Technical learning article for RF, microwave, mmWave, PCB, and hardware engineers

When engineers first compare Rogers laminates, dielectric constant is often the first number they look at.

That is understandable. Dk directly affects transmission-line impedance, phase velocity, wavelength, resonator dimensions, antenna geometry, and the physical size of distributed RF structures.

But Dk alone is a poor way to select a high-frequency PCB material.

Two laminates with similar dielectric constants can behave very differently once copper roughness, dielectric loss, laminate thickness, thermal coefficient of Dk, Z-axis expansion, moisture absorption, surface finish, and fabrication processes are considered.

At microwave frequencies, those differences affect insertion loss and impedance accuracy. At millimeter-wave frequencies, they can alter phase length, antenna resonance, conductor loss, and production consistency enough to determine whether a design works at all.

Rogers’ own portfolio illustrates this clearly. A comparison of Rogers material properties across families such as RO3000, RO4000, and RT/duroid shows that materials with similar high-frequency positioning can differ substantially in Dk, Df, thermal behavior, CTE, and fabrication requirements. Their resin systems, reinforcement, copper options, mechanical properties, fabrication requirements, and temperature behavior are fundamentally different.

Rogers Is a Material Family, Not a Single PCB Technology

The first distinction is the material system itself.

The RO4000 family uses ceramic-filled hydrocarbon thermoset resin reinforced with glass fabric. One of its major advantages is manufacturability: RO4003C and RO4350B can largely be processed using conventional FR-4 fabrication equipment and do not require the specialized hole-wall preparation normally associated with PTFE materials.

RO3000 materials use ceramic-filled PTFE formulations. They provide lower dielectric loss and excellent electrical performance, but their PCB fabrication process is closer to traditional PTFE processing. Rogers specifies special hole-wall preparation and processing controls for RO3000-series materials before metallization.

RT/duroid 5880 is another PTFE-based system, reinforced with randomly oriented microfibers. It has an exceptionally low Dk of approximately 2.20 and a very low dissipation factor of 0.0009 at 10 GHz, making it attractive for low-loss broadband circuits. Mechanically, however, it behaves very differently from glass-reinforced RO4000 materials.

RO3010 goes in the opposite electrical direction. Its process Dk is approximately 10.2, allowing physically smaller transmission lines, filters, resonators, and antenna structures, while still maintaining relatively low dielectric loss.

A representative comparison shows how quickly the differences extend beyond Dk:

Property	RT/duroid 5880	RO3003G2	RO4003C	RO4350B	RO3010
Process Dk	2.20 ±0.02	3.00 ±0.04	3.38 ±0.05	3.48 ±0.05	10.20 ±0.30
Published Design Dk	2.20	3.07	3.55	3.66	11.2
Df @ 10 GHz	0.0009	0.0011	0.0027	0.0037	0.0022
TCDk	−125 ppm/°C	−35 ppm/°C	+40 ppm/°C	+50 ppm/°C	−395 ppm/°C
Thermal conductivity	0.20 W/m·K	0.43 W/m·K	0.71 W/m·K	0.69 W/m·K	0.95 W/m·K
X-axis CTE	31 ppm/°C	16 ppm/°C	11 ppm/°C	10 ppm/°C	13 ppm/°C
Y-axis CTE	48 ppm/°C	17 ppm/°C	14 ppm/°C	12 ppm/°C	11 ppm/°C
Z-axis CTE	237 ppm/°C	18 ppm/°C	46 ppm/°C	32 ppm/°C	16 ppm/°C
Water absorption	0.02%	0.06%	0.04%	0.05%	0.05%

These are published typical or specification values, and the test conditions behind individual properties are not necessarily identical across all materials. They should therefore be treated as engineering inputs, not as a simple ranking table.

The most important observation is that there is no universally “better” Rogers material.

The correct material depends on which electrical and manufacturing errors the design can tolerate.

Process Dk and Design Dk Are Not the Same Number

One of the most common mistakes in high-frequency PCB design is copying the first Dk value from a datasheet directly into an electromagnetic field solver.

Rogers often publishes both a Process Dk and a Design Dk.

They exist for different reasons.

Process Dk is useful for material manufacturing control and lot-to-lot specification. Design Dk is intended to give engineers a dielectric constant that better represents how a fabricated transmission line behaves.

