PCB Copper Thickness: Base Copper vs. Finished Copper
How starting foil, copper plating, and process losses affect finished copper thickness and the requirements recorded in a PCB stackup.
A copper specification can describe either the foil used at the start of production or the copper remaining on the finished PCB. These values are related, but they are not interchangeable. The distinction matters when the drawing, stackup, and quotation use different assumptions.
Base Copper and Finished Copper
Base copper is the starting copper foil on the laminate. Finished copper is the conductor thickness after the relevant manufacturing processes. Cleaning and micro-etching remove some copper; plating adds copper to layers that undergo that process.
On conventional unplated inner layers, the finished thickness comes from the starting foil less processing losses. On plated outer layers, it includes both the remaining base foil and deposited copper. Consequently, finished copper is not always thinner than base copper.
Copper Weight and Thickness Units
The familiar designation “1 oz copper” means a copper mass per unit area of 1 oz/ft². It corresponds to approximately 35 µm, or 1.37 mil, of uniform copper. This is a nominal conversion, not a guaranteed minimum finished thickness.
For uniform foil, the dimensional relationship is thickness = mass ÷ (area × density), using consistent units. Mass divided by area alone gives an areal mass, not a thickness. A finished conductor calculation also needs to account for the actual plating and processing losses.
Why the Distinction Matters in Manufacturing
Copper thickness influences current-carrying capacity and thermal performance. It also affects the practical relationship between trace width, spacing, and etching. Thicker copper needs more etch compensation, which can make closely spaced features harder to manufacture. Increasing copper weight therefore involves a review of the circuit pattern as well as the electrical requirement.
A useful manufacturing review separates these items:
- The starting foil specified for each layer.
- The required finished conductor thickness, including whether it is a nominal target or a minimum acceptance value.
- The plated-hole wall copper requirement, recorded separately from surface conductor thickness.
- The trace widths and spaces that must remain after fabrication.
Verification on the Finished Board
Microsection analysis of a board or representative coupon can reveal inner and outer conductor thickness and plated-hole construction. Where appropriate, it can also distinguish base foil from deposited copper. Surface finishes are separate coatings and should not be counted as conductor copper.
Clear layer-by-layer requirements connect the material choice to the acceptance check. They also avoid treating an approximate ounce-to-micrometre conversion as proof that a finished board meets a specified minimum.
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