Arlon IsoClad 918 PTFE random glass fiber laminate: full specs (Dk 2.18, Df 0.0013), mechanical properties, fabrication tips, and RF/microwave applications explained.
If you’ve spent any time designing RF or microwave circuits, you’ve probably run into the question of substrate selection more than once โ and it’s never a trivial decision. The Arlon IsoClad 918 is a nonwoven PTFE/random glass fiber laminate that sits in a very specific sweet spot for engineers who need ultra-low loss, near-isotropic electrical behavior, and the mechanical flexibility to accommodate conformal or wrap-around designs. This guide breaks down everything you need to know about IsoClad 918 from a PCB engineer’s perspective: its material composition, key electrical and mechanical specs, fabrication considerations, and the real-world applications where it outperforms the competition.
What Is Arlon IsoClad 918?
Arlon IsoClad 918 is a PTFE-based microwave laminate reinforced with randomly oriented nonwoven glass fibers โ hence the “random glass fiber” designation. It belongs to Arlon’s IsoClad product family, a line of nonwoven fiberglass/PTFE composite materials engineered specifically for use as printed circuit board substrates in high-frequency applications.
The “Iso” in IsoClad is not marketing fluff. It directly refers to the isotropic electrical and mechanical behavior that the random fiber architecture delivers. Unlike woven fiberglass reinforcements โ where the fiber weave introduces measurable differences in the X and Y directions โ the nonwoven random glass structure of IsoClad 918 distributes glass uniformly in all in-plane directions. The result is a substrate that behaves consistently whether your transmission line runs along the length or width of the panel.
IsoClad products use longer random fibers and a proprietary process to provide greater dimensional stability and better dielectric constant uniformity than competitive nonwoven fiberglass/PTFE laminates of similar dielectric constants.
The dielectric constant (Dk/Er) of IsoClad 918 sits at approximately 2.18 at 10 GHz, positioning it between the ultra-low Dk of pure PTFE and the higher Dk values associated with ceramic-loaded or heavily glass-reinforced substrates. This makes it a practical choice when you need the electrical performance of PTFE with slightly better mechanical handling than a pure PTFE film laminate.
IsoClad 918 Material Composition and Construction
Understanding what this material is actually made of helps you make smarter decisions both in design and during fabrication. IsoClad 918 is a two-component composite:
PTFE (Polytetrafluoroethylene): The matrix resin is PTFE, a fluoropolymer thermoplastic renowned for its exceptionally low dielectric loss, broad chemical inertness, and wide operating temperature range. PTFE, or polytetrafluoroethylene (also known by its DuPont trade name Teflonยฎ), is used in laminates for microwave and RF because its dielectric properties are ideal for high frequency applications.
Nonwoven Random Glass Fibers: The reinforcement is short glass fibers dispersed randomly throughout the PTFE matrix. This is a fundamentally different architecture from woven glass cloth. There’s no orthotropic bias introduced by a weave pattern, so the in-plane electrical and mechanical properties are essentially isotropic. The tradeoff compared to woven reinforcement is slightly lower dimensional stability under etching and thermal stress โ something to account for during panel-level fabrication.
The copper cladding is typically electrodeposited (ED) copper, available in 1/2 oz, 1 oz, or 2 oz weights, though rolled annealed (RA) copper is also available for applications where lower surface roughness and reduced conductor loss at very high frequencies are priorities.
Arlon IsoClad 918 Key Electrical Specifications
This is usually the section engineers jump to first โ and rightfully so. The electrical properties of IsoClad 918 are what justify its use over cheaper FR-4 alternatives in RF and microwave designs.
| Property | Value | Test Method / Condition |
| Dielectric Constant (Dk) @ 10 GHz | ~2.18 | IPC TM-650 2.5.5.5, C23/50 |
| Dissipation Factor (Df) @ 10 GHz | โค0.0013 | IPC TM-650 2.5.5.5, C23/50 |
| Thermal Coefficient of Er (ppm/ยฐC) | ~-155 | IPC TM-650 2.5.5.5 Adapted, -10ยฐC to +140ยฐC |
| Dk Stability vs. Frequency | Very High | Stable from 1 GHz to 30+ GHz |
| Volume Resistivity (Mฮฉยทcm) | ~1.5 ร 10ยนโฐ | IPC TM-650 2.5.17.1, C96/35/90 |
| Surface Resistivity (Mฮฉ) | ~1.0 ร 10โน | IPC TM-650 2.5.17.1, C96/35/90 |
| Dielectric Breakdown (kV) | >45 | ASTM D-149, D48/50 |
Engineer’s Note: The dissipation factor of โค0.0013 at 10 GHz is one of the lowest you’ll find in a glass-reinforced PTFE laminate. For context, standard FR-4 has a Df of roughly 0.020 at 1 GHz โ already about 15x worse. At 10 GHz, that gap widens significantly.
