F4B Metal Base PTFE Laminate: Aluminum & Copper-Backed PCB for High-Power RF Thermal Management

If you’ve spent any time designing power amplifiers, base station antennas, or phased-array radar modules, you already know the painful compromise at the center of every RF board: the better the high-frequency performance of your dielectric, the worse its ability to shed heat. PTFE does wonders for signal integrity up into the millimeter-wave range, but on its own it’s a poor thermal conductor. Bolt a precision-machined aluminum or copper heat spreader onto the back of that PTFE laminate, however, and you get a substrate that genuinely solves both problems at once. That hybrid structure is what the F4B metal base PTFE laminate family — designated F4B-1/AL and F4B-1/CU in manufacturer nomenclature — is all about.

This article breaks down the material science, key electrical and thermal specs, design rules, manufacturing considerations, and real-world application scenarios. It’s written from the bench, not a marketing deck.

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What Is F4B Metal Base PTFE Laminate, and Why Does It Exist?

The Core Concept

F4B metal base PTFE laminate is a sandwich structure. Starting from the top: a copper circuit foil (0.5 oz, 1 oz, or 2 oz), a PTFE-woven glass fibre dielectric layer, and then a rigid metal backplane — either an aluminium alloy plate or a copper plate. The metal backplane serves two simultaneous roles: it acts as a ground plane or electromagnetic shield, and it is the primary heat extraction path for components mounted on the circuit side.

The standard model designations follow this logic:

• F4BM***-AL — F4BM dielectric with aluminium backing
• F4BME***-AL — F4BME dielectric (RTF copper foil, better PIM) with aluminium backing
• F4BM***-CU — F4BM dielectric with copper backing
• F4BME***-CU — F4BME dielectric with copper backing

The three digits in the middle encode the nominal dielectric constant multiplied by 100. So F4BM220-AL is an F4BM laminate with Dk = 2.20 on an aluminium base.

Why Not Just Use a Standard Metal-Core PCB?

Standard aluminium-core PCBs (IMS boards) typically use an epoxy-based thermally conductive dielectric interlayer. That works great for LED drivers and power supplies — but the dissipation factor of those epoxy dielectrics is catastrophically high above 1 GHz. You cannot run a 5G power amplifier or a radar transmit/receive module on an IMS board with an epoxy dielectric.

F4B metal base PTFE laminate solves this by replacing the epoxy interlayer with a PTFE-woven glass fibre composite. PTFE’s dissipation factor (Df) is 0.001–0.003 @ 10 GHz, compared to 0.015–0.025 for standard epoxy thermally conductive dielectrics. That is an order-of-magnitude improvement, and at 10 GHz or 24 GHz, it makes a real, measurable difference in insertion loss.

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F4B PTFE Family Overview: Variants and Their Differences

Before diving into the metal-base-specific configurations, it helps to understand the broader F4B laminate family produced by Chinese manufacturers (principally Wangling, which supplies many board houses globally). The sub-variants have distinct electrical and processing characteristics.

Variant	Copper Foil Type	Dk Range	Key Advantage	Typical Use
F4BM	ED (Electrodeposited)	2.17 – 3.0	Low dielectric loss, broad Dk range	RF/microwave general purpose
F4BME	RTF (Reverse-treated)	2.17 – 3.0	Excellent PIM performance	Base station antennas, repeaters
F4BMX	Imported glass fabric + ED	2.17 – 3.0	Tighter Dk consistency	Phased array, aerospace
F4BT	Nano-ceramic filled PTFE	Higher Dk	Higher thermal conductivity, miniaturisation	mmWave, compact filters
F4BTME	Nano-ceramic + RTF foil	Higher Dk	PIM + thermal	Active antenna arrays
F4B-1/AL(CU)	ED or RTF	2.17 – 3.0	Metal-backed for heat spreading	High-power RF, PA modules

The -AL and -CU suffix variants share the same dielectric core as their non-metal-backed counterparts. The difference is purely on the back side: instead of copper foil, you get a solid metal plate pressed and bonded during the lamination process.

