High-layer-count HDI PCB: supplier and stackup review

What to consider

Use additional copper layers only where they solve routing, reference-plane or power-distribution requirements. For a 22-layer board, 2+18+2 is one arithmetic possibility; a 28-layer board could be described as 2+24+2. Neither notation specifies all buried-hole spans or press steps. At 32, 36, 38, 40, 42, 44, 48 or 52 copper layers, request a construction-specific engineering review rather than extrapolating a lower-layer capability table. Finished thickness and the smallest through-hole diameter can become more restrictive than the outer microvia geometry. Registration, resin flow, copper balance and cumulative thermal exposure require attention across the complete panel.

A practical review example

Compare a proposed 40-layer construction with a smaller layer-count alternative that uses a different placement or breakout strategy. Keep the connector constraints, channel-loss budget and acceptance criteria identical. Ask each supplier to identify which portions of the proposed process are demonstrated production and which require development coupons. A successful sample on another material or thickness does not qualify this stackup.

Checks before release

1. Show the full copper allocation and every hole span.
2. Calculate drilled-hole aspect ratio from the proposed finished thickness and agreed diameter convention.
3. Request relevant registration, electrical-test and thermal-stress evidence.
4. Separate tooling and qualification costs from the recurring board price.

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.