The near-constant Dk across a wide frequency band is critical for broadband designs. When you’re designing a microstrip filter or coupler that needs to perform consistently from 2 GHz to 18 GHz, a substrate whose Dk drifts with frequency will degrade your circuit’s bandwidth and center frequency accuracy.
Arlon IsoClad 918 Mechanical and Thermal Properties
Electrical specs don’t tell the whole story. Here’s how IsoClad 918 behaves mechanically and thermally, which governs fabrication quality and long-term reliability.
| Property | Value | Test Method / Condition |
| Tensile Modulus (kpsi) | ~133 (MD), ~120 (TD) | ASTM D-638, A, 23ยฐC |
| Tensile Strength (kpsi) | ~4.3 (MD), ~3.8 (TD) | ASTM D-882, A, 23ยฐC |
| Compressive Modulus (kpsi) | ~182 | ASTM D-695, A, 23ยฐC |
| Flexural Modulus (kpsi) | ~213 | ASTM D-790, A, 23ยฐC |
| Peel Strength (lbs/inch) | 10 (after thermal) | IPC TM-650 2.4.8 |
| CTE X-Axis (ppm/ยฐC) | ~46 | IPC TM-650 2.4.24, 0ยฐC to 100ยฐC |
| CTE Y-Axis (ppm/ยฐC) | ~47 | IPC TM-650 2.4.24, 0ยฐC to 100ยฐC |
| CTE Z-Axis (ppm/ยฐC) | ~236 | IPC TM-650 2.4.24, 0ยฐC to 100ยฐC |
| Thermal Conductivity (W/mK) | 0.263 | ASTM E-1225, 100ยฐC |
| Density (g/cmยณ) | ~2.23 | ASTM D-792, Method A |
| Water Absorption (%) | 0.04 | MIL-S-13949H / IPC TM-650 2.6.2.2, E1/105 + D24/23 |
| Flammability | UL94-V0 | UL 94 Vertical Burn |
The low water absorption of 0.04% is worth emphasizing. Moisture ingress shifts Dk upward โ sometimes significantly โ and degrades insertion loss. IsoClad 918’s excellent moisture resistance makes it reliable in outdoor or humid environments, including base station hardware exposed to seasonal humidity swings.
The Z-axis CTE of ~236 ppm/ยฐC is higher than what you’d see in woven-glass laminates and is a known consideration when designing plated-through holes (PTHs) in thicker stackups. For single-layer and thin double-sided designs, this is typically a non-issue.
Outgassing Properties of Arlon IsoClad 918
Space and vacuum applications have strict outgassing requirements. IsoClad 918 meets those demands.
| Outgassing Property | IsoClad 918 Typical Value | Limit |
| Total Mass Loss (TML) % | 0.02 | Max 1.00% |
| Collected Volatile Condensable Material (CVCM) % | 0.00 | Max 0.10% |
| Water Vapor Regain (WVR) % | 0.02 | โ |
| Visible Condensate | None | โ |
Test conditions: 125ยฐC, โค10โปโถ torr
These numbers make IsoClad 918 a qualified candidate for aerospace and satellite PCB applications where outgassing can contaminate sensitive optical systems or instrument sensors.