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Key Electrical Specifications of F4B Metal Base PTFE Laminate

The electrical performance of the PTFE dielectric layer in a metal-base laminate is essentially identical to the non-metal-backed version. The metal backplane doesn’t touch the signal layer; it sits below the dielectric, acting as a reference plane or heat sink.

Dielectric Constant (Dk) and Dissipation Factor (Df)

Parameter	Typical Value	Test Condition
Dielectric Constant (Dk)	2.17 / 2.20 / 2.45 / 2.55 / 2.65 / 2.94 / 3.0	10 GHz, IPC-TM-650 2.5.5.5
Dissipation Factor (Df)	0.001 – 0.003	10 GHz
Dk Tolerance	±0.03 to ±0.05	Depending on model
Thermal Coefficient of Dk (TCDk)	~-45 to -100 ppm/°C	Typical for PTFE/glass composite
Volume Resistivity	≥ 10⁷ MΩ·cm	IPC-TM-650 2.5.17
Surface Resistivity	≥ 10⁶ MΩ	IPC-TM-650 2.5.17
Dielectric Breakdown Voltage	≥ 30 kV/mm	IPC-TM-650 2.5.6

The low Dk options (2.17–2.20) are common in antenna feed networks where you want a wide characteristic impedance range without excessively narrow trace widths. The higher Dk options (2.65–3.0) help shrink patch antenna footprints or resonator dimensions.

Copper Foil Options

Copper Foil	Thickness Options	Notes
ED Copper (F4BM)	0.5 oz / 1 oz / 1.5 oz / 2 oz	Standard RF traces, good adhesion
RTF Copper (F4BME)	0.5 oz / 1 oz	Low conductor roughness, lower PIM, better signal integrity at mmWave

For designs where passive intermodulation (PIM) is a specification — 3GPP base station antennas, for example — always specify the RTF foil. The smoother copper surface reduces micro-arcing under high-power conditions, which is the dominant source of passive intermodulation distortion in passive RF components.

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Thermal Performance: Where the Metal Base Earns Its Keep

This is the reason the laminate exists. Let’s look at the numbers honestly.

Thermal Conductivity Comparison

Material / Layer	Thermal Conductivity (W/m·K)
Standard PTFE laminate alone	0.25 – 0.35
Epoxy thermally conductive IMS dielectric	1.0 – 3.0
Aluminium 6061-T6 backplane	155 – 170
Copper backplane (OFC)	385 – 400
F4B PTFE dielectric interlayer	0.26 – 0.30

The PTFE dielectric itself remains a bottleneck in absolute thermal terms — PTFE never conducts heat well. What the metal base adds is a low thermal resistance heat-spreading plate that accepts heat conducted through the thin dielectric and rapidly distributes it laterally to the board edges, a heat sink, or an enclosure wall. This is analogous to the thermal interface material and heat slug arrangement inside an RF power transistor package.

Designers typically keep the PTFE dielectric layer thin (0.1 mm to 0.5 mm common in metal-base variants) to minimise the thermal resistance of that layer, while the aluminium or copper plate is 0.5 mm to 3.0 mm thick for mechanical rigidity and lateral heat spreading.

Thermal Performance Targets in Practice

Configuration	Typical θ (dielectric)	Notes
0.1 mm PTFE on 1.5 mm Al base	~0.3 °C·cm²/W	Excellent for power transistor mounting
0.254 mm PTFE on 1.5 mm Al base	~0.8 °C·cm²/W	Balanced RF + thermal design
0.508 mm PTFE on 2.0 mm Al base	~1.5 °C·cm²/W	Practical for most PA modules
0.1 mm PTFE on 2.0 mm Cu base	~0.15 °C·cm²/W	Best thermal, premium cost

Operating temperature range for the laminate assembly is -50°C to +260°C, which comfortably covers the reflow soldering window (peak 250°C for lead-free) and extended operational temperature ranges required by telecom and defence specifications.

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Aluminium-Backed vs. Copper-Backed: Which Should You Choose?

This is one of the most common questions RF hardware engineers face when specifying a metal-base PTFE laminate. The short answer: aluminium for most RF power applications; copper when thermal budget is extremely tight or when EMI shielding needs a conductive path through the board.

