Rogers CLTE PCB | Low CTE Microwave Laminate

What Is Rogers CLTE?

Rogers CLTE is a PTFE-based, woven-glass-reinforced microwave laminate whose defining feature is a coefficient of thermal expansion (CTE) closely matched to copper in the X-Y plane—approximately 16–18 ppm/°C compared to copper’s 17 ppm/°C. Its dielectric constant (Dk) of 2.94 ± 0.04 at 10 GHz and dissipation factor (Df) of 0.0012 place it among the lower-loss PTFE laminates suitable for circuits operating from L-band through Ka-band. If your design faces repeated thermal cycling or must survive a wide operating temperature range, Rogers CLTE PCB eliminates the pad-lifting and micro-cracking failures that plague laminates with mismatched CTE.

Key Electrical and Thermal Properties

The X-Y CTE near 17 ppm/°C is the critical specification. Standard woven-glass PTFE laminates typically land around 20–24 ppm/°C, and unreinforced PTFE can exceed 100 ppm/°C. That 3–7 ppm/°C gap may sound small, but over hundreds of thermal cycles in aerospace or satellite hardware it accumulates enough shear stress to fracture plated-through holes and lift surface-mount pads.

Rogers CLTE vs CLTE-MW: What Changed?

Rogers later introduced CLTE-MW, the microwave-grade evolution, which keeps the same CTE philosophy but tightens Dk tolerance, reduces Df to roughly 0.0020 at 10 GHz on a slightly different resin system, and offers improved lot-to-lot consistency. If you are starting a new design, CLTE-MW is the current production recommendation from Rogers. CLTE remains available for legacy programs and repair-and-replace situations where a qualified material cannot be substituted without requalification.

Why CTE Matching Matters for Microwave PCBs

Every solder joint on a PCB is a mechanical bridge between the copper conductor and the laminate beneath it. When those two materials expand at different rates during thermal excursions, the joint sees cyclic shear strain. The wider the CTE mismatch and the larger the board, the higher the cumulative displacement at the corners. For a 150 mm × 150 mm board cycling between −55 °C and +125 °C, a CTE difference of just 6 ppm/°C generates roughly 160 µm of differential displacement along the diagonal—enough to crack BGA solder balls or fracture via barrels after a few hundred cycles.

Rogers CLTE PCB was engineered specifically to keep that differential near zero in the X-Y plane. This makes it a go-to choice for:

• Satellite transponder filters and multiplexers that see eclipse thermal cycling.
• Airborne radar modules behind the radome, exposed to rapid altitude-driven temperature swings.
• Phased-array antenna feed networks where hundreds of identical elements must maintain phase coherence across temperature.
• Test-and-measurement fixtures that must hold calibration over an extended temperature range.

Check CTE Compatibility for Your Stackup

Use the calculator below to compare the CTE of your chosen laminate against copper and any heat-sink or carrier material in your assembly. Enter the CTE values for the laminate and the constraining material, along with the temperature range, to see the predicted differential displacement.

For example, entering 17 ppm/°C for CLTE, 17 ppm/°C for copper, and a ΔT of 180 °C (the −55 °C to +125 °C military range) yields a near-zero differential—exactly the point of this laminate. Swap in a generic woven-glass PTFE at 24 ppm/°C and the displacement jumps to a value that demands careful pad design and possibly stress-relief features in the copper. If the calculator shows a differential above 50 µm on your board diagonal, consider switching to a CTE-matched laminate or adding mechanical compliance in the attachment scheme.

Fabrication Guidelines for Rogers CLTE PCB

Drilling

PTFE-based laminates are softer than FR-4, and CLTE’s woven-glass reinforcement adds abrasiveness. Use carbide drills with a positive rake angle, entry speeds of 150–200 SFM (surface feet per minute), and chip loads of 0.05–0.08 mm per revolution. Back-up boards should be phenolic or aluminum; standard FR-4 entry/exit material works, but aluminum entry reduces burring. Inspect the first panel’s hole quality under 30× magnification before committing a production run.

Plating and Surface Preparation

PTFE is inherently non-wettable, so the drilled hole walls must be treated before electroless copper deposition. Sodium-naphthalene etch or plasma desmear are the two proven methods. Plasma (CF₄/O₂ mix) is preferred for tighter-pitch vias because it does not leave chemical residues that can shift Dk locally. After activation, standard electroless-then-electrolytic copper plating proceeds normally. Target a minimum 25 µm (1 mil) barrel copper for reliability through thermal cycling.

Lamination and Hybrid Stackups

CLTE bonds well with FEP or PTFE-based bonding films at 290–315 °C under 200–350 psi. For hybrid stackups combining CLTE with a different RF laminate—say a CuClad 233 layer for a lower-Dk antenna element—use a shared bonding film chemistry and verify that the Z-axis CTE of the bond film does not create a weak link. Pre-bake all PTFE materials at 150 °C for 2–4 hours to drive off absorbed moisture before lamination; even 0.02 % moisture can cause delamination blisters at bonding temperatures.

Etching and Dimensional Stability

One advantage of the CTE-matched construction is excellent dimensional stability during etching and reflow. Registration accuracy of ±25 µm (±1 mil) is achievable on standard tooling, which matters for fine-pitch microstrip and coupled-line filters. Etch compensation should follow standard copper-thickness tables; no special factors are needed beyond those for any PTFE laminate.