For example:

• RO4003C: Process Dk ≈ 3.38, Design Dk ≈ 3.55
• RO4350B: Process Dk ≈ 3.48, Design Dk ≈ 3.66
• RO3003G2: Process Dk ≈ 3.00 at 10 GHz, Design Dk ≈ 3.07 at 77 GHz

Those differences are too large to dismiss as datasheet rounding.

The reason is that a transmission line does not interact with an abstract bulk dielectric constant.

Its phase response also depends on conductor geometry, copper roughness, frequency, field distribution, substrate thickness, and transmission-line structure.

Rogers has demonstrated experimentally that the same dielectric material laminated to copper with different surface roughness can produce substantially different extracted effective Dk values. The dielectric did not change; the propagation behavior did.

This matters whenever phase length is important.

For an ideal homogeneous stripline:

β = (2πf / c) · √εᵣ

where:

• β is phase constant,
• f is frequency,
• c is the speed of light,
• εᵣ is dielectric constant.

Suppose, purely as an illustrative homogeneous-stripline calculation, an engineer models a 10 mm line at 77 GHz with Dk = 3.00 instead of 3.07.

The accumulated phase difference is approximately 18.6° over only 10 mm.

A real microstrip line will not follow that exact result because part of its electromagnetic field exists in air and its effective dielectric constant is lower than the substrate Dk. But the calculation demonstrates why a seemingly small Dk difference becomes important at millimeter-wave frequencies.

At 1 GHz, a few hundredths of Dk may be manageable.

At 77 GHz, it can become a meaningful phase error.

Df Describes Only One Part of Insertion Loss

Dissipation factor is another property engineers frequently use as a shortcut.

Lower Df generally means lower dielectric loss.

For example:

• RT/duroid 5880: 0.0009
• RO3003: 0.0010
• RO3003G2: 0.0011
• RO4003C: 0.0027
• RO4350B: 0.0037

It would therefore be tempting to conclude that insertion-loss performance follows exactly the same order.

It does not.

Total transmission-line loss can contain several components:

α_total = α_d + α_c + α_r + α_leakage

where:

• α_d = dielectric loss,
• α_c = conductor loss,
• α_r = radiation loss,
• α_leakage = leakage or other structure-dependent losses.

Df primarily addresses α_d.

At millimeter-wave frequencies, conductor effects can become comparable to or greater than the difference in dielectric loss between two candidate laminates. Rogers specifically notes that conductor roughness and circuit construction become increasingly important as operating frequency rises.

This is why selecting a laminate using Df alone can produce disappointing measured results.

Copper Roughness Becomes a Material Parameter at Millimeter Wave

At low frequency, engineers may treat copper as an almost ideal conductor with a fixed bulk conductivity.

At microwave and millimeter-wave frequencies, current is concentrated near the conductor surface because of skin effect. This is why copper roughness in high-frequency PCB design has to be treated as part of the conductor-loss model rather than as a fabrication detail.

The classical skin depth is:

δ = √[ 2ρ / (ωμ) ]

Using nominal room-temperature copper properties gives approximately:

Frequency	Copper skin depth
1 GHz	2.09 µm
10 GHz	0.66 µm
24 GHz	0.43 µm
60 GHz	0.27 µm
77 GHz	0.24 µm

That final number is important.

At 77 GHz, the current is concentrated within roughly 0.24 µm of the conductor surface under the simplified smooth-conductor model

Now compare that with real copper.

Rogers reports an average RMS roughness around 2.0 µm for standard ED copper used in one RO3003 comparison and approximately 0.35 µm for rolled copper.

At 77 GHz, a 2 µm roughness scale is many times the nominal skin depth.

The current no longer travels along an ideally flat interface.

It follows a longer and more complex surface path, increasing conductor loss and propagation delay.

This is why Rogers developed products such as RO3003G2 using Very Low Profile ED copper. The laminate is specifically optimized for 77/79 GHz automotive radar, combining controlled dielectric properties with smoother copper behavior.

Substrate Thickness Determines How Much Copper Roughness Matters

Copper roughness does not affect every stackup equally.

Rogers has published a useful comparison using RO3003.

For a 5 mil substrate, changing from rougher standard ED copper to smoother rolled copper produced approximately 0.35 dB/in difference in insertion loss at 25 GHz.

