The Problem: Material Choice Makes or Breaks RF Designs
Standard FR-4 works fine for digital circuits, but RF and microwave designs demand more. Above 1 GHz, FR-4’s high loss tangent (Df ~0.02) attenuates signals noticeably. By 10 GHz, you’re losing over 1 dB per inch. Worse, FR-4’s dielectric constant varies unpredictably with frequency and temperature, making impedance control unreliable.
For engineers designing at microwave frequencies, this creates a genuine problem: how do you choose from dozens of specialty materials with overlapping specifications and wildly different costs?
The Solution: Match Material Category to Your Frequency and Application
RF substrate materials fall into distinct categories, each suited to specific frequency ranges and applications. Understanding these categories simplifies selection considerably.
Key benefits of proper material selection:
- Predictable impedance control across production volumes
- Acceptable insertion loss at your operating frequency
- Thermal stability for outdoor and power applications
- Manufacturing processes your fabricator can actually handle
- Cost optimisation through hybrid stackup strategies
The right material balances electrical performance against manufacturing complexity. The “best” material electrically often creates fabrication challenges that increase cost and risk.
Material Categories at a Glance
| Category | Dk Range | Df @ 10 GHz | Processing | Cost vs FR-4 | Best For |
|---|---|---|---|---|---|
| Standard FR-4 | 4.3-4.7 | 0.018-0.020 | Standard | 1× | Below 1 GHz |
| Hydrocarbon ceramic (RO4000) | 3.38-3.66 | 0.0027-0.0037 | FR-4 compatible | 3-5× | 500 MHz - 15 GHz |
| Ceramic-filled PTFE (RO3000) | 3.0-10.2 | 0.0010-0.0023 | Specialised | 5-7× | 15-77 GHz, radar |
| Pure/glass PTFE (RT/duroid) | 2.1-2.65 | 0.0009-0.0018 | Most difficult | 8-10× | Satellite, lowest loss |
| Advanced thermoset (Astra MT77) | 3.0 | 0.0017 | FR-4 compatible | 3-5× | mmWave with easier processing |
Frequency Determines When Material Choice Becomes Critical
FR-4 has a practical ceiling of 1-2 GHz for RF work. Beyond this, Dk instability and dielectric loss become limiting factors.
| Frequency Range | Viable Materials | Key Constraint |
|---|---|---|
| DC - 1 GHz | Standard FR-4 | Cost dominates |
| 1 - 5 GHz | Enhanced FR-4, RO4000 series | Dk stability |
| 5 - 20 GHz | RO4350B, RO3003, Astra MT77 | Dielectric loss |
| 20 - 40 GHz | RO3003, RT/duroid 5880 | Dk tolerance, Df |
| 40 - 77 GHz | RO3003, RT/duroid 5880, Astra MT77 | All properties critical |
| Above 77 GHz | PTFE, advanced thermosets | Specialised design |
The crossover from conductor-dominated to dielectric-dominated loss occurs between 1-10 GHz for most materials. Above this range, material Df directly limits system performance.
PTFE-Based Materials: Lowest Loss, Highest Complexity
Pure and glass-reinforced PTFE materials offer Dk values of 2.1-2.65 and Df as low as 0.0009 - the lowest dielectric loss available in commercial laminates. Rogers RT/duroid 5880, with Df of 0.0009 at 10 GHz, represents the gold standard for phase-sensitive applications like phased arrays and satellite communications.
However, PTFE’s soft, non-stick properties create significant fabrication challenges. Not all PCB fabricators can process these materials.
PTFE Processing Requirements
| Process Step | Requirement |
|---|---|
| Drilling | 180,000-250,000 RPM, slow feed (20-60 µm/rev), fresh carbide bits |
| Surface prep | Plasma treatment or sodium naphthalene etch |
| Plating | Thorough bakeout (115-125°C, 15-30 min) before copper |
| Solder mask | Apply within 12 hours of etching |
| Routing | Diamond-coated or carbide tools |
Drill bits should not be resharpened - ceramic fillers damage cutting edges after 2,000-3,000 hits.
