Distinguishing Base Copper from Finished Copper on a PCB Drawing

A note such as “one-ounce copper” can mean different things to a designer and a fabricator if the stage of manufacture is not stated. Inner and outer layers may also follow different process routes. Make the intended finished conductor requirement explicit where it affects current, thermal behavior, or controlled impedance.

Identify the manufacturing stage

Eurocircuits distinguishes starting foil from finished copper and explains that processing can remove copper while plating can add it. Its examples show why an outer-layer finished thickness cannot always be inferred directly from the initial foil designation. The supplier’s numerical tolerances describe its stated process and acceptance basis, not a universal guarantee for another shop. Eurocircuits: copper-thickness terminology.

Review every layer in the stackup. State whether its thickness entry is base foil, nominal finished copper, or a required minimum finished value. Keep hole-wall plating requirements separate from surface conductor thickness; they describe different features even when the same plating operation contributes to both.

Connect thickness to the design calculation

• Use the intended finished geometry for resistance and thermal evaluation.
• Check the copper-thickness assumption in controlled-impedance models.
• Review minimum trace and spacing requirements for the proposed copper construction.
• Identify local features whose performance depends on a minimum cross-section.
• Confirm the agreed inspection method and acceptance basis.

For example, a power trace calculation may assume a particular thickness while the fabrication order specifies only a starting foil. Ask the fabricator to reconcile those requirements before manufacture. Do not resolve the ambiguity by increasing all copper weights without checking fine-pitch routing and impedance consequences.

Avoid conflicting notes

A drawing, stackup table, online order form, and purchase specification should describe a compatible build. If one document says “finished” and another says “base,” clarify which governs and update the package. A verbal understanding with one engineer may not survive a later reorder or supplier transfer.

Review evidence at the right location

Ask how the supplier measures the critical thickness and how sampling relates to the delivered lot. A measurement on a convenient coupon is useful when its representation is understood. Keep minimum and average values distinct in the report.

Once accepted, retain the layer-by-layer copper definition with the electrical and thermal assumptions that depend on it. That makes a future cost reduction or process change reviewable: the team can see which finished features must remain equivalent and which starting-material choices belong to the fabricator’s qualified process.

For a critical high-current region, identify the particular layer and feature on the drawing instead of applying an unexplained global copper note.