How IsoClad 918 Compares to Related Arlon Laminates
One of the most practical questions a design engineer faces is: “Which grade do I actually need?” Here’s a head-to-head comparison of IsoClad 918 against closely related laminates.
| Material | Type | Dk @ 10 GHz | Df @ 10 GHz | Glass Reinforcement | Best For |
| Arlon IsoClad 918 | Nonwoven PTFE/Glass | ~2.18 | ~0.0013 | Random (nonwoven) | Ultra-low loss, conformal designs |
| Arlon IsoClad 917 | Nonwoven PTFE/Glass | 2.17โ2.20 | 0.0013 | Random (nonwoven) | Lowest Dk IsoClad variant |
| Arlon IsoClad 933 | Nonwoven PTFE/Glass | 2.33 | 0.0016 | Random (nonwoven) | Better mechanical strength |
| Arlon DiClad 527 | Woven PTFE/Glass | 2.40โ2.65 | ~0.0019 | Woven (unidirectional) | Higher dimensional stability |
| Arlon CuClad 250 | Cross-plied PTFE/Glass | 2.40โ2.60 | ~0.0017 | Woven (cross-plied) | Phased array antennas |
| Rogers RO3003 | PTFE/Ceramic | 3.00 | 0.0010 | Woven glass/ceramic | Low CTE, stable Dk |
IsoClad 917 uses a low ratio of fiberglass/PTFE to achieve the lowest dielectric constant and dissipation factor available in a combination of PTFE and fiberglass. IsoClad 933 uses a higher fiberglass/PTFE ratio for a more highly reinforced combination that offers better dimensional stability and increased mechanical strength.
The IsoClad 918 occupies the low end of the Dk range in the IsoClad family, making it the preferred selection when signal propagation speed and insertion loss minimization outweigh the need for high mechanical rigidity.
Key Advantages of Nonwoven vs. Woven Reinforcement
This distinction matters more than most datasheets make obvious. Here’s why the nonwoven random glass architecture of IsoClad 918 matters:
Isotropy in the XY plane: Woven glass introduces a periodic dielectric structure. The nonwoven reinforcement allows these laminates to be used more easily in applications where the final circuit will be bent to shape. Conformal or “wrap-around” antennas are a good example. With random fibers, you don’t have that weave-induced Dk variation between 0ยฐ and 90ยฐ, which simplifies impedance control in circuits that route traces in multiple orientations.
Flex-friendliness: The absence of a rigid woven structure means IsoClad 918 bends more gracefully without delamination or cracking, enabling it to wrap around cylindrical structures or conform to shaped housings. Woven glass laminates are stiffer and more prone to micro-cracking under bending stress.
Uniform Dk distribution: Because the glass fibers are randomly distributed rather than concentrated in yarn bundles, IsoClad 918 has more consistent local Dk than woven alternatives. This uniformity is valuable for tight impedance tolerances (ยฑ2% or better) in precision microwave circuits.
Primary Applications for Arlon IsoClad 918
Understanding where IsoClad 918 actually gets specified โ not just where it theoretically works โ helps you benchmark it against your own application requirements.
Conformal and Wrap-Around Antenna Systems
This is the application that nonwoven PTFE laminates like IsoClad 918 were essentially built for. Missile nose cones, aircraft fuselage antennas, and wearable military electronics all require substrates that conform to curved surfaces. The mechanical flexibility of IsoClad 918 allows the circuit to be formed into cylindrical or doubly-curved shapes without compromising electrical performance. The isotropic Dk also ensures that the radiation pattern remains predictable regardless of how the substrate is oriented.
Radar and Electronic Warfare (EW) Systems
Typical applications for IsoClad laminates include missile guidance systems and radar and electronic warfare systems. These systems commonly demand operation across very wide frequency bands โ sometimes 2 GHz to 40 GHz or beyond โ where a substrate with stable Dk vs. frequency is not optional but mandatory. IsoClad 918’s Dk stability across the full microwave band makes it a go-to material in EW receiver front ends, electronic countermeasures (ECM), and radar signal processing boards.
Microstrip and Stripline Transmission Circuits
Filters, couplers, power dividers, and combiners all benefit from IsoClad 918’s combination of low loss and stable Dk. These materials are used in high frequency applications where low loss and controlled dielectric constant are required, such as filters, couplers, low noise amplifiers, power dividers, and combiners. With a Df of 0.0013, the insertion loss contribution from the substrate remains negligible even over long transmission line runs, which is critical in power distribution networks for phased array systems.
Satellite and Space-Grade Electronics
The outgassing performance (TML โค 0.02%) and UL94-V0 flammability rating make IsoClad 918 suitable for space-qualified PCBs. Combined with its stability across a wide temperature range โ from cryogenic to high-temperature extremes โ it’s a credible choice for satellite subsystem boards, downlink/uplink electronics, and sensor electronics in space environments.