Side-by-Side Comparison

Property	Aluminium Base	Copper Base
Thermal Conductivity	~155–170 W/m·K	~385–400 W/m·K
Density	~2.7 g/cm³	~8.9 g/cm³
Weight Impact	Low	High (~3.3× heavier)
CTE (X/Y)	~23 ppm/°C	~17 ppm/°C
CTE Match to PTFE	Moderate mismatch	Better match
Machinability	Excellent	Good (harder to machine)
Cost	Lower	Significantly higher
Direct Thermal Attachment	Good (bolt to heatsink)	Excellent
Electrical Conductivity Through Base	Good (Al ground plane)	Excellent
Corrosion Resistance	Good (anodising)	Requires plating

For most 5G macro base station PA modules, 77 GHz automotive radar transmitters, and solid-state radar T/R modules, aluminium-backed is the default. It is lighter, cheaper, machines cleanly for connector mounting holes, and anodises well for environmental protection. Copper-backed configurations appear most often in space-grade or military radar applications where every degree of junction temperature matters, or in compact modules where the copper base is also the RF ground chassis.

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Physical and Mechanical Specifications

Parameter	Specification
Available Panel Sizes	300×250 mm, 350×380 mm, 500×500 mm, 840×840 mm, up to 1000×1500 mm
Minimum Dielectric Thickness (Dk ≤ 2.65)	0.1 mm
Minimum Dielectric Thickness (Dk 2.7–3.0)	0.2 mm
Metal Base Thickness Range	0.5 mm – 3.0 mm (Al or Cu)
Total Board Thickness	0.2 mm – 5.0 mm
Copper Foil Peel Strength	≥ 8 N/cm (1 oz Cu)
Moisture Absorption	≤ 0.02% (24 h, 20°C distilled water)
Thermal Resistance (solder dip test)	Passes 288°C / 10 s
Flammability Rating	UL 94 V-0
Dimensional Stability	Shrinkage < 0.0002% after 2 h in boiling water

The exceptionally low moisture absorption (0.02%) is a critical advantage for outdoor RF infrastructure. Base station antennas mounted on towers are exposed to high humidity cycles. A substrate that absorbs moisture will show Dk drift, which shifts the operating frequency of patch antennas and filters in a way that’s very difficult to predict and compensate for in simulation.

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Design and Fabrication Considerations for F4B Metal Base PCBs

Working with PTFE-based laminates, especially metal-backed ones, is genuinely different from FR-4 processing. If your board house doesn’t have experience with PTFE, you will encounter problems. Here’s what to watch for.

Etching and Trace Geometry

PTFE etches with conventional ferric chloride or cupric chloride processes, but the lower adhesion of copper to PTFE compared to FR-4 requires careful process control. Minimum trace widths of 0.1 mm are achievable, but 0.15 mm is a more conservative and reliable minimum for production. Impedance tolerances of ±5% are standard; ±2% requires extra process qualification.

Drilling and Machining the Metal Backplane

The aluminium or copper base cannot be drilled by standard PCB drill bits designed for glass-fibre. Carbide-tipped tooling is required. Through-hole vias in metal-base laminates need counter-bore clearance holes in the metal base to maintain electrical isolation of the plated via from the metal backing. This is a fabrication step that many general-purpose PCB factories are not set up for — always qualify your board house’s experience with this process before placing a production order.

Connector mounting holes in the metal base are often countersunk for flathead screws. For SMA or 2.92 mm connectors, the transition from the connector flange to the PTFE substrate requires careful co-planning of the via transition geometry and connector launch design, particularly above 10 GHz.

Surface Finish Selection

Surface Finish	Suitability for RF	Notes
ENIG (Electroless Ni / Immersion Au)	Excellent	Most common for RF; flat, solderable, controlled skin depth
Immersion Silver	Very Good	Good RF, lower cost than ENIG, tarnishes if not assembled promptly
HASL (Lead-Free)	Acceptable	Surface non-uniformity causes minor impedance variations; avoid above 10 GHz
Bare Copper (OSP)	Limited	Oxidation risk; only acceptable for protected indoor environments

For anything above 6 GHz, ENIG or immersion silver is strongly recommended. The thin gold layer on ENIG (typically 0.05–0.1 µm Au over 3–5 µm Ni) has minimal effect on skin depth at microwave frequencies, and the surface planarity ensures consistent trace geometry and solder joint quality on QFN and flip-chip packages.