PTFE and Ceramic-Filled Materials
| Material | Dk @ 10 GHz | Df @ 10 GHz | Construction | CTE ppm/°C (X;Y;Z) | Best For |
|---|---|---|---|---|---|
| RT/duroid 5870 | 2.33 | 0.0012 | Glass PTFE | 22;28;173 | Low-loss, phase stable |
| RT/duroid 5880 | 2.20 | 0.0009 | Glass PTFE | 31;48;237 | Lowest Df available |
| RT/duroid 6002 | 2.94 | 0.0012 | Ceramic PTFE | 16;16;24 | High reliability, aerospace |
| RT/duroid 6006 | 6.15 | 0.0019 | Ceramic PTFE | 17;17;24 | Circuit miniaturisation |
| RT/duroid 6010LM | 10.2 | 0.0023 | Ceramic PTFE | 24;24;24 | Maximum miniaturisation |
| RO3003 | 3.00 | 0.0010 | Ceramic PTFE | 17;17;24 | 77 GHz radar standard |
| RO3006 | 6.15 | 0.0020 | Ceramic PTFE | 17;17;24 | High-Dk applications |
| RO3010 | 10.2 | 0.0022 | Ceramic PTFE | 13;11;16 | Tight CTE match |
Ceramic-Filled PTFE: The 77 GHz Standard
Adding ceramic fillers to PTFE creates materials with engineered Dk values (2.94-10.2), improved mechanical stability, and reduced CTE. Rogers RO3003 (Dk = 3.0, Df = 0.0013) has become the dominant choice for 77 GHz automotive radar because it eliminates glass weave effects while matching copper’s thermal expansion closely.
These materials still require specialised processing but offer tighter Dk tolerances (±0.04) than pure PTFE.
Why RO3003 Dominates Automotive Radar
Six properties matter critically at 77 GHz:
- Dk tolerance ±0.04 for consistent antenna patterns
- Very low Df (0.0013) for acceptable link budgets
- Smooth copper surfaces (VLP or HVLP foils) to reduce conductor loss
- Stable TCDk (-3 ppm/°C) for temperature swings
- Low moisture absorption (<0.04%) for outdoor reliability
- No glass weave effects that cause signal variation
Materials must meet AEC-Q200 passive component qualification, with extended temperature range testing (-40°C to +125°C), 1500+ thermal cycles, and 85°C/85% RH humidity resistance.
Hydrocarbon Ceramics: FR-4 Processing with RF Performance
The Rogers RO4000 series revolutionised RF PCB manufacturing by delivering good high-frequency performance (Dk 3.38-3.66, Df 0.0027-0.0037) with FR-4-compatible processing. No plasma treatment, no sodium etch, no specialised equipment - just standard PCB fabrication.
This makes RO4003C and RO4350B the workhorses for commercial applications from 500 MHz to 15 GHz, including 5G sub-6 GHz infrastructure and power amplifiers.
Rogers RO4000 Series Comparison
| Material | Dk (Process) | Dk (Design) | Df @ 10 GHz | CTE (X;Y;Z) | Key Feature |
|---|---|---|---|---|---|
| RO4003C | 3.38 | 3.55 | 0.0027 | 11;14;46 | Lowest loss in series |
| RO4350B | 3.48 | 3.66 | 0.0037 | 14;16;32 | UL 94 V-0 rated |
| RO4835 | 3.48 | 3.66 | 0.0031 | 14;16;32 | High oxidation resistance |
| RO4360G2 | 6.15 | 6.40 | 0.0038 | 10;12;30 | High-Dk, UL rated |
Note on Dk values: Rogers provides both “process” Dk (for manufacturing) and “design” Dk (for simulation). Use the design value for impedance calculations.