Base Station and Telecommunications Infrastructure
Arlon microwave materials deliver the electrical performance needed in frequency-dependent circuit applications, including base station antennas, phased array radars, power amplifier boards, communications systems, and various other antenna applications. IsoClad 918’s low loss directly translates to lower noise figures in receive chains and better efficiency in transmit amplifier circuits โ both of which affect system link budget and operational cost.
Low Noise Amplifier (LNA) Boards
LNA designs are especially sensitive to substrate loss because any loss in the input network directly adds to noise figure on a 1:1 dB basis. Using IsoClad 918 with a Df of ~0.0013 instead of a lossy FR-4 substrate can deliver several tenths of a dB improvement in noise figure โ which at system level translates directly to detection range or sensitivity margin.
Fabrication Guidelines for Arlon IsoClad 918
If you’ve ever built PTFE-based PCBs before, IsoClad 918 follows a mostly familiar playbook with a few important differences from FR-4 processing.
Handling and Storage
PTFE laminates should be stored flat in a clean, dry environment. Avoid contamination of the copper surface prior to processing โ even fingerprints can compromise copper adhesion and etch uniformity. Handle panels with clean cotton gloves.
Drilling
PTFE is a soft thermoplastic at elevated temperatures, which means standard drill feeds and speeds used for FR-4 will smear the PTFE around hole walls rather than cutting cleanly. Use sharp, carbide-tipped drills with higher feed rates and controlled entry/exit speeds. Minimize heat buildup at the drill tip. Coolant-assisted drilling is recommended for high-aspect-ratio holes.
Chemical Etching and Sodium Treatment
The PTFE surface requires chemical activation (sodium etching or sodium naphthalene treatment) before metallization, because PTFE’s low surface energy means standard adhesives and electroless copper don’t bond well to an untreated surface. This step is non-negotiable for reliable PTH formation.
Copper Etching
Use ammoniacal etch or cupric chloride etch โ both work well with IsoClad 918. Avoid over-etching, as PTFE has some tendency to absorb etchant if exposed for excessive time.
Lamination for Multilayer Designs
When building multilayer stackups incorporating IsoClad 918, use PTFE-compatible bonding plies. Arlon offers prepreg options specifically designed for PTFE-to-PTFE bonding. Temperature and pressure profiles must be carefully controlled during lamination โ PTFE’s sintering temperature (~370ยฐC) is significantly higher than standard FR-4 cure temperatures, so vacuum lamination with precise thermal profiling is required.
Material Availability and Ordering Information
IsoClad 918 is available in the following standard configurations:
| Parameter | Options |
| Copper Cladding | 1/2 oz, 1 oz, 2 oz ED copper (standard) |
| Copper Foil Type | Electrodeposited (ED) or Rolled Annealed (RA) |
| Ground Plane Options | Aluminum, brass, or copper plate (integral heat sink) |
| Master Sheet Size | 36″ ร 48″ (standard); other sizes available |
| Custom Configurations | Heavy metal plate, Ohmega-Plyยฎ resist foil, specialty foils |
When ordering, always specify: dielectric constant, dielectric thickness, copper cladding weight, foil type, panel size, and any special requirements.
For Arlon PCB fabrication inquiries, confirm with your PCB manufacturer that they have PTFE processing capability, including sodium etch treatment and PTFE-compatible lamination equipment. Not all shops are equipped for PTFE โ this is a qualification question to ask upfront.
Useful Resources for Engineers
The following reference materials and databases will help you work more effectively with Arlon IsoClad 918:
| Resource | Description | Link |
| Arlon IsoClad Official Datasheet | Full property tables, frequency response graphs | arlon-med.com |
| Arlon Fabrication Guidelines (DiClad, CuClad, IsoClad) | Drilling, etching, lamination, plating procedures | RF Globalnet |
| Arlon Microwave & RF Materials Guide (PDF) | Full portfolio comparison, Dk/Df tables | Integrated Test (PDF) |
| Arlon Everything You Wanted to Know (Laminate Guide PDF) | Deep-dive technical guide covering PTFE, thermosets, Dk/Df science | Arlon EMD (PDF) |
| MatWeb IsoClad Material Data | Searchable mechanical/electrical property database | matweb.com |
| IPC TM-650 Test Methods | Official test method standards referenced in all datasheets | ipc.org |
| NW Engineering RF PCB Materials Comparison | Independent comparison table of Rogers, Taconic, Arlon laminates | nwengineeringllc.com |
