Describe what the resin must fill
Give the fabricator copper coverage and feature distribution for the layers surrounding each bonding interface. Dense planes and sparse routing areas create different local geometries. A nominal sheet thickness cannot describe that entire situation. Isola's FR408HR processing guide treats prepreg handling and lamination as controlled processes requiring fabrication-specific adjustment. Use the guide as background for a construction discussion, not as a substitute for the fabricator's assessment of your actual artwork.
Distinguish delivered prepreg from cured dielectric
The stackup should identify the finished separation used for electrical modeling. Ask how the proposed glass and resin combination will achieve that separation after pressing. If an initial CAD stackup uses a catalog thickness, mark it provisional until the pressed construction is agreed. This matters particularly for a microvia span: an unintended increase in dielectric thickness affects the drilling and plating geometry as well as the impedance of adjacent traces.
Treat electrical inputs as construction-specific
Changing the resin-to-glass balance can alter the effective dielectric behavior of a composite. Do not preserve old Dk and Df inputs merely because the resin family name is unchanged. Request the construction table corresponding to the chosen prepreg and cured condition. For an impedance-controlled stripline, review both surrounding dielectric regions. When a board contains very different copper densities, ask whether the predicted local thickness variation needs to be represented in a tolerance study.
Resolve alternatives while routing is still flexible
Suppose a fabricator proposes an additional thin prepreg ply to improve lamination fill. Ask for the revised pressed thickness, electrical properties, and effect on symmetry before accepting it. The change may be reasonable, but its consequences should be assessed explicitly. Record the approved ply construction in the stackup release and define whether equivalent alternatives require engineering review. This makes later procurement decisions traceable and reduces the chance of rediscovering a thickness conflict after routing is complete.
- Review copper distribution around each bonding interface.
- Confirm cured thickness rather than relying on an unpressed value.
- Update impedance geometry when the construction changes.
A construction is more than its laminate name
Dielectric · geometry + material
Core or build-up dielectric
Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.
Common questions
Is higher resin content always preferable?
No. The appropriate construction depends on fill requirements, thickness, glass reinforcement, electrical behavior, and the qualified process.
Can two prepregs of equal nominal thickness be interchangeable?
Not necessarily. Glass, resin content, flow, and cured properties may differ. Compare the finished construction before approving an alternative.
References & further reading
Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.