An inner layer is a conductor layer
Inner PCB layers may carry signals, power distribution or reference planes. They are patterned copper within the board, separated by dielectric. The number of copper layers is different from the number of dielectric sheets, and a plane layer still needs a complete released definition of clearances and connections.
Map every layer name to its position in the stackup. Verify reference planes, copper pours, split regions and antipads around holes. An incorrect internal clearance may not be apparent from a photograph of the finished outer surface.
Pattern the copper before bonding
In a conventional subtractive process, resist defines the copper pattern on the core, unwanted copper is etched away and the temporary resist is removed. The detailed sequence varies with construction and process. Do not assume an outer-layer pattern-plating sequence describes every inner layer.
Surface treatment prepares the patterned cores for bonding. These preparation steps must suit the laminate and downstream process. A material substitution can affect more than the dielectric data sheet; review its processing and bonding requirements as well.
Control registration and dimensions
The core can change dimension during processing. Manufacturers use registration references and process compensation to keep layers aligned with drilling and later operations. Critical pad-to-hole relationships require sufficient design allowance and an agreed tolerance budget.
For controlled-impedance structures, use the finished copper geometry and approved dielectric construction. Nominal CAD line width alone does not describe the etched cross-section. Ask how the fabricator will handle critical conductors and what changes require your approval.
Inspect before the layer is hidden
Optical comparison against released data can identify copper-pattern discrepancies before lamination. Suspected opens, shorts, residual copper and damaged features need disposition at this stage. Agree the applicable acceptance requirements and how any permitted repair is documented.
After bonding, additional evidence may come from electrical testing, registration checks or destructive samples. These methods answer different questions and do not replace the opportunity to inspect the exposed inner pattern. Include inspection and traceability requirements in the purchase package, together with the approved stackup and netlist reference.