Frequently Asked Questions About Arlon IsoClad 918
1. What is the operating frequency range of Arlon IsoClad 918?
IsoClad 918 is suitable for use from DC through at least 30 GHz, with published data showing stable Dk and Df up to that point. In practice, it is widely used in L-band (1โ2 GHz), S-band (2โ4 GHz), C-band (4โ8 GHz), X-band (8โ12 GHz), and Ku-band (12โ18 GHz) systems. Its extremely stable Dk versus frequency makes it particularly attractive for broadband designs spanning multiple frequency bands.
2. How does IsoClad 918 handle soldering and assembly temperatures?
PTFE does not have a glass transition temperature (Tg) in the conventional thermoset sense. Its melting point is well above standard solder reflow temperatures (โค260ยฐC for lead-free), so IsoClad 918 is compatible with both tin-lead and lead-free solder processes. However, PTFE is dimensionally sensitive to thermal stress, so slow, controlled ramp rates during reflow are recommended to minimize warpage.
3. Can IsoClad 918 be used in multilayer PCB stackups?
Yes, but with important caveats. Multilayer construction requires PTFE-compatible prepreg bonding plies and high-temperature vacuum lamination equipment. The Z-axis CTE of ~236 ppm/ยฐC also needs to be accounted for in PTH reliability analysis, particularly for thick multilayer boards with high aspect-ratio holes. Many RF engineers limit PTFE multilayer constructions to 4โ6 layers for this reason, using hybrid stackups where IsoClad 918 is the RF signal layer and FR-4 or other materials handle power and digital layers.
4. What’s the difference between IsoClad 918, DiClad, and CuClad materials?
All three are PTFE/glass composite laminates from Arlon’s microwave portfolio, but the glass reinforcement architecture differs. IsoClad uses nonwoven random glass fibers, providing isotropy and flex-friendliness. DiClad uses woven glass cloth in a unidirectional orientation, while CuClad uses woven glass cloth in a cross-plied (alternating 90ยฐ) configuration, giving CuClad superior dimensional stability and true XY isotropy. DiClad and CuClad are stiffer, more dimensionally stable, and better suited to rigid multilayer builds, while IsoClad is preferred when conformability or near-isotropic behavior from a non-woven structure is required.
5. How does IsoClad 918 compare to Rogers RT/duroid 5880?
Both are PTFE/glass laminates with similar Dk values (IsoClad 918: ~2.18; RT/duroid 5880: 2.20) and comparable Df (~0.0009 for 5880 vs ~0.0013 for IsoClad 918). RT/duroid 5880 uses PTFE with microfiber glass reinforcement and is often considered the benchmark in ultra-low-loss substrates. IsoClad 918 is slightly lossier but offers Arlon’s proprietary random-fiber process for improved dimensional stability over competitors’ nonwoven products. The right choice between them depends on your loss budget, dimensional control requirements, and supplier relationship.
Summary
Arlon IsoClad 918 is a high-performance PTFE/nonwoven random glass fiber laminate engineered for demanding RF and microwave circuit applications. Its combination of ultra-low dissipation factor (~0.0013 at 10 GHz), near-isotropic electrical properties, mechanical flexibility for conformal designs, and excellent moisture resistance makes it a strong contender in applications from missile guidance and radar EW systems to satellite electronics and 5G infrastructure hardware.
For PCB engineers, the critical differentiators are its Dk stability across frequency (minimizing design uncertainty in broadband circuits), its conformability (enabling wrap-around antenna designs impossible with rigid woven laminates), and its established track record in aerospace-grade applications where outgassing and thermal performance matter as much as loss tangent. Pair it with a manufacturer experienced in PTFE processing, and IsoClad 918 will deliver the performance its datasheet promises.
Note: All property values listed are typical properties and should not be used as specification limits. Verify with the current Arlon datasheet before finalizing a design. Properties may vary depending on design and application.
