use less of the X-Y Dk and it reports an
overall lower Dk. A loosely coupled feature
uses more of the X-Y Dk and reports an
overall Dk that is higher; assuming X-Y Dk
is higher than the Z-axis Dk. 26

27. • Design Dk Test Method Defined
Page
• Rogers has considered and evaluated many test methods in an effort to find the most
appropriate method for determining the Design Dk.
• There are a tremendous amount of different microwave circuit applications.
• As it has been shown, different circuit designs can affect the perceived Dk value of the
laminate.
• The best case scenario for a Design Dk test method would be to:
• use a real circuit and not a “sample” or “testing structure”
• use the most common microwave circuit configuration employed in the industry
• have the testing done at higher frequency to account for the copper effects
• have the testing done over a wide range of frequencies to help account for many
different microwave applications
• The only test method found to fit these needs was the microstrip differential phase
length method (previously discussed). This is the test method that Rogers
Corporation has adapted to define the Design Dk for our materials.
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28. • Dk values from different test methods
Page
The following information is from one of the support documents previously mentioned:
“General Information of Dielectric Constants for Circuit Design using Rogers
High Frequency Materials”
Clamped stripline test at
different frequencies
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29. • Dk values from different test methods
Page
The following information is from one of the support documents previously mentioned:
“General Information of Dielectric Constants for Circuit Design using Rogers
High Frequency Materials”
Dk comparisons of FSR and SPDR FSR test
results, can be viewed as an approximate results only
estimate for anisotropic effects of material
FSR Dk values are good
for understand Z-axis Dk
at lower frequencies 29

30. • Dk values from different test methods
Page
The following information is from one of the support documents previously mentioned:
“General Information of Dielectric Constants for Circuit Design using Rogers
High Frequency Materials”
Different Design Dk Design Dk values for various materials and different thickness
values for different
thicknesses are
due to the copper
effects and not a
change in the raw
substrate
properties
RO4400TM prepreg
Dk values are an
average of 2 and 3
ply constructions
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31. Page
Thank You
The world runs better with Rogers., the Rogers' logo, RT/duroid, RO3000, RO3003, RO3006, RO3010, RO3035, RO3203, RO3206, RO3210, RO3730,
RO4000, RO4350B, RO4003C, RO4350, LoPro, RO4360, RO4730, RO4450B, RO4450F, XT/duroid, ULTRALAM, RO4400, RO4003 and TMM are licensed
trademarks of Rogers Corporation
The information in this presentation is intended to assist you in working with Rogers' High-Frequency Materials. It is not intended to and does not create any
warranties, express or implied, including any warranty of merchantability or fitness for a particular purpose or that any results show in this presentation will be
achieved by a user for a particular purpose. The user is responsible for determining the suitability of Rogers' High Frequency Materials for each application.
Prolonged exposure in an oxidative environment may cause changes to the dielectric properties of hydrocarbon based materials. The rate of change increases
at higher temperatures and is highly dependent on the circuit design. Although Rogers’ high frequency materials have been used successfully in innumerable
applications and reports of oxidation resulting in performance problems are extremely rare, Rogers recommends that the customer evaluate each material and
design combination to determine fitness for use over the entire life of the end product.
This presentation may not be reproduced, copied, published, broadcast, transmitted or otherwise distributed without the written approval of Rogers
Corporation.
©2010 Rogers Corporation. All rights reserved
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