Design Considerations: Where CLTE Excels and Where It Does Not

Strengths

• Thermal-cycle reliability: Unmatched among PTFE laminates for solder-joint longevity.
• Low moisture absorption: 0.02 % means negligible Dk shift in humid environments.
• Proven heritage: Decades of flight history on military and commercial satellite programs.

Limitations

• Z-axis CTE is still high: At 173 ppm/°C, via reliability in thick multilayer builds (> 6 layers) requires careful via-diameter and aspect-ratio control. Microvias or back-drilled stubs may be needed.
• Cost: CLTE is a specialty PTFE laminate priced well above lower-cost alternatives such as Taconic RF-35 or hydrocarbon-ceramic options. Typical lead times are 4–8 weeks for non-stock thicknesses.
• Not the lowest loss: At Df 0.0012, CLTE is good but not best-in-class. For millimeter-wave circuits above 40 GHz where every 0.0005 in Df matters, evaluate ultra-low-loss laminates like Elite EM-891 or Rogers RT/duroid 5880.

Cost Factors for Rogers CLTE PCB

The price of a Rogers CLTE PCB is driven by raw laminate cost, panel utilization, layer count, and the PTFE-specific processing steps described above. PTFE laminates carry a significant material premium over FR-4 and even over hydrocarbon-ceramic RF laminates like Ventec VT-47. Sodium-naphthalene or plasma desmear adds a process step (and sometimes a separate vendor qualification), and FEP bonding requires higher-temperature presses with tighter process windows.

Use the cost index tool below to get a relative sense of where CLTE sits compared to other laminates in your candidate set. Enter the laminate family and thickness, and the tool returns a normalized cost index against a baseline FR-4 panel.

A typical result places Rogers CLTE at roughly 6–10× the cost index of standard FR-4, and about 1.5–2× that of a mid-range hydrocarbon-ceramic laminate. That premium is justified when thermal-cycle reliability is a hard requirement—replacing a failed satellite transponder filter is infinitely more expensive than the laminate upgrade. For ground-based commercial products with benign thermal environments, the cost rarely makes sense, and a lower-CTE woven-glass PTFE or a thermoset hydrocarbon laminate will serve better.

Comparing Rogers CLTE to Alternative Low-CTE Laminates

Choose Rogers CLTE when your program already has it qualified and you need drop-in replacement panels. Choose RT/duroid 6002 when you need equivalent CTE performance with tighter Dk tolerance on a new design. Choose Isola IS680-AG when your stackup mixes high-speed digital and RF layers and you want a thermoset (non-PTFE) process flow with inherently lower CTE.

Application Examples

Satellite Multiplexer Filters

C-band and Ku-band output multiplexers on communications satellites use microstrip or suspended-stripline filter topologies on CLTE. The spacecraft’s eclipse cycle imposes roughly 5,000 thermal cycles over a 15-year mission. CTE matching keeps filter center frequencies within ±1 MHz across the full temperature range, eliminating the need for post-launch frequency trimming.

Airborne Phased-Array Feed Networks

Corporate feed networks for X-band phased arrays can span 300 mm or more. On a laminate with 24 ppm/°C CTE, the differential expansion against the aluminum cold plate below would warp the feed and shift element-to-element phase. CLTE’s copper-matched expansion keeps phase error below 2° over the operational temperature envelope.

Test Fixtures and Calibration Standards

Precision TRL (Thru-Reflect-Line) calibration kits for vector network analyzers are often built on CLTE or RT/duroid 6002 because dimensional stability directly impacts measurement uncertainty. A calibration substrate that drifts with lab temperature is worse than useless.

Frequently Asked Questions

Is Rogers CLTE the same as RT/duroid 6002?

No. Both share a Dk of 2.94 and a CTE near 16–17 ppm/°C, but they use different PTFE/glass constructions and have separate Rogers datasheets. RT/duroid 6002 is the more commonly stocked grade today; CLTE is maintained primarily for legacy programs.

Can I use Rogers CLTE for multilayer builds above 4 layers?

Yes, but the Z-axis CTE of 173 ppm/°C means via reliability must be carefully managed. Keep aspect ratios below 8:1, use via-in-pad with filled and capped vias where possible, and consider sequential lamination to reduce individual press-cycle stress.

What is the maximum operating frequency for CLTE?

With a Df of 0.0012, CLTE supports circuits well into Ka-band (26–40 GHz). Above 40 GHz, conductor losses dominate and the laminate loss becomes less critical, but designers should evaluate whether tighter Dk tolerance materials like Rogers’ newer product lines offer better phase stability at millimeter-wave frequencies.

Does CLTE require special solder paste or reflow profiles?

No special paste is needed. Standard SAC305 or SnPb eutectic profiles work. However, pre-bake the panels at 150 °C for 2 hours before reflow to prevent moisture-driven delamination—a standard precaution for all PTFE laminates.

How does Rogers CLTE compare on cost to hydrocarbon-ceramic laminates?

CLTE typically costs 1.5–2× more than hydrocarbon-ceramic RF laminates of similar thickness. The premium buys CTE matching and very low moisture absorption, both critical for space and military environments. For commercial telecom, hydrocarbon-ceramic laminates usually offer a better cost-performance trade-off.