Processing Advantage
The major advantage of hydrocarbon ceramic materials is FR-4-compatible processing:
- Standard drilling parameters work (300-500 SFM surface speed, 0.002-0.004”/rev chip load)
- No plasma treatment or special desmear required
- Compatible with standard develop-etch-strip systems
- Lower fabricator qualification barriers
Advanced Thermoset Materials: The Best of Both Worlds
Isola’s Astra MT77 exemplifies a new generation of ultra-low-loss thermoset materials achieving Df of 0.0017 with FR-4-compatible processing - approaching PTFE electrical performance without PTFE fabrication headaches.
| Material | Manufacturer | Dk | Df @ 10 GHz | Processing | Target Application |
|---|---|---|---|---|---|
| Astra MT77 | Isola | 3.0 | 0.0017 | FR-4 compatible | 77 GHz radar, mmWave 5G |
| mmWave77 | Shengyi | 3.06 | 0.001 | FR-4 compatible | 77 GHz radar (cost-optimised) |
| TerraGreen 400G | Isola | 3.05-3.15 | 0.0018 | FR-4 compatible | Halogen-free mmWave |
These materials support frequencies to 110 GHz and integrate well with high-speed digital materials for hybrid RF/digital designs.
Critical Electrical Properties Explained
Dk Tolerance Determines Impedance Repeatability
The Dk tolerance - not just the Dk value - determines whether designs will meet impedance specifications across production volumes. A material with Dk = 3.50 ± 0.05 enables predictable 50Ω traces. A material with Dk = 4.3 ± 0.5 (like standard FR-4) creates impedance swings that cannot be compensated through trace width adjustments alone.
Practical guideline: For frequencies above 10 GHz, specify materials with Dk tolerance ≤±0.05. For mmWave beam-forming applications, tighten this to ±0.02-0.04.
| Material | Dk @ 10 GHz | Tolerance | Impedance Impact |
|---|---|---|---|
| RT/duroid 5880 | 2.20 | ±0.02 | Best available |
| RO3003 | 3.00 | ±0.04 | Excellent control |
| RO4350B | 3.48 | ±0.05 | ±2-3Ω on 50Ω |
| Standard FR-4 | 4.3-4.7 | ±0.2-0.5 | Unusable for RF |
Dissipation Factor Thresholds by Application
Different applications have dramatically different Df requirements:
| Application | Frequency | Maximum Df | Typical Material |
|---|---|---|---|
| Consumer WiFi/BLE | 2.4-5 GHz | 0.02 | FR-4 acceptable |
| 5G sub-6 GHz | 2.4-6 GHz | 0.004 | RO4350B, RO4003C |
| 5G mmWave | 24-39 GHz | 0.002-0.0035 | RO3003, Astra MT77 |
| 77 GHz automotive radar | 76-81 GHz | 0.0015 | RO3003 (Df = 0.0013) |
| Satellite Ka-band | 26.5-40 GHz | 0.001 | RT/duroid 5880 |
Temperature Stability (TCDk)
The temperature coefficient of Dk (TCDk) determines how much impedance drifts with temperature swings. RO3003’s exceptional TCDk of -3 ppm/°C means Dk barely changes from -40°C to +150°C. Standard FR-4 at +200 to +400 ppm/°C causes substantial drift.
Target TCDk < 50 ppm/°C for outdoor base stations, automotive radar, and power amplifiers.
Hybrid Stackup Strategies
Combining RF materials with FR-4 in hybrid stackups can reduce material costs by 40-70% while maintaining RF performance where it matters. The key is placing high-frequency materials only on layers carrying RF signals.
Example: Symmetric RF Outer Layers
Layer 1: RO4003C (0.020") — RF signals
Layer 2: Ground plane
Layer 3-4: FR-4 core — digital/power
Layer 5: Ground plane
Layer 6: RO4003C (0.020") — RF signalsCritical: Keep stackups symmetric to prevent warping. Asymmetric constructions cause boards to bow during lamination and thermal cycling.
