RO4350B combines the RO4000 thermoset processing approach with a flame-retardant formulation. It is a useful reference material for RF boards that need controlled impedance, multilayer interconnection and compatibility with an established glass-epoxy fabrication line. The advertised material name alone does not define the thin-core dielectric or copper construction.
Material system
Ceramic-filled hydrocarbon thermoset reinforced with woven glass
Technical properties
| Property | Reported value | Conditions / source |
|---|---|---|
| Process dielectric constant | 3.48 ± 0.05 | Typical, 10 GHz, 23°C; IPC-TM-650 2.5.5.5 clamped stripline; thickness direction. Source |
| Design dielectric constant | 3.66 | Typical, 8–40 GHz; differential phase length method. Average across the common constructions in the cited datasheet. Source |
| Dissipation factor | 0.0037 | Typical, 10 GHz, 23°C; IPC-TM-650 2.5.5.5; thickness direction. Source |
| Thermal conductivity | 0.69 W/(m·K) | Typical; ASTM C518, 80°C; through-thickness thermal conduction. Source |
| CTE, X axis | 10 ppm/°C | Typical; -55°C to 288°C, IPC-TM-650 2.4.41; average expansion over the interval, not necessarily linear. Source |
| CTE, Y axis | 12 ppm/°C | Typical; -55°C to 288°C, IPC-TM-650 2.4.41; average expansion over the interval, not necessarily linear. Source |
| CTE, Z axis | 32 ppm/°C | Typical; -55°C to 288°C, IPC-TM-650 2.4.41; average expansion over the interval, not necessarily linear. Source |
| Thermal coefficient of dielectric constant | 50 ppm/°C | Typical; -50°C to 150°C, IPC-TM-650 2.5.5.5 (modified temperature measurement); 10 GHz. Source |
| Water absorption | 0.06 % | Typical; 48 h at 50°C, 60 mil specimen, ASTM D570. Source |
| Density | 1.86 g/cm³ | Typical; 23°C, ASTM D792. Source |
Typical reported values are not a purchase specification. Process Dk, design Dk and values measured at different frequencies are not interchangeable.
Selection and design notes
- Select it when the combination of fabrication practicality and RF performance meets the channel budget; do not choose it solely because a schematic operates above an arbitrary frequency threshold.
- A long feedline and a short matching network can justify different loss budgets even at the same frequency. Estimate dielectric loss separately from conductor and finish losses.
Thickness and copper options
Standard datasheet offerings include 4, 6.6, 10, 20, 30 and 60 mil; the 4 mil dielectric requires the specific Dk footnote.
18 and 35 µm ED copper are listed. LoPro adds a resin/foil construction and must be ordered and modelled explicitly.
Fabrication considerations
- Agree the actual core, prepreg and finished copper stackup with the fabricator. Include an impedance coupon that represents the RF layer and reference plane, rather than borrowing acceptance limits from a different layer pair.
Application fit
- RF front ends
- Power-amplifier matching
- Multilayer microwave boards
Limits and substitution checks
- The 4 mil standard material has a separate process Dk of 3.33 ±0.05 in the datasheet; the headline 3.48 value is not universal. RO4350B LoPro also has a distinct UL qualification note, so the base grade's status must not be transferred automatically.