Microwave and Millimeter-Wave High Frequency Circuit Material Performance (Up to 110 GHz)

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Presented by: John Coonrod

Microwave and Millimeter-Wave High Frequency Circuit Material Performance (up to 110 GHz)

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Agenda:

• Overview of frequency dependent dielectric behavior

• Dissecting insertion loss

• Copper surface roughness influences

• Comparing insertion loss of different designs and plated finishes

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Overview of frequency dependent dielectric behavior


D = εE

D is electric displacement vector, E is electric field intensity, ε is complex permittivity

Specific to material considerations:

• When an electric field is applied to a dielectric material, electric dipole moments are established

• The dipole moments augment the total displacement flux

• Polarization (P) is due to the material properties and the related dipole moments

D = ε0E + P

ε0 is free space permittivity

Bold letters are vector variables

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c is electric susceptibility of the material

• Dielectrics used in the high frequency PCB industry are typically a “linear dielectric”

• Or P is linear with an applied E so:

P = ε0 c E

D = ε0E + P


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D = ε0E + P = ε0 (1+ c) E = εE

• Finally, the displacement flux, including material effects:

ε = ε’ – jε” = ε0 (1 + c)

• ε’ is the real (storage) and ε” is the imaginary (dissipative)

• ε’ is associated with dielectric constant and ε” is associated with dissipation factor (Df) of the material

Dk = εr = ε’/ε0

Df = Tan() = ε”/ε’

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• Dipole displacement contributes to the Dk (εr)

• Molecular friction due to dipole rotation contributes to tan() or Df

• Depending on material properties, from about 10 MHz to 300 GHz, most interaction between electric fields and substrate is due to displacement and rotation of the dipoles

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• Dispersion is how much the Dk will change with a change in frequency

• Dipole moment relaxation is another issue which contributes to dispersion

• At low radio frequencies the dipole relaxation has little effect on Dk dispersion

• At microwave frequencies dipole relaxation has significant effect on dispersion

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ε’

1 GHz 300 GHzFrequency vs. Dk curve for a generic dielectric material

Dipolar and related

relaxation phenomena

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Low loss materials have much less Dk-Frequency slope

ε”

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Using 3 different test methods on the same piece of material, the Dk-Frequency trend is validated; the Dkdecreases with an increase in frequency

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Dissecting insertion loss

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αT is total insertion lossαC is conductor lossαD is dielectric lossαR is radiation lossαL is leakage loss

Dielectric loss (αD) is mostly due to the substrate, prepreg or soldermask

Conductor loss (αC) is due to several issues related to the conductor of the circuit

Radiation loss (αR) is due to many issues related to energy radiating off of the circuit

Leakage loss (αL) is mostly due to electrical leakage, through the dielectric and between conductor layers

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• Three circuit sets made from same material but different thicknesses

• Circuit material used was RO4350B™ laminate

• The dominate loss component can be different, for the different circuit thicknesses


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Using same material of different thicknesses and different copper types, thin circuits are more impacted by difference in conductor loss than thick circuits

Copper with smoother surface has lower conductor loss

Rolled copper has smoother surface than standard ED copper

RO3003 laminate is a very low loss material: typical Df @ 10 GHz is 0.001

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Copper surface roughness influences

Exaggerated surface roughness example

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• A higher Dk material will slow an electromagnetic wave

• In other words…

as a wave is slowed the “circuit perceived effective Dk” is assumed to be higher

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• There are other things that can slow the wave besides a substrate with higher Dk

• A rough copper surface can slow the wave propagation

• Again, a slower wave translates into higher Dk even if the substrate is unchanged

Excerpt to the right is from:

“Conductor Profile Effects on the Propagation Constant of Microstrip Transmission Lines”, Allen F. Horn, III, * John W. Reynolds* and James Rautio+

*Rogers Corporation, Lurie R&D Center, Rogers, CT 06259-0157 USA

+Sonnet Software, North Syracuse, NY 13212 USA

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• Shown are circuits with the same substrate, but using different copper types with different surface roughness

• Circuits with rougher copper surface (higher RMS) have higher effective Dk

The LCP laminate used in this study is the Rogers ULTRALAM® 3850 laminate

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• A thinner circuit will be dominated by conductor properties for loss and phase