For a 20 mil substrate, a similar comparison produced only about 0.10 dB/in difference.

The dielectric material was essentially the same.

The importance of conductor loss changed because the electromagnetic field distribution and transmission-line geometry changed with substrate thickness.

This creates a design tradeoff.

Thin substrates are attractive at millimeter-wave frequencies because they help suppress unwanted modes, reduce radiation-related problems, and permit compact transmission-line structures.

But thinner circuits also tend to make conductor loss more dominant.

Rogers notes that 5 mil RO3003-family materials are commonly used in 77 GHz applications for exactly this reason, while also pointing out their increased sensitivity to conductor effects.

Therefore:

Material thickness and copper type should be selected together.

Choosing RO3003G2 and only later deciding copper profile and core thickness is not a complete mmWave material decision.

Surface Finish Can Undo the Benefit of Smooth Copper

Even after a designer chooses low-loss dielectric and low-profile copper, the finished PCB conductor may no longer be copper alone.

Surface finish matters.

ENIG is popular in conventional electronics because it provides excellent solderability, flatness, and oxidation protection.

For mmWave RF traces, however, the nickel layer creates a problem.

Rogers notes that nickel has roughly one-third the conductivity of copper and that ENIG increases conductor loss as frequency rises. The effect also varies with nickel thickness.

This becomes especially significant for tightly coupled grounded coplanar waveguide structures, where fields interact strongly with conductor sidewalls and plated surfaces.

Rogers’ mmWave guidance therefore notes that many high-frequency designs avoid ENIG on critical RF paths and instead consider lower-loss alternatives such as immersion silver, immersion tin, or suitable OSP processes.

That does not mean ENIG should never be used in Rogers PCB manufacturing.

It means the surface finish should be included in the RF model and loss budget rather than selected only from an assembly perspective.

A board can even use selective finishes when electrical and assembly requirements justify the additional process complexity.

Temperature Changes Dk and Phase

A room-temperature Dk specification is not sufficient for equipment that must operate across a wide temperature range.

Rogers therefore publishes the thermal coefficient of dielectric constant, usually abbreviated TCDk.

A first-order approximation is:

Δεᵣ / εᵣ ≈ TCDk × ΔT

with TCDk expressed in ppm/°C.

Because phase constant is approximately proportional to √εᵣ, a small dielectric change produces approximately:

Δβ / β ≈ ½ · (Δεᵣ / εᵣ)

Consider a temperature excursion of 125°C purely as a first-order comparison:

Material	TCDk	Approx. relative Dk change	Approx. relative phase change
RO3003G2	−35 ppm/°C	−0.44%	−0.22%
RO4003C	+40 ppm/°C	+0.50%	+0.25%
RO4350B	+50 ppm/°C	+0.63%	+0.31%
RT/duroid 5880	−125 ppm/°C	−1.56%	−0.78%
RO3010	−395 ppm/°C	−4.94%	−2.47%

These calculations are illustrative rather than guaranteed circuit shifts because TCDk can vary with frequency and temperature and the field of a real transmission line may not reside completely inside the dielectric. The published material values nevertheless show why temperature behavior cannot be inferred from nominal Dk.

For phase-sensitive applications such as:

• phased-array antennas,
• automotive radar,
• beamforming networks,
• narrowband filters,
• couplers,
• oscillators,
• delay lines,

TCDk can be more important than a small difference in room-temperature Df.

RO3003G2 Shows Why Frequency-Specific Data Matter

RO3003G2 provides a useful real-world example.

Rogers specifies:

• Process Dk: 3.00 ±0.04 at 10 GHz
• Design Dk: 3.07 at 77 GHz
• Df: 0.0011 at 10 GHz
• TCDk: approximately −35 ppm/°C
• thermal conductivity: approximately 0.43 W/m·K
• Z-axis CTE: approximately 18 ppm/°C

Rogers also reports approximately 1.3 dB/in insertion loss for a 5 mil laminate using its microstrip differential-phase-length measurement method.

That dataset is much more useful for a 77 GHz radar engineer than simply being told:

RO3003G2 has a Dk of about 3.

The second statement hides the details that actually determine whether an antenna or transmission line will correlate with simulation.

CTE Determines Whether the PCB Survives the Manufacturing Process

Electrical performance is only half the material-selection problem.

