Rogers CLTE-XT PCB | Dk 2.94 Low Loss Multilayer

What Is Rogers CLTE-XT?

Rogers CLTE-XT is a woven-glass-reinforced PTFE composite laminate with a dielectric constant (Dk) of 2.94 ± 0.04 and a dissipation factor (Df) of 0.0012 at 10 GHz. It was designed specifically for multilayer RF and millimeter-wave boards where low Z-axis CTE and dimensional stability matter as much as electrical performance. The “CLTE” name stands for Controlled Low Thermal Expansion—the material’s Z-axis CTE is approximately 24 ppm/°C below T_g, which is far lower than most standard PTFE laminates and close enough to copper’s 17 ppm/°C to dramatically improve plated-through-hole (PTH) reliability in thick multilayer stackups.

Key Electrical and Mechanical Properties

With a Dk of 2.94, CLTE-XT sits between the lower-Dk PTFE family (Dk ≈ 2.20–2.55) and ceramic-filled hydrocarbon laminates like RO4350B (Dk 3.48). That mid-range Dk yields moderate trace widths—wider than pure PTFE for the same impedance, which improves etch tolerance—while keeping wavelength short enough for compact antenna elements up through Ka-band.

Why Z-Axis CTE Matters for Multilayer CLTE-XT PCBs

Standard unreinforced PTFE laminates can exhibit Z-axis CTE values exceeding 200 ppm/°C above T_g. During reflow soldering (peak temperatures around 260 °C for lead-free), that expansion stresses barrel copper in plated vias, leading to micro-cracks and open circuits. CLTE-XT’s 24 ppm/°C Z-axis CTE is roughly 8× lower than unreinforced PTFE, making it suitable for 10+ layer hybrid stackups that must survive multiple reflow cycles without via failures—a requirement called out in IPC-6012 Class 3 reliability testing.

This CTE advantage is the main reason designers choose CLTE-XT over a pure unreinforced PTFE substrate. Where a two-layer patch antenna on bare PTFE works fine, a 12-layer phased-array feed network with blind and buried vias demands the dimensional stability that CLTE-XT provides.

Dk Stability Across Temperature and Frequency

CLTE-XT’s Dk remains within ± 0.04 from −40 °C to +140 °C, a tighter window than many glass-reinforced hydrocarbon laminates. For designs operating outdoors—automotive radar modules, base-station antennas, satellite terminals—this thermal Dk stability keeps impedance and phase velocity predictable across the full operating range.

Use the estimator below to see how much Dk drift your stackup may experience over a given temperature swing. Enter the nominal Dk, the temperature range, and the material’s Dk thermal coefficient (approximately 40 ppm/°C for CLTE-XT).

For example, a CLTE-XT board swinging from −40 °C to +85 °C (a 125 °C span) shifts Dk by roughly 0.015—small enough that a 50 Ω microstrip line changes by less than 0.3 Ω. Compare that with a filled-PTFE laminate whose Dk thermal coefficient may be 100+ ppm/°C: the same swing could push impedance more than 1 Ω off target, enough to degrade return loss at millimeter-wave frequencies.

Loss Budget: Dielectric and Copper Roughness

Dielectric Loss

At Df = 0.0012 (10 GHz), CLTE-XT is among the lowest-loss glass-reinforced PTFE laminates available. The dielectric loss contribution in a 50 Ω microstrip at 28 GHz on 5 mil CLTE-XT is roughly 0.15 dB/inch—low enough that conductor and roughness losses dominate the total insertion loss.

Copper Roughness Loss

Because dielectric loss is so low, copper surface roughness becomes the primary loss mechanism above about 10 GHz. Standard electrodeposited (ED) copper has an RMS roughness (Rq) of 1.5–2.0 µm. At 28 GHz the skin depth in copper is only about 0.39 µm, so the current crowds into the rough surface profile, increasing effective resistance. Switching to HVLP (hyper-very-low-profile) copper foil with Rq ≈ 0.3 µm can cut conductor loss by 30–40 % at Ka-band.

Estimate the additional loss contributed by copper roughness using the calculator below. Enter your operating frequency, copper roughness (Rq), and trace geometry.

A typical result: on CLTE-XT at 28 GHz with standard 1 oz ED foil (Rq ≈ 1.8 µm), roughness adds approximately 0.25 dB/inch on top of the smooth-conductor baseline. Switching to HVLP foil (Rq ≈ 0.3 µm) drops that penalty to roughly 0.05 dB/inch. Over a 4-inch feed network, that difference is nearly 0.8 dB—significant in a phased-array power budget. When specifying CLTE-XT for frequencies above 20 GHz, always call out the copper foil grade on your fabrication drawing.

CLTE-XT vs. Related Low-Loss Laminates

Choose CLTE-XT when you need the absolute lowest loss in a glass-reinforced PTFE system with robust multilayer processability—think 24–40 GHz phased arrays, SATCOM feed networks, and high-reliability military radar.

Choose CLTE-AT laminate when a Dk of 3.00 fits your impedance targets and you can tolerate a marginally higher Df of 0.0013. The two materials share the same resin system and bond well in hybrid stackups.

