The RO4350B figures that hold across the whole thickness range, plus a thickness selection guide that maps 4 mil to 60 mil onto the applications that actually use them.
RO4350B is one of the most widely used high-frequency laminates in the world, and the reason is not that it wins on any single figure. It is a low-loss, high-reliability material that sits in a RO4000 series designed for RF, microwave and automotive radar work, and it earns its place by balancing electrical performance against processability and cost. Compared with a pure PTFE substrate it is easier to drill, easier to plate and considerably cheaper, while still delivering the loss behaviour that RF front ends need.
What follows covers the figures that hold across the whole thickness range, then how the available thicknesses map onto real applications, and finally what changes in the process when the panel gets thick or very thin. The thickness decisions are the interesting part, because RO4350B is supplied from 4 mil up to 60 mil and the right choice depends almost entirely on whether the constraint is board area or heat.
The electrical figures barely move with thickness, which is what makes RO4350B predictable to design with. The dielectric constant is 3.48 (±0.05) at 10 GHz and the dissipation factor is 0.0037 at the same frequency. Both stay close to those values whichever panel you order, so an impedance model built once does not have to be rebuilt for every thickness.
The mechanical and thermal figures matter just as much when the board goes through assembly. They are collected below.
| Parameter | Value | Why It Matters |
|---|---|---|
| Dielectric constant (Dk) | 3.48 ±0.05 at 10 GHz | Little variation with thickness, so impedance control remains accurate across the range |
| Dissipation factor (Df) | 0.0037 at 10 GHz | Consistently low loss, independent of how thick the panel is |
| CTE, X and Y axes | 11 ppm/°C | Close to copper, so in-plane stress during thermal cycling stays manageable |
| CTE, Z axis | 32 ppm/°C | Keeps the risk of delamination and pad cracking low |
| Thermal conductivity | 0.62 W/m·K | Enough heat spreading for high-power devices without a metal core |
| Water absorption | 0.06% | Electrical behaviour stays stable in humid conditions |
| Copper foil options | Standard and low-profile LoPro | Low-profile foil trims insertion loss where the link budget is tight |
| Flammability rating | UL 94 V-0 | Meets the safety requirement for commercial and automotive equipment |
Six thicknesses cover most of the demand: 0.1016 mm (4 mil), 0.2032 mm (8 mil), 0.254 mm (10 mil), 0.508 mm (20 mil), 0.762 mm (30 mil) and 1.524 mm (60 mil). Thin panels suit miniaturised designs where the electrical length has to stay short. Thick panels suit high-power and multilayer work, where the extra dielectric provides both a thermal path and physical rigidity.
Long-term performance in high-temperature and high-humidity conditions is good across the range, which is why automotive and industrial applications feature so heavily in the list below.
| Thickness | Typical Use | Why This Thickness |
|---|---|---|
| 0.1016 mm (4 mil) | Antenna feedlines and filters in compact front ends | Lowest profile available; supports the narrow line widths a dense feed network needs |
| 0.2032 mm (8 mil) | 77 GHz automotive radar, thin transmission lines | Short electrical length with impedance values that remain practical to etch |
| 0.254 mm (10 mil) | High-frequency filters and power amplifiers | Balances low loss against enough body to handle routinely |
| 0.508 mm (20 mil) | 77 GHz radar boards, satellite RF modules, RF power distribution | More mechanical support for a larger circuit without giving up loss performance |
| 0.762 mm (30 mil) | Low-pass filters, mixers, multilayer cores | Used as the core in a multilayer build, combined with other low-loss substrates |
| 1.524 mm (60 mil) | Power distribution networks, multilayer base station boards | Thermal mass and rigidity for circuits that carry real current |
Thickness does not change the material, but it changes what the material does in a circuit. Two effects matter most, and they pull in opposite directions.
Because Dk stays flat across the range, the design variable that shifts with thickness is geometry. For a fixed characteristic impedance, a thicker dielectric needs a wider trace, and a thin panel needs a narrower one. That matters on dense layouts, where the line width a thin panel demands may sit at the edge of what the etch process can hold repeatably. The relationship between geometry and impedance, and the tolerances that keep it under control, is covered in these steps for impedance control in high-frequency boards.
