This is a material-system decision as well as an RF comparison. RO4350B is a hydrocarbon ceramic thermoset candidate; RT/duroid 5880 is a filled-PTFE candidate. Their different geometry and processing requirements should be evaluated with the same circuit objective.
Translate the loss budget into a route
A long microwave feed and a short matching section do not assign equal importance to dielectric loss. Estimate the propagation loss over the actual line length, then include conductor roughness, transitions and radiation. A lower Df does not guarantee a proportionate improvement in total insertion loss. Identify which contributor dominates before investing in a material change.
Rebuild the geometry comparison
Open RO4350B and RT/duroid 5880 with their conditions and thickness options. The lower-Dk route changes line widths, resonator lengths and spacing. It may also affect the transition into a fixed device package or connector. A fair comparison uses separately optimized circuits that both satisfy the same impedance, board envelope and operating-band constraints.
Example: a mixed RF and control board
A board with many digital-control layers and a short RF path may favor a familiar thermoset multilayer process. A different board with a long low-loss feed may justify investigating PTFE despite the additional fabrication discussion. A hybrid stackup is another candidate, provided its bonding system, thermal behavior and reference planes are supported. These examples are decision patterns, not universal frequency rules or price claims.
Qualify the process you intend to buy
Confirm drilling, hole preparation, copper adhesion, handling and lamination for the selected construction. Ask the fabricator for the achievable finished dimensions and evidence relevant to the proposed stack. Include coupons that separate impedance control from propagation loss. Review flame-performance and any project-specific material requirements independently of RF performance. Compare quotations only after the geometry and acceptance package are equivalent; otherwise, a cheaper quote may omit the very test or construction that makes the comparison meaningful.
A procurement decision should identify who owns the stackup and impedance compensation. If the fabricator changes thickness to use available stock, revisit both electrical length and launch geometry. An approved laminate name cannot authorize an unreviewed change in the physical construction.
Source-aware property comparison
RO4350B
| 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.
RT/duroid 5880
| Property | Reported value | Conditions / source |
|---|---|---|
| Process dielectric constant | 2.20 ± 0.02 | Specification-labelled process Dk; 10 GHz, C24/23/50; IPC-TM-650 2.5.5.5, Z direction. Datasheet footnote 1 describes 1 oz ED copper acceptance testing. Source |
| Design dielectric constant | 2.2 | Typical, 8–40 GHz; differential phase length method. Average across the common constructions in the cited datasheet. Source |
| Dissipation factor | 0.0009 | Typical; 10 GHz, C24/23/50; IPC-TM-650 2.5.5.5, Z direction. Source |
| Thermal conductivity | 0.2 W/(m·K) | Typical; ASTM C518, 80°C; through-thickness thermal conduction. Source |
| CTE, X axis | 31 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| CTE, Y axis | 48 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| CTE, Z axis | 237 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| Thermal coefficient of dielectric constant | -125 ppm/°C | Typical; -50°C to 150°C, IPC-TM-650 2.5.5.5 (modified temperature measurement); 10 GHz. Source |
Typical reported values are not a purchase specification. Process Dk, design Dk and values measured at different frequencies are not interchangeable.
Select the material system that meets the circuit budget with a supported fabrication and verification route.