Bondply Selection
Rogers RO4450F bondply provides reliable bonding between RO4000 laminates and FR-4 cores:
- Bond temperature: 177°C (FR-4 compatible)
- Lamination pressure: 200-750 PSI
- High Tg (>280°C) allows multiple lamination cycles
For dissimilar materials, target Dk difference ≤0.3 between bonded layers and use modified low-Dk FR-4 (such as Megtron 6 or I-Speed) for digital layers.
Bonding Films and Prepregs Reference
| Type | Supplier | Thickness (mils) | Dk @ 10 GHz | Df @ 10 GHz | Lamination Temp | Use |
|---|---|---|---|---|---|---|
| PTFE adhesive | DuPont | 1.2 | 2.08 | 0.0005 | 327°C | PTFE bonding |
| FEP | DuPont | 0.5-2.5 | 2.06 | 0.001 | 260-280°C | PTFE bonding |
| R700 | Arlon | 1.5-3 | 2.35 | 0.0025 | 100°C | Low-temp bond |
| 6250 | Arlon | 1.5 | 2.32 | 0.0013 | 100°C | Low-temp bond |
| RO4450F | Rogers | 3-4 | 3.23 | 0.004 | 200-400°C | RO4000/FR-4 hybrid |
| 6002 Bond-Ply | Rogers | - | 2.94 | 0.0012 | 1700 psi | RO6000 bonding |
| Speedboard C | Gore | 1.5-4.5 | 2.7 | 0.004 | 100-350°C | High-reliability |
| 25N/25FR | Arlon | 6.5 | 3.25 | 0.0024 | 110°C | Halogen-free option |
CTE Management and Via Reliability
Z-axis CTE mismatch between the laminate and copper causes plated through-hole barrel cracking during thermal cycling. Standard FR-4 with Z-CTE of 55-70 ppm/°C stresses copper barrels. RO4350B at 32 ppm/°C significantly improves reliability.
Target Z-axis CTE < 50 ppm/°C for high-reliability applications. For boards experiencing many thermal cycles (automotive, aerospace), consider increased copper plating thickness (>1.5 oz).
RT/duroid 6002 demonstrates exceptional reliability with more than 5,000 thermal cycles without via failure - its Z-axis CTE of 24 ppm/°C closely matches copper.
Application Selection Guide
5G Infrastructure
Sub-6 GHz massive MIMO (2.4-6 GHz): RO4835 and RO4350B dominate commercial deployments. Key requirements include low passive intermodulation (PIM) for antenna arrays and thermal dissipation for power amplifiers.
mmWave 5G (28 GHz, 39 GHz): Tighter Dk tolerance (±0.02) becomes essential. RO3003 and RT/duroid 5880 provide the Df < 0.002 needed for acceptable link budgets.
Automotive Radar (77 GHz)
The market has standardised on RO3003 and equivalent materials. Alternatives include:
- RO3003G2 - Optimised for 77-81 GHz with smaller filler particles
- Isola Astra MT77 - FR-4 process compatible
- Shengyi mmWave77 - Cost-effective alternative
Aerospace and Defence
Beyond electrical performance, applications demand MIL-PRF-31032 compliance, extended service life (15+ years), temperature cycling from -55°C to +125°C for 5000+ cycles, and full material traceability.
Satellite Communications
Space-qualified materials must pass NASA ASTM E595 outgassing testing: Total Mass Loss < 1.0% and Collected Volatile Condensable Materials < 0.1%. PTFE and polyimide materials generally excel.
Common Mistakes to Avoid
Design Errors
Asymmetric stackup: Causes warping during lamination. Always mirror RF layers symmetrically.
Wrong Dk in calculations: Use “process Dk” or “design Dk” from manufacturer datasheets, not “measured Dk.” These values can differ by 5-10%.
Via stubs above 5 GHz: Quarter-wave resonances degrade signals. Specify back-drilling for via stubs in any RF path above 5 GHz.