Choose Taconic RF-35 when budget is a primary driver and the design operates below 18 GHz, where RF-35’s higher Df is still manageable.

For high-speed digital-RF hybrid boards where you need low-loss cores only in the RF layers, pairing CLTE-XT signal layers with a Megtron 6 digital section is a proven hybrid approach. The key is matching prepreg flow and lamination pressure to avoid resin starvation at the interface.

Fabrication and Stackup Guidelines

Drilling

CLTE-XT’s woven-glass reinforcement makes it far easier to drill than unreinforced PTFE. Standard carbide drill bits work well, though pecking cycles and reduced feed rates (compared to FR-4) help minimize smear. For microvias, laser drilling with a CO₂ laser at 9.4 µm wavelength is effective; UV lasers can be used but are slower on PTFE-based resins.

Plasma Desmear

PTFE-based laminates do not respond to standard permanganate desmear. A CF₄/O₂ plasma etch cycle is required after drilling to clean the via barrel and promote copper adhesion. Typical plasma parameters are 200–400 W RF power, 200–400 mTorr pressure, and a 5–15 minute cycle depending on smear severity.

Lamination

CLTE-XT laminates bond with Rogers CuClad or CLTE-series bondplies, or with low-flow prepregs such as RO4450F. Lamination temperature is typically 375–385 °F (190–196 °C) at 200–400 psi for 60–90 minutes. Because PTFE composites are thermoplastic, exceeding recommended temperature or pressure risks resin squeeze-out and thickness variation.

Surface Finish

ENIG, immersion silver, and ENEPIG are all compatible. For wire-bondable pads in bare-die MMIC attach, electrolytic soft gold over nickel is standard. HASL is generally avoided on PTFE boards because the thermal shock can delaminate the copper-to-dielectric interface.

Typical Applications

• Phased-array antennas (24–40 GHz): Low Df keeps array efficiency high; low CTE preserves via integrity across hundreds of elements.
• SATCOM terminals (Ka-band, 26.5–40 GHz): Dk stability over temperature prevents beam squint in outdoor-mounted flat-panel antennas.
• Military and aerospace radar: Meets the thermal-cycling and vibration demands of MIL-PRF-31032 and Rogers’ own CLTE-XT qualification data for high-reliability builds.
• 5G mmWave base-station filters and combiners: Tight Dk tolerance enables repeatable filter center frequencies across production lots.
• Test and measurement fixtures: Stable Dk and low loss make CLTE-XT a reliable reference material for characterizing connectors and devices to 50 GHz.

Design Checklist for CLTE-XT PCBs

1. Confirm Dk = 2.94 in your field solver; use the Rogers MWI impedance calculator for a quick cross-check.
2. Specify copper foil grade on the fab drawing—HVLP or rolled for frequencies above 20 GHz.
3. Call out plasma desmear (not permanganate) in the drill notes.
4. Define bondply material and lamination profile; do not assume the fabricator will default to the right prepreg.
5. Include a test coupon with impedance and insertion-loss requirements per IPC-TM-650 2.5.5.7.
6. If hybridizing with a different resin system (e.g., Isola I-Tera MT40 for digital layers), run a CTE mismatch analysis to verify via reliability under thermal cycling.
7. For builds thicker than 100 mil (2.54 mm), request a cross-section coupon to verify barrel-crack-free PTHs after 6× reflow simulation.

Frequently Asked Questions

What frequency range is CLTE-XT suitable for?

CLTE-XT performs well from UHF through millimeter-wave, with its ultra-low Df of 0.0012 providing the greatest advantage above 10 GHz. It is commonly specified for 24 GHz, 28 GHz, and 39 GHz 5G and radar designs, and has been characterized by Rogers up to 50 GHz.

Can CLTE-XT be used in hybrid stackups with FR-4?

Yes, but with care. CLTE-XT’s X/Y CTE (10–12 ppm/°C) differs from FR-4’s (14–17 ppm/°C), which can cause registration shift during lamination. Use a low-flow bondply and keep the FR-4 layers thin relative to the CLTE-XT layers to minimize warpage. A hydrocarbon laminate like Rogers Kappa 438 in the non-RF layers is a better CTE match than standard FR-4.

How does CLTE-XT compare to RO3003 for multilayer builds?

Both share similar Dk (2.94 vs 3.00) and Z-axis CTE (24 ppm/°C). The key difference is reinforcement: CLTE-XT’s woven glass gives it superior dimensional stability and drill accuracy, while RO3003’s ceramic-filled PTFE (no glass) offers slightly lower Df (0.0010) but is softer and more prone to drill wander. For boards with more than four layers or via densities above 20 vias/cm², CLTE-XT is the safer choice.

Is CLTE-XT lead-free solder compatible?

Yes. CLTE-XT withstands peak reflow temperatures of 260 °C for lead-free assembly. Its low Z-axis CTE and high decomposition temperature (Td > 500 °C) ensure that multiple reflow cycles do not compromise via or laminate integrity.