Start with the actual routing problem

Identify why the board needs HDI before choosing the dielectric. A compact controller may need microvias only to escape an area-array package, while a communications board may need them to shorten electrical transitions. These situations can share a via geometry yet have different material requirements. Prepare a layer-by-layer list of critical signals, reference planes, dielectric gaps, and intended via spans. Use that list to ask for a producible construction before detailed routing begins.

Separate the material name from build-up suitability

Isola describes 370HR as an FR-4 material compatible with HDI and sequential lamination. That is evidence for a named material family, not a blanket statement about every FR-4 sheet. A useful purchasing record also identifies which thin cores and prepregs will be used. Ask the fabricator to explain whether its proposed laser drilling, cleaning, copper filling, and lamination sequence has been qualified for those exact constructions.

Evaluate electrical and thermal needs independently

Do not select a thermally robust material and assume the loss budget is solved. Conversely, favorable dielectric loss does not establish the durability of the via structure. For a short sensor connection, material loss may leave comfortable margin; a long connector channel on the same board needs its own calculation. Compare alternate builds with the same channel geometry and operating assumptions. Keep assembly heat exposure and expected field temperature cycling as separate inputs to the review.

Write a release decision that can be audited

An effective release note records the selected material system, construction table revision, copper option, and allowed substitutions. Attach the proposed stackup and identify which nets require impedance or loss evidence. If a lower-cost construction is offered, ask what physically changes before accepting it. A modest change to a build-up gap can affect both microvia geometry and routing width, so approval should consider the design as a connected set of constraints.

  • Resolve uncertain thicknesses before finalizing dense escape routing.
  • Record assembly cycles and the intended qualification vehicle.
  • Preserve the approved stackup with the manufacturing release.
DESIGN NOTE / SCHEMATIC

A construction is more than its laminate name

Signal copper

Dielectric · geometry + material

Reference copper

Core or build-up dielectric

Signal copper

Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.

Common questions

Does every HDI board require a specialty low-loss laminate?

No. Establish the channel budget and physical construction first. Some boards need HDI for density without needing an unusually low-loss dielectric.

Can the drawing specify only FR-4?

That leaves important choices open. Add the agreed material designation, construction requirements, and approval process for any proposed alternative.

References & further reading

Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.