Handling PTFE Thermal Expansion

PTFE has a significantly higher coefficient of thermal expansion than its glass-fibre reinforcement, and a substantial CTE mismatch with aluminium (23 ppm/°C) or copper (17 ppm/°C). In a metal-base construction, the rigid metal plate constrains the PTFE dielectric, which reduces in-plane expansion — but shear stresses at the bond interface need to be considered for designs that will experience wide thermal cycling (e.g., outdoor base stations cycling from -40°C to +85°C under power). Adhesive bond line integrity should be validated through thermal shock testing per IPC-TM-650 2.6.7 for mission-critical applications.

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Application Scenarios: Where F4B Metal Base PTFE Laminate Fits

5G Massive MIMO Power Amplifier Modules

This is arguably the highest-volume application today. A 64T64R massive MIMO radio unit contains dozens of GaN or LDMOS PA driver stages, each dissipating 5–20 W. Running those on standard PTFE would create a severe thermal problem. The F4B metal base PTFE laminate solution mounts the PA die or packaged transistor directly onto the circuit layer, with the aluminium base conducting heat to the radio unit chassis. The combination of sub-0.003 Df at 3.5 GHz or 28 GHz and effective thermal management through the aluminium back is difficult to achieve any other way at production cost.

Phased Array Radar T/R Modules

Active electronically scanned arrays (AESAs) pack many transmit/receive modules into a tight aperture. Each module includes a PA, a low-noise amplifier, phase shifters, and control circuitry. Thermal density in the aperture is extreme. Metal-base PTFE substrates allow the entire RF layer to be thermally bonded to a liquid-cooled or conduction-cooled chassis structure, making the RF substrate itself part of the thermal management architecture.

Base Station Antenna Feed Networks

For passive antenna elements where PIM performance is critical, F4BME-AL or F4BME-CU (with RTF foil) is specified. The RTF copper foil dramatically reduces passive intermodulation distortion, and the metal base provides mechanical rigidity and acts as an integrated ground/shielding layer for the combiner network.

Automotive 77 GHz Radar

Automotive radar sensors operating at 77 GHz demand extremely low-loss substrates and tight dimensional stability over a wide temperature range. The F4BT series (ceramic-filled PTFE) on an aluminium base combines higher Dk (2.94–3.0) for compact resonators with the thermal management needed behind the power stages of FMCW radar transceivers.

Military and Aerospace RF Front Ends

Defence-grade designs often require substrates that survive MIL-STD-810 temperature, humidity, vibration, and shock profiles. The PTFE matrix is inherently chemically inert and radiation-tolerant. Copper-backed configurations are common here because the copper base can be directly soldered or brazed to a copper chassis for hermetic-level thermal bonding.

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F4B Metal Base PTFE Laminate vs. Rogers Metal-Backed Laminates

A fair comparison, because Rogers Corporation (now part of DuPont) also offers metal-backed RF laminates such as RT/duroid 5880 and RO4000 series with aluminium or copper carriers.

Factor	F4B Metal Base (Wangling / Chinese)	Rogers Metal-Backed
Df @ 10 GHz	0.001 – 0.003	0.0009 – 0.003 (RT/duroid 5880)
Dk Range	2.17 – 3.0	2.2 – 11+ (broad portfolio)
Cost	Significantly lower	2–5× higher per panel
Panel Availability	Good, large-format panels available	Good, slightly limited sizes
Fab Ecosystem	Very strong in China, growing globally	Global ecosystem, widely qualified
Datasheet Traceability	Good (IPC / national standards)	Excellent (UL, IPC, aerospace quals)
Certification	Commercial / IPC-4103	IPC-4103, MIL, aerospace grades
PIM Performance (RTF)	Excellent (F4BME)	Excellent (low-PIM grades)

For commercial telecom and industrial RF applications, F4B metal base laminate is a compelling and cost-effective alternative. For programmes requiring UL or aerospace material qualification traceability, Rogers or Taconic materials may carry lower qualification risk. In practice, many EMS factories now run both material families and can offer direct comparison builds.