• Conversely a thicker circuit will be less affected by copper roughness regarding the

“circuit perceived effective Dk” or the Rogers’ term of “Design Dk”

• Example: 50 ohm microstrip transmission line circuits evaluated for Design Dk,

using the same substrate and same copper, but different thickness

Cross-sectional view of a thin circuit, with exaggerated copper surface roughness

Cross-sectional view of a thick circuit

4mil RO4350BTM laminate has Design Dk = 3.95

30mil RO4350B laminate has Design Dk = 3.68

The Design Dk on the data sheet for RO4350B laminate is 3.66 because the substrate Dk, without copper effects (thick substrate), is 3.66

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Results from using same substrate and copper type, however circuits using different substrate thicknesses

All circuits were 50 ohm microstrip transmission lines


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Results from same substrate and thickness (5mil), however using different copper types which have very different copper surface roughness

This ED copper has an average roughness of ≈ 1.8 microns RMS andthe rolled copper has ≈ 0.3 microns RMS

The RO3003 laminate has a nominal bulk Dk of 3.0

Smooth copper has less influence on the propagation constant

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Comparing insertion loss of different designs and plated finishes

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Comparing insertion loss of different designs and plated finish

• Most final plated finish is less conductive than copper

• Due to this fact, when most final plated finish is added to copper the conductor loss will increase, which increases the insertion loss

• The exception is silver

• Typically the silver used in the PCB industry is applied very thin so the skin effects benefit of silver may not be obvious unless at very high frequencies

Conductivity of pure metals

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There is increased current density at the edges of the conductor

The increased loss is due to the current at the edges using the metal that is less conductive

Example: ENIG finish on microstrip circuit

ENIG is Electroless Nickel Immersion Gold

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• copper-nickel-gold at low microwave frequencies

• nickel-gold at moderate microwave frequencies

• Gold at high microwave frequencies

Simplified thought process for current density – skin effect

At conductor edge, majority of current density is within:

There are many other loss effects varying across this range of frequencies which complicates this simple thought exercise; Df increases with frequency, copper roughness increases loss, fields concentrate more between signal plane and ground plane at higher frequencies, etc. 24

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Microstrip, final plated finish impacts the conductor loss due to high current density at the edge of the conductor

Grounded coplanar waveguide (GCPW) has fields and current densities using 4 edges of the ground-signal-ground conductors

Since GCPW has 4 edges using the plated finish, it will have more conductor loss (and insertion loss) due to the finish than microstrip

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• Bare copper vs. ENIG on Microstrip vs. GCPW

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• Reminder: a thinner circuit is more dominated by conductor loss than a thick circuit

• The added loss due to final plated finish causes additional conductor loss

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The increased loss due to the final plated finish, depends on the thickness of the circuit

A thinner circuit will be dominated by conductor loss more than a thick circuit

When final plated finish is added to the copper it adds to the conductor loss


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A high loss (FR-4) material was used in a different study and showed little difference in insertion loss between circuits with bare copper and circuits with ENIG plated finish

This is because the FR-4 circuit was dominated by dielectric loss and the additional conductor loss due to ENIG was a very minor effect for the overall insertion loss


• Above charts show a comparison of 20mil FR-4 to 20mil RO4003C laminate

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• The following slides are insertion loss curves comparing plated finish over very wideband

• All circuits were 50 ohm microstrip transmission line circuits

• All circuits used a thin, very low loss thin laminate: 5mil RT/duroid® 6002 with rolled copper

• Basic material electrical properties when tested at 10 GHz is:

• Dk = 2.94

• Df = 0.0012

• The simple loss trend compared to copper, from least to most loss, is the following:• Immersion silver (no significant difference)• OSP• Immersion Tin• ENIPEG• ENIG

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No significant difference between bare copper circuits and circuits using immersion silver

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No significant difference between bare copper circuits and circuits using OSP

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There is a difference in loss between these two finishes, however experience suggests that this immersion tin curve has slightly more loss than expected. This curve should be considered a worst case scenario for immersion Tin

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Summary of insertion loss curves overlaid up to 40 GHz

The cluster of insertion loss curves around copper are not significantly different

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Thank You to Enthone for their support on the final plating finish studies


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Microwave and Millimeter-Wave High Frequency Circuit Material Performance (Up to 110 GHz)

Documents

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