Dk describes dielectric response relevant to propagation and impedance; Df describes dielectric loss. Both need a frequency, test method and construction context.
What to consider
Effective dielectric constant for a route is influenced by geometry and the surrounding media. It is not always identical to a laminate datasheet value. Copper roughness also contributes to insertion loss, so a lower Df by itself does not describe the entire channel. Use the data appropriate to the field solver and design frequency.
A practical review example
For a surface microstrip, part of the field is outside the laminate. Using bulk laminate Dk directly as the route’s effective Dk can distort a simple delay estimate. A proper stackup solver accounts for the geometry.
Checks before release
- Keep units, frequency and test method with the values.
- Distinguish process, design and effective Dk.
- Include conductor loss in channel comparisons.
Connect this decision to fabrication
Use the material selector to organize the initial discussion. Keep proposed values separate from the approved drawing and document the effect of any change on the rest of the construction.
- Identify the exact grade, glass style, resin content and copper foil.
- Compare Dk and Df only with their frequency and test method.
- Confirm the material system is qualified for the proposed via structure.
Technical references
FR408HR material and processing resourcesMEGTRON6 material family and test conditionsRO4000 laminate data and processing resourcesReferences checked 2 October 2026. Manufacturer data describes its stated construction and is not an HDIPCB.org manufacturing commitment.