If you’re evaluating manufacturers, Wangling PCB has built a solid reputation in the F4B laminate space, offering F4BM and F4BME series substrates in both standard and metal-backed configurations across a wide range of Dk values and panel sizes.

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Stackup Configurations for F4B Metal Base PCBs

Metal-base PTFE laminates are inherently single-signal-layer substrates in their basic form. However, more complex configurations are used in practice.

Single-Layer Metal Base (Most Common)

[ Circuit Copper (1 oz) ][ PTFE-Glass Dielectric (0.1 – 0.8 mm) ][ Al or Cu Base (1.0 – 3.0 mm) ]

This is the standard configuration for power amplifier boards, patch antenna arrays, and combiner networks. The metal base is the ground plane and the heat spreader in one.

Hybrid Stack with FR-4 Carrier

Some designs bond an F4B metal-base sub-board to a multilayer FR-4 carrier board for the digital control and power management circuitry. The RF section (F4B on metal) and the digital section (FR-4) are interconnected by press-fit connectors or solder-bridged castellations at the board edge. This hybrid approach keeps the RF dielectric environment clean while leveraging FR-4’s multi-layer routing density for the control circuitry.

Dual-Dielectric Buried Metal Core

For more complex assemblies where the metal plate must carry both thermal and moderate electrical current, the aluminium or copper core can be configured as a buried power or ground bus, with PTFE dielectric layers on both sides. This structure is considerably more expensive to fabricate but enables a self-shielded, thermally managed RF module with minimal external metalwork.

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Useful Datasheets and Technical Resources

Engineers working with F4B metal base PTFE laminate should bookmark these resources:

Resource	Description	Link
IPC-4103	Specification for High Speed/High Frequency Base Materials	ipc.org
IPC-TM-650	Test Methods Manual (Dk, Df, peel strength, thermal)	ipc.org/TM
Wangling F4BM/F4BME Datasheet	Official laminate specifications (Dk, Df, panel sizes, CTE)	pcbapeak.com PDF
Wangling PCB Manufacturer Profile	F4B PCB production capabilities and stackup options	pcbsync.com/Wangling-pcb
Rogers Material Comparison	Reference for Dk/Df benchmarking against international materials	rogerscorp.com
Altium Designer Material Library	Reference for modelling PTFE laminates in PCB EDA tools	altium.com
MIL-PRF-55110	Military specification for printed circuit boards	everyspec.com
IPC-2221B	Generic Standard on Printed Board Design (trace width, thermal)	ipc.org

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Common Mistakes Engineers Make with F4B Metal Base Laminates

Having seen these boards fail in production and in the field, here are the mistakes that cause the most pain:

1. Specifying insufficient copper-to-metal-base isolation clearance. If the via hole in the metal base isn’t correctly counter-bored and filled before lamination, you get a hard short between the via and the aluminium base at the drill stage. This kills entire production panels.

2. Ignoring the PTFE surface activation step before bonding. PTFE is notoriously non-adhesive. Metal-base PTFE laminates are manufactured with proper surface activation, but any secondary bonding operations (potting, adhesive dam fill) require a sodium-naphthalene or plasma etch activation step first. Skip it and the adhesive peels off.

3. Applying standard FR-4 DRC rules to PTFE traces. Minimum trace widths and spacing that are safe on FR-4 may not be achievable on PTFE at the same factory, due to different copper adhesion and etch undercut characteristics.

4. Assuming the aluminium base is your only ground path. The aluminium base is a ground plane in DC terms, but RF currents predominantly flow in the copper foil on the circuit side. Ground stitching vias back to the metal base need to be planned and correctly electrically isolated through the base thickness.

5. Not specifying bow and twist tolerances. Metal-base PCBs are inherently stiff, but residual stress from the lamination bond can create bow in large-format panels. Specify bow/twist per IPC-6012 and validate with your board house before mass production.

