Prove why two levels are insufficient

Capture the failed routes in a two-level study. Separate blocked package escape from insufficient central routing channels, poor placement, and reference-plane constraints. A third access level helps only some of these problems.

For an illustrative high-pin-count module, the inner pin groups may require another independently reachable routing region. Show their complete paths with realistic lands and clearances. A diagram with arrows between layers does not establish that neighboring nets can occupy the same area.

Review combinations, not isolated vias

Three build-up levels allow more ways to stack, offset, or mix transitions. Assign a name to each permitted structure and exclude combinations that have not been reviewed. Where a path joins a central via, make the relationship visible in the cross-section.

An approved single microvia dimension does not approve every three-level chain made from it. Ask what construction-specific evidence supports the proposed interfaces and material system, and record any restrictions on stacking or landing positions.

Preserve the electrical reference strategy

Repeated layer changes can move a signal among different reference environments. Review the return path at each transition and keep nearby reference connections available. Do not spend all newly available space on signal vias before reserving what the electrical design needs.

  • Map reference layers for the critical signal groups.
  • Check power delivery and decoupling access at every depth.
  • Evaluate intermediate copper that blocks neighboring routes.
  • Keep test structures representative of the deepest relevant paths.

Compare against a different architecture

An illustrative twelve-layer 3+6+3 construction and a twelve-layer 2+8+2 construction allocate copper differently even though the total is identical. Compare their reachable channels and verification needs; also evaluate any-layer only if broader access has a clear purpose. Altium's stackup discussion is useful background for distinguishing build-up and central regions, but the final sequence needs fabricator confirmation.

DESIGN NOTE / SCHEMATIC

Adjacent-layer access, one connection at a time

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 the third level require three vertically aligned vias?

No. Alignment depends on the approved connection path. The build-up notation does not require one particular geometry.

Can qualification from a shallower stack be reused?

It can inform the review, but identify the new interfaces and exposures. Additional evidence may be needed for the changed construction.

References & further reading

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