Prepreg fills the spaces around copper during pressing and becomes part of the finished dielectric system. Its final thickness depends on the construction and process.
What to consider
A glass-style label is not a universal thickness specification. Resin content, copper coverage and the press process influence the cured result. Fine HDI features need a material and drilling process that work together; selecting an arbitrary thin prepreg without factory review can create voiding, registration or via problems.
A practical review example
When a core layer has a large etched area, ask how resin flow affects the dielectric above it. Use the proposed pressed thickness in impedance calculations instead of a nominal unpressed sheet value.
Checks before release
- Request pressed dielectric values.
- Review local copper coverage and resin needs.
- Confirm laser drilling and desmear compatibility.
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.