RT/duroid 5870 uses random glass fibers in PTFE and provides a slightly higher dielectric constant than RT/duroid 5880. It is relevant to low-loss microwave lines and antennas where this particular permittivity gives useful geometry. The random reinforcement differs from the woven-glass architecture of CuClad or DiClad products.
Material system
PTFE reinforced with random glass fibers
Technical properties
| Property | Reported value | Conditions / source |
|---|---|---|
| Process dielectric constant | 2.33 ± 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 | 2.33 | Typical, 8–40 GHz; differential phase length method. Average across the common constructions in the cited datasheet. Source |
| Dissipation factor | 0.0012 | Typical; 10 GHz, C24/23/50; IPC-TM-650 2.5.5.5, Z direction. Source |
| Thermal conductivity | 0.22 W/(m·K) | Typical; ASTM C518, 80°C; through-thickness thermal conduction. Source |
| CTE, X axis | 22 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| CTE, Y axis | 28 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| CTE, Z axis | 173 ppm/°C | Typical; 0°C to 100°C, IPC-TM-650 2.4.41. Source |
| Thermal coefficient of dielectric constant | -115 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.
Selection and design notes
- Choose between 5870 and 5880 by examining the required trace width, radiator dimensions, loss budget and available thickness. Their similar names do not mean the same artwork produces the same impedance or phase delay.
- On a thin feedline, compare copper loss as well as dielectric loss. A change in Dk changes the width needed for a target impedance and can alter the conductor-loss comparison.
Thickness and copper options
Standard cores include 5, 10, 20, 31 and 62 mil with thickness-specific tolerances.
The datasheet lists both ED and rolled copper, including 18 and 35 µm. Select the actual foil before modelling loss.
Fabrication considerations
- Confirm the PTFE handling, drilling and hole activation process with the fabricator. Keep core thickness measurement separate from finished copper thickness, and use a representative RF coupon if the impedance or phase budget is tight.
Application fit
- Microwave antennas
- Low-loss feedlines
- RF test structures
Limits and substitution checks
- The low Df does not confer high thermal conductivity or eliminate temperature drift. RT/duroid 5870's thermal expansion is direction dependent, so multilayer reliability and mechanical mounting deserve separate review. Published rolled and ED foil options also should not be treated as electrically identical.