Dielectric loss is a property of the material, so it does not grow with thickness in the way copper loss does. What changes is heat. A thicker panel spreads heat from a hot component over a wider area, which lowers the temperature rise for the same dissipation. At 0.62 W/m·K RO4350B is not a thermal conductor in the way a metal core is, but thickness turns a mediocre conductivity into a usable thermal path when the power density is moderate.
The application list almost writes itself once the thickness logic is clear, and the divisions are consistent across the industry.
Thin boards from 0.1016 mm to 0.254 mm carry antenna feedlines, high-frequency filters and power amplifiers, where the priority is a compact, low-loss line. Thicker boards from 0.508 mm to 1.524 mm take the power distribution networks and the multilayer structures of a base station, where current and mechanical stability matter more than minimum loss. RO4350B’s role in this sector is examined in more depth in this article on RO4350B laminates in AI and communication systems.
77 GHz millimetre-wave radar is dominated by 0.2032 mm and 0.508 mm panels. The 8 mil option keeps the antenna geometry compact; the 20 mil option gives a more robust board where the radar sits in a harsh location. Both benefit from the material’s stability over temperature, which is a safety-relevant property in a vehicle that has to work from a cold start in winter to a hot engine bay in summer.
0.508 mm panels are common in RF satellite communication modules, where the loss budget is tight and the mechanical envelope is fixed. In industrial control and sensing, the material’s reliability under humidity is the deciding factor rather than its loss figure. For multilayer designs, panels above 0.762 mm are used as core material bonded with other low-loss substrates, which is where the fabrication process and its key considerations become the limiting factor rather than the datasheet.
Cutting should use laser or precision tooling, because edge damage on a high-frequency laminate shows up electrically as well as mechanically. Drilling speeds need adjusting in both directions: thin panels heat up and deform, thick panels demand more cutting force. Lamination runs between 175°C and 210°C with pressure adjusted upward for thicker stacks. Impedance is best simulated before the build with a tool such as ADS or HFSS and verified after it, and low-temperature soldering protects the material through assembly. Where the assembly process is the larger concern, the comparison in this article on RO4350B against FR-4 is a useful reference point.
None of these steps is exotic. They are the ordinary controls that a fabricator handling high-frequency materials applies as a matter of routine, and their absence is usually visible in yield rather than in a specification sheet.
RO4350B’s value is that it behaves consistently from 4 mil to 60 mil, so the thickness decision can be made on mechanical and thermal grounds without reopening the electrical design. Thin panels buy board area and short electrical length; thick panels buy heat spreading and rigidity. Pick the thickness the application actually needs, confirm that the line widths stay inside the etch window, and the rest of the build follows the standard process.
Frequently Asked Questions
Dielectric constant of 3.48 (±0.05) and dissipation factor of 0.0037, both at 10 GHz. In-plane CTE is 11 ppm/°C against 32 ppm/°C in the Z axis, thermal conductivity is 0.62 W/m·K, water absorption is 0.06% and the material is rated UL 94 V-0.
0.2032 mm (8 mil) or 0.508 mm (20 mil). The 8 mil panel keeps the antenna geometry compact, while the 20 mil panel provides more mechanical body for radar units mounted in exposed positions.
Only marginally. Dk varies very little with thickness, which is why an impedance model built for one panel thickness generally holds for the others and only the line geometry needs to be recalculated.
Between 175°C and 210°C, with pressure adjusted upward for thicker stacks. Thin panels need gentler handling to avoid deformation, and thick panels need uniform pressure to bond completely.
When the link budget is tight. Low-profile LoPro foil reduces skin-effect loss compared with standard foil, at the cost of slightly more demanding adhesion control during imaging and lamination.
Send us your impedance targets, layer count and thermal load and we will confirm which RO4350B thickness and copper foil combination suits the build.
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