The PCB still has to survive drilling, copper plating, lamination, soldering, thermal cycling, and service conditions.

Z-axis coefficient of thermal expansion is especially important for plated through-holes.

Consider the published Z-axis CTE values:

• RO3003G2: ~18 ppm/°C
• RO3010: ~16 ppm/°C
• RO4350B: ~32 ppm/°C
• RO4003C: ~46 ppm/°C
• RT/duroid 5880: ~237 ppm/°C

The difference is substantial.

As a simple dimensional illustration, assume a 1 mm dielectric thickness experiences a 235°C temperature increase.

Using a linear CTE estimate:

ΔL = α · L · ΔT

the unconstrained Z-direction dimensional change would be approximately:

• RO3003G2: 4.2 µm
• RO4350B: 7.5 µm
• RO4003C: 10.8 µm
• RT/duroid 5880: 55.7 µm

This is not a direct plated-barrel strain calculation. Real multilayer structures are mechanically constrained, copper itself expands, and material CTE is not necessarily perfectly linear through the entire thermal range.

But the scale of the difference explains why material system and via reliability must be considered together.

An excellent low-loss RF material is not automatically the easiest material for a thick multilayer PCB with thousands of plated holes.

High-Dk Materials Do More Than Shrink the Circuit

RO3010 is a useful example of another common simplification.

Its high Dk, approximately 10.2 process and 11.2 design, allows much smaller distributed RF structures than a Dk 3 material.

For an ideal wavelength in dielectric:

λ = c / ( f · √εᵣ )

So at the same frequency, increasing Dk from approximately 3 to approximately 10 reduces wavelength substantially.

That can be extremely useful for:

• patch antennas,
• filters,
• couplers,
• resonators,
• matching networks,
• compact RF modules.

But high Dk also concentrates more field energy inside the dielectric.

The electrical consequences include:

• stronger sensitivity to dielectric-property variation;
• smaller physical dimensional tolerances relative to wavelength;
• more compact fields;
• potentially narrower bandwidth for certain resonant structures;
• different conductor widths for the same impedance.

RO3010’s published Df remains relatively low at about 0.0022, and its Z-axis CTE is excellent at approximately 16 ppm/°C, but its TCDk is much larger in magnitude than that of RO3003G2.

That may be acceptable for one application and unacceptable for another.

Again, Dk by itself cannot answer the question.

Thermal Conductivity Is Often Misinterpreted

Another datasheet number that deserves care is thermal conductivity.

Representative values include:

• RT/duroid 5880: ~0.20 W/m·K
• RO3003G2: ~0.43 W/m·K
• RO4350B: ~0.69 W/m·K
• RO4003C: ~0.71 W/m·K
• RO3010: ~0.95 W/m·K

These values are meaningfully different.

But PCB thermal performance cannot be predicted by laminate conductivity alone.

A board-level thermal path normally includes:

Rθ,total = Rθ,package + Rθ,pad + Rθ,via + Rθ,copper + Rθ,dielectric + Rθ,interface + Rθ,ambient

In many practical power layouts, copper planes and thermal vias dominate lateral and vertical heat spreading.

A low-loss RT5880 RF layer can therefore coexist with a separate FR-4 or high-thermal-performance power section in a hybrid stackup.

The correct question is not:

Which Rogers laminate has the highest thermal conductivity?

It is:

Where is the heat generated, and what complete thermal resistance path carries it out of the system?

Moisture Matters More as Wavelength Becomes Smaller

Water has very different dielectric behavior from low-loss PCB laminates.

Even relatively small changes in absorbed moisture can therefore alter RF behavior.

Published Rogers moisture absorption values are generally low, but they are not identical.

For example:

• RT/duroid 5880: ~0.02%
• RO4003C: ~0.04%
• RO4350B: ~0.05%
• RO3003G2: ~0.06%

For indoor equipment with stable environmental control, the difference may have little practical consequence.

For radar, telecom outdoor infrastructure, aerospace electronics, or sensors operating over humidity and temperature cycling, environmental material stability becomes more significant.

At millimeter-wave frequencies, small electrical changes represent a larger fraction of wavelength and phase.

Material selection should therefore include the actual environmental envelope rather than only room-temperature RF data.

RO4000 and RO3000 Also Require Different Fabrication Strategies

An RF schematic does not show how the laminate will behave in a PCB factory.

