A 1+N+1 HDI stackup places one build-up layer on each side of a core structure containing N copper layers. It is useful when surface-to-adjacent-layer microvias solve the critical escape-routing problem. It should be selected from a routing study, not from the assumption that every compact board needs several build-up stages.
Read the notation correctly
In an illustrative 1+6+1 construction, the total is eight copper layers. The top and bottom build-up layers surround the six-layer inner structure. The label alone does not say how a net entering layer 2 reaches deeper layers. A drawing must identify any buried or through-hole connection used for that part of the path.
Check the actual available route under the densest package. A microvia can free space at the surface while the next connection consumes space on an inner layer. Moving a bottleneck below the surface is not the same as eliminating it.
Review the layer functions
| Location | Review question |
|---|---|
| Top build-up | Which pads need microvias and which can escape on the surface? |
| First inner layer | Is this a routing layer, a reference plane or a deliberately managed combination? |
| Core structure | How do signals reach deeper layers without blocking critical channels? |
| Bottom build-up | Does the opposite side have a genuine routing requirement or a construction-balancing role? |
When the first build-up pair is enough
Map the nets requiring layer changes before selecting the structure. If one transition opens enough channels and preserves references, extra build-up stages may add complexity without changing the outcome. Conversely, a package with inaccessible inner-row signals may require a different arrangement even if the overall board has generous area.
Create a small layout experiment using the actual footprint, pad dimensions and proposed design rules. Route representative power, ground and signal groups. Include the large clearances and keepouts that are easy to omit in a simple fanout sketch. Ask the fabricator to review the experiment together with the full stackup.
Release package and common omissions
State the surface-to-inner via spans, finished dielectric targets and fill or cap requirements. Show any connection between the build-up and the core. A generic “microvia” drill label cannot substitute for these details. If impedance is controlled, identify the relevant layers and reference planes rather than listing only an impedance value.
Before ordering, obtain a written response to each exception in the supplier's construction proposal. The approved response should travel with the manufacturing data and the purchase order. This guide describes a design approach; supported layer counts, geometry limits and delivery commitments require project-specific confirmation.
Continue the engineering review
- HDI PCB stackup selection
- Fine-pitch BGA fanout with microvias
- HDI vs standard PCB construction
- HDI PCB engineering hub
Open the related planning tool. Review its assumptions before using an estimate in a design decision.
Review your HDI requirements
Share the board revision, proposed stackup, quantity and open questions. Include the requirements discussed in this guide so the project review starts with the same assumptions.