Allocate electrical roles first
Begin with the critical interfaces and power rails. Mark which routing layers need an adjacent continuous reference, where power must travel, and where the component connections enter those regions. Avoid assigning all six layers as signal resources before discovering that the design has no coherent return path.
One illustrative role plan is signal, ground, signal, signal, ground, signal. This is an architectural sketch, not a recommended finished stack. Dielectric spacing, copper pattern, power distribution, and via access still determine whether it works.
Do not confuse a ground layer with an escape layer
In a 1+4+1 concept, the first surface microvia may land on a layer intended largely as ground. Signal lands must remain isolated from that copper and need a practical onward path. Confirm that those paths and clearances do not fragment the reference beneath important surface routes.
If the local escape requires extensive routing on that layer, revisit its role and the surrounding reference strategy. Retaining the ground label does not preserve plane behavior once the copper is heavily interrupted.
Budget power distribution explicitly
Texas Instruments' Sitara PDN guidance emphasizes that plane placement affects power-path inductance. For a six-layer design, review power delivery early because there may be little spare plane area after routing. Assign rail paths and decoupling connections before declaring the layer count sufficient.
- Estimate routing demand beyond the package escape boundary.
- Check the return path for each critical layer transition.
- Reserve copper and connections for major power rails.
- Include connector and test access in the placement study.
Recognize the point of diminishing returns
If every remaining route needs a minimum-width neck or an awkward detour, compare an eight-layer option. More central routing or better reference allocation may be preferable to increasingly restrictive geometry. The right conclusion may also be a placement change or slightly larger outline; evaluate these with the same electrical requirements.
Adjacent-layer access, one connection at a time
Dielectric · geometry + material
Core or build-up dielectric
Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.
Common questions
Is six-layer HDI automatically cheaper than eight layers?
No. Fine geometry, interconnect complexity, and acceptance requirements also matter. Compare reviewed constructions rather than layer count alone.
Can I copy a six-layer stack from another product?
Use it as a starting concept only. Package escape, power rails, reference needs, materials, and thickness must match your own design.
References & further reading
Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.