This distinction is particularly important when comparing RO4000 with RO3000.

RO4000

RO4003C and RO4350B are rigid thermoset materials.

Rogers specifically designed the family so it can be fabricated using equipment and processes similar to standard epoxy/glass PCBs. Specialized PTFE hole preparation such as sodium etching is not required.

Their Tg is greater than 280°C, and their relatively low Z-axis CTE makes them suitable for multilayer structures and plated-through-hole construction.

This manufacturing compatibility is one reason RO4000 materials are widely attractive for commercial RF products where cost and volume matter.

RO3000

RO3000 is PTFE based.

After drilling, hole-wall preparation becomes more important because untreated PTFE is chemically inert and provides poor adhesion for conventional metallization.

Rogers provides specific plasma and chemical-treatment procedures for these materials.

For example, its RO3000 fabrication guidance calls for baking panels before plasma treatment and gives a range of 110–125°C for at least one hour prior to the plasma process. It also recommends a copper flash plate of approximately 0.0001–0.0003 in., or about 2.5–7.6 µm, during metallization preparation.

Those are manufacturing details that never appear in a microwave simulation.

Yet they influence:

• via reliability;
• hole-wall adhesion;
• process yield;
• cost;
• supplier capability;
• minimum practical via size.

Material selection therefore becomes partly a supplier-process decision.

Hybrid Rogers/FR-4 Stackups Need More Than a Layer Diagram

A common cost-control strategy is to use Rogers only where RF performance requires it.

For example:

• top RF layer: Rogers laminate;
• internal digital layers: conventional FR-4;
• power distribution: FR-4;
• bottom control layer: FR-4.

This can dramatically reduce material cost compared with building the entire PCB from high-frequency laminate.

But hybrid construction introduces additional variables:

• different CTE values;
• different resin flow behavior;
• different lamination temperatures;
• different dimensional movement;
• different Dk values;
• bondply thickness after lamination;
• different copper roughness;
• different drill behavior.

The bonding layer itself also has electrical properties.

For example, Rogers publishes approximately:

• RO4450F Dk = 3.52 ±0.05
• RO4450F Df = 0.004 at 10 GHz

for its RO4000-family bonding material.

That means a transmission line interacting with a bondply cannot simply be modeled as though the entire dielectric region were the RO4003C or RO4350B core.

The actual electromagnetic field distribution determines which materials contribute to effective Dk and loss.

Glass Reinforcement Can Matter Even When Average Dk Is Correct

RO4000 materials use woven glass reinforcement.

That improves dimensional stability and mechanical processability, but woven glass creates local microscopic dielectric inhomogeneity.

The electromagnetic field may locally encounter more glass in one region and more resin in another.

For ordinary RF dimensions this may average out.

For very fine high-speed differential pairs or extremely short wavelengths, local field interaction with the glass weave can become relevant.

The PCB engineer therefore needs to distinguish between:

• bulk average material Dk;
• local dielectric distribution;
• transmission-line effective Dk;
• circuit-extracted Design Dk.

This is one reason simply entering a datasheet number into a field solver does not guarantee exact correlation with the finished PCB.

Impedance Control Is a Manufacturing Problem as Well as a Simulation Problem

Even if the correct Design Dk is chosen, characteristic impedance still depends on manufactured geometry.

For a microstrip or stripline, relevant variables include:

• finished trace width;
• etched sidewall angle;
• copper thickness;
• plating thickness;
• dielectric thickness;
• Dk;
• solder mask;
• copper roughness.

At millimeter-wave frequencies, conductor shape becomes particularly important.

Rogers notes that trapezoidal conductor geometry caused by etching can influence circuit behavior significantly around 77 GHz, especially in tightly coupled structures such as grounded coplanar waveguide.

Therefore an RF drawing that says:

50 Ω ±10%

is incomplete unless the fabricator also understands which stackup, material batch, finished copper thickness, and etching geometry the impedance model assumes.

For demanding designs, the impedance model should be based on finished dimensions, not nominal artwork dimensions.

A 77 GHz Radar Stackup Shows How the Variables Interact

Consider a 77 GHz automotive radar PCB built around a 5 mil RO3003G2 RF layer.

A design review should not stop at:

Dk = 3.0.