Fabrication Errors
Mechanical scrubbing of PTFE: Damages surface uniformity. Use only chemical or plasma treatment.
Reusing drill bits: Ceramic fillers damage carbide edges. Use fresh drills; expect only 2,000-3,000 hits before replacement.
Skipping bakeout: Moisture or absorbed chemicals cause PTH failures. Bake panels at 115-125°C for 15-30 minutes before copper plating.
Specification Errors
Over-specifying tolerance: Requesting ±5% impedance when ±10% suffices increases cost and scrap.
Under-specifying material: Calling out “Rogers” without a specific part number leaves material selection to the fabricator.
Information Fabricators Need
When specifying RF materials, provide:
- Exact material part numbers (e.g., “RO4003C 0.020” LoPro”)
- Complete layer-by-layer stackup with materials per layer
- Controlled impedance values and tolerance (±10% standard, ±5% for >10 GHz)
- Operating frequency range
- Reference plane assignments for each controlled impedance trace
- Special requirements (back-drilling, plasma treatment, surface finish)
Frequently Asked Questions
When does FR-4 stop working for RF?
Standard FR-4 has a practical ceiling around 1-2 GHz. Above this, its high Df (~0.02) causes unacceptable insertion loss, and its Dk instability (±10% variation) makes impedance control unreliable. For applications above 2 GHz, specify purpose-built RF materials.
What’s the difference between RO4003C and RO4350B?
RO4003C has lower loss (Df 0.0027 vs 0.0037) and is preferred when UL flammability rating isn’t required. RO4350B carries UL 94 V-0 certification, making it necessary for consumer products and applications with flammability requirements. Both process like FR-4.
Can any fabricator process PTFE materials?
No. PTFE processing requires specialised drilling equipment (high-RPM spindles), plasma or sodium naphthalene surface treatment, and modified plating processes. Verify your fabricator’s PTFE capabilities before specifying RT/duroid or RO3000 series materials.
How do I choose between RO3003 and Astra MT77 for 77 GHz?
RO3003 remains the industry standard with proven automotive radar heritage. Astra MT77 offers similar electrical performance (Df 0.0017 vs 0.0013) with FR-4-compatible processing - potentially simplifying fabrication and reducing cost. If your fabricator can process RO3003 reliably, it’s the safe choice. If PTFE processing is a concern, Astra MT77 is worth evaluating.
What Dk value should I use for impedance calculations?
Use the “design Dk” value from manufacturer datasheets, measured at a frequency close to your operating frequency. For Rogers materials, design Dk is optimised for impedance calculations, while process Dk is for manufacturing. The difference can be 5-10%.
Are Asian material suppliers competitive with Rogers and Isola?
Yes. Shengyi (mmWave77), ITEQ, and Doosan now offer materials genuinely competitive for high-frequency applications. About 70% of high-speed CCL production is now based in Asia. For cost-sensitive high-volume applications, these alternatives deserve evaluation.
When to Consider Each Material Type
RO4003C/RO4350B:
- Frequency range 500 MHz - 15 GHz
- Standard PCB fabrication available
- Cost-performance balance required
- Mixed RF/digital designs
RO3003/RO3003G2:
- Frequency above 20 GHz
- 77 GHz automotive radar
- Tight Dk tolerance critical
- Fabricator has PTFE capability
RT/duroid 5880:
- Lowest possible loss required
- Satellite and space applications
- Phase-sensitive antenna arrays
- Cost secondary to performance
Astra MT77/mmWave77:
- mmWave frequencies with FR-4 processing
- PTFE capability limited
- Cost optimisation for high-volume
- Hybrid RF/digital integration
Related Articles
- PCB Materials and Laminates Guide - Complete laminate specifications
- Characteristic Impedance in PCBs - How Dk affects impedance
- PCB Capability Limits - Manufacturing specifications
Need help selecting RF materials? Request a quote - we’ll review your frequency requirements and recommend the right substrate for your application.