One build-up layer on each side of an N-layer core provides a simple starting point for HDI routing.
What to consider
The outer microvias normally connect each surface to the adjacent core surface. Additional connections through the core require a separate strategy. A short blind via into L2 is not automatically a path to every inner signal layer. Power and ground placement can make the available escape depth more or less useful.
A practical review example
For eight copper layers, 1+6+1 uses a six-layer core. Mark the L1–L2 and L7–L8 transitions and then show how any signal that must reach L3 or L6 continues through the core.
Checks before release
- Confirm whether core holes are buried, through, or both.
- Reserve route channels around larger core lands.
- Compare the need for a second build-up against placement changes.
Connect this decision to fabrication
Use the stackup planner 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.
- Give every signal layer a defined return-path strategy.
- Review material and copper balance about the board center.
- Specify every via span and the factory-approved lamination sequence.
Technical references
HDI construction types and definitionsFabrication design guidelinesReferences checked 2 October 2026. Manufacturer data describes its stated construction and is not an HDIPCB.org manufacturing commitment.