It should establish at least:

Dielectric

• material: RO3003G2;
• nominal thickness: 5 mil;
• Process Dk: 3.00 ±0.04;
• relevant Design Dk near 77 GHz: approximately 3.07;
• Df: approximately 0.0011.

Copper

• foil type;
• copper profile;
• initial copper weight;
• final plated copper thickness;
• expected etched conductor geometry.

Transmission line

• microstrip, stripline, or GCPW;
• finished conductor width;
• gap;
• ground-via arrangement;
• launch geometry;
• field-solver assumptions.

Surface finish

• bare/OSP, immersion silver, immersion tin, ENIG, or selective finish;
• metal thickness;
• whether nickel exists along the RF current path.

Manufacturing

• etch compensation;
• dielectric-thickness tolerance;
• artwork scaling;
• via drilling;
• PTFE hole-wall treatment;
• copper plating.

Verification

• impedance coupon;
• differential phase-length coupon where appropriate;
• RF insertion-loss coupon;
• cross-section;
• finished dielectric and copper measurements.

This is the level at which material selection becomes meaningful.

Material Selection Should Start With the Failure Mode

A practical way to choose Rogers material is to begin with what the design cannot afford to lose.

If insertion loss dominates

Prioritize:

• low Df;
• smooth copper;
• appropriate dielectric thickness;
• low-loss surface finish;
• transmission-line topology.

RO3003G2 and RT/duroid 5880 become attractive candidates depending on frequency, mechanical requirements, and fabrication strategy.

If phase stability dominates

Prioritize:

• Design Dk consistency;
• TCDk;
• moisture stability;
• copper roughness consistency;
• tight dielectric-thickness control.

This is common in radar and phased-array systems.

If PCB manufacturing cost dominates

RO4003C or RO4350B can be attractive because they provide RF performance while remaining much closer to conventional FR-4 fabrication processes.

RO4350B also provides a UL 94 V-0 rating, while RO4003C itself is not rated V-0, which can be decisive in commercial product qualification.

If miniaturization dominates

A high-Dk material such as RO3010 can reduce distributed-circuit dimensions substantially.

But the resulting design must tolerate its different temperature behavior and field concentration.

If plated-hole reliability dominates

CTE becomes a major selection variable.

A low Z-axis CTE can be more important than achieving the absolute minimum possible Df.

The Material Datasheet Is the Beginning of the Model

For professional RF engineering, Rogers material selection should follow a sequence closer to this:

1. Define the operating frequency and bandwidth
10 GHz and 77 GHz cannot be treated as the same material problem.

2. Define the required transmission-line architecture
Microstrip, stripline and GCPW interact differently with dielectric, copper and surface finish.

3. Use the appropriate Design Dk
Do not automatically substitute Process Dk into the EM model.

4. Include the real copper
Model copper thickness, conductor roughness, plating, and etched geometry.

5. Evaluate temperature and environment
Review TCDk, water absorption, operating temperature, and thermal cycling.

6. Review mechanical manufacturability
Include X/Y/Z CTE, PCB thickness, via aspect ratio, multilayer construction, and material processing.

7. Build verification structures into the panel
Use coupons appropriate to the electrical requirement rather than relying entirely on incoming laminate certification.

The Lowest-Loss Rogers Material Is Not Automatically the Best Material

The instinct to rank RF laminates by Df is understandable.

But PCB engineering rarely rewards one-variable optimization.

RT/duroid 5880 offers extremely low dielectric loss, but RO4000 materials are substantially easier to integrate into conventional multilayer manufacturing.

RO3010 can make RF structures dramatically smaller, but its temperature coefficient of Dk is very different from RO3003G2.

RO4350B may have higher Df than RO3003G2, but its thermoset construction and FR-4-compatible processing can make it a more practical solution for many commercial multilayer RF products.

RO3003G2 is highly attractive for 77 GHz radar, but obtaining its intended performance still depends on copper profile, conductor geometry, surface finish, thickness control, and PCB processing.

That is the central engineering lesson:

A Rogers part number does not define the electrical performance of a finished RF PCB. The finished stackup does.

Material Dk is one input.

The actual circuit sees a combined system consisting of dielectric, copper, geometry, plating, temperature, moisture, and manufacturing variation.

At microwave frequencies, understanding that system improves correlation between simulation and measurement.

At millimeter-wave frequencies, it becomes essential.