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FAQs: F4B Metal Base PTFE Laminate

Q1: Can F4B metal base PTFE laminate be used for multilayer RF designs?

In its standard form, F4B metal base laminate is a single-signal-layer substrate — the metal plate occupies the position a second dielectric layer would in a multilayer. True multilayer RF designs are possible by laminating multiple PTFE prepreg layers above the metal base, but this is a specialist process and typically reserved for high-value military and aerospace programmes. For commercial RF designs needing multiple routing layers, most engineers use a hybrid approach: F4B metal base for the RF signal layer, with a separate multilayer FR-4 board for control and power routing.

Q2: What is the difference between F4BM and F4BME in the context of metal-base laminates?

Both use the same PTFE-woven glass dielectric. The critical difference is the copper foil bonded to the circuit side. F4BM uses standard ED (electrodeposited) copper, which is cost-effective and reliable for general RF use. F4BME uses reverse-treated foil (RTF), which has a smoother bond interface and significantly lower passive intermodulation (PIM) — a key metric in 3GPP-compliant base station antennas. If your application has a PIM specification (typically ≤ -150 dBc), specify F4BME. If not, F4BM is the right choice.

Q3: How does F4B metal base PTFE laminate compare to Rogers RT/duroid 5880 on an aluminium carrier?

The PTFE dielectric performance is very similar — both offer Df in the 0.001–0.003 range at 10 GHz and Dk in the 2.2 range. The practical differences are cost (F4B is substantially less expensive), supply chain (F4B is widely available from Chinese manufacturers with fast lead times), and qualification status (Rogers materials carry broader aerospace and defence qualification traceability). For commercial telecom and industrial radar, F4B is a proven and cost-effective substitute. For programmes requiring AS9100 or MIL-spec material traceability to a US-origin material, Rogers remains the safer qualification path.

Q4: What copper thickness should I specify for high-power RF applications?

For power amplifier boards where transistors dissipate 10 W or more per device, 2 oz copper (70 µm) on the circuit layer helps spread heat laterally before it enters the thin PTFE dielectric. However, heavier copper creates etching challenges for fine-geometry RF traces — trace widths below 0.2 mm become difficult to control consistently at 2 oz. Many designers use 1 oz copper on the RF traces and specify localised coin-insert or copper-filled via arrays under the transistor mounting pads to improve thermal conductivity directly under the hotspot.

Q5: Is F4B metal base PTFE laminate compatible with lead-free reflow soldering?

Yes. The PTFE matrix and the metal base both survive the 250–260°C peak temperatures of SAC305 lead-free reflow profiles. The critical constraint is cycle count — PTFE’s CTE mismatch with the metal base generates cyclic stress with each thermal excursion through reflow. For assemblies that will undergo more than three reflow passes (common in complex assemblies built up over multiple soldering operations), verify adhesive bond integrity with your laminate supplier’s thermal cycling qualification data.

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Summary: Is F4B Metal Base PTFE Laminate Right for Your Design?

If you are designing a circuit that simultaneously needs microwave-frequency dielectric performance (Df < 0.005 at 10 GHz), substantial heat removal capability, and mechanical rigidity — and you need it at a competitive production cost — then F4B metal base PTFE laminate belongs on your short list of substrate candidates.

It is not the right answer for every RF design. Pure PTFE laminates without the metal base cost less when thermal management isn’t a constraint. FR-4 remains the right answer for anything below 1–2 GHz where loss budget is comfortable. Rogers materials remain the reference for the most demanding performance or qualification requirements.

But for the growing category of designs that sit at the intersection of high RF frequency and high power — 5G massive MIMO radios, automotive radar front ends, active phased arrays, solid-state radar power modules — the F4B metal base PTFE laminate is a well-characterised, production-proven solution with an expanding supply chain and a price point that makes high-volume production economics viable.

Specify your Dk value based on your impedance and antenna geometry requirements. Choose aluminium base unless your thermal budget demands copper. Specify RTF copper foil (F4BME) if PIM matters. Keep your dielectric layer as thin as your process allows for best thermal performance. And work with a board house that has validated PTFE experience — because the manufacturing steps that differ from FR-4 are the ones that will bite you if they’re not properly controlled.