Heavy copper disputes rarely start in design. They surface at first-article review: the drawing calls for a 40 mil conductor, the microsection reports 33 mil at the top and nearly 42 mil at the base, and both parties can argue the part either conforms or fails. Nobody measured incorrectly. The drawing simply never stated which location the width refers to.
The thicker the copper, the more expensive that gap becomes. At conventional weights, the lateral loss from etching is small relative to the conductor width. Once copper enters the 10 oz and above range, the copper volume to be removed grows sharply, etch dwell time increases, sidewalls cannot stay vertical, and the difference between top and bottom width can approach the width of the tolerance band itself. Building to print only works if the print says enough.
1. Agree on what "copper thickness" means
A heavy copper data package can carry at least four different copper values: base copper as supplied on the laminate, plated thickness, finished conductor copper, and hole-wall copper. They are different numbers formed at different process steps. Inner layers are usually governed by base copper, while outer layers are base copper plus pattern plating, so whether an outer-layer "12 oz" means base or finished directly changes the required compensation.
The data set should state, per layer: the copper value, whether it is base or finished, the unit (oz or µm), and whether hole-wall plating is included. The applicable build range for a project can be raised through the heavy copper PCB page, with engineering confirming the feasible window. SKPCB's maximum copper capability is a conditional item: builds above 12 oz require engineering review to confirm feasibility and applicable scope, and must not be treated as standard production terms. Parameter definitions can be aligned first on the manufacturing capability page, with the project files then freezing the actual values.
2. An etched conductor is a trapezoid, not a rectangle
Wet etching acts from the copper surface downward. Copper closer to the base sees shorter exposure to the etchant and more shadowing from the resist, so a typical cross-section is narrower at the top and wider at the base. That sidewall taper depends on copper weight, equipment, chemistry condition, conveyor speed, pattern density and open-area ratio. It is not a fixed geometric constant.
Three consequences follow. First, one conductor has at least two widths, top and bottom, with a mid-height width also definable. Second, current-carrying and temperature-rise behaviour depends on effective cross-sectional area, not on a single width reading. Third, a design that estimates area as nominal width multiplied by nominal thickness may be assuming more copper than the finished part contains, because the sidewall consumed the margin.
3. Decide who compensates, and record it
Etch compensation adds the expected lateral loss to the artwork so that the etched result lands on target. It can be applied by the design side before releasing manufacturing data, or by the manufacturer during CAM using its own process factors. Either is workable. Doing both is not, and neither is each party assuming the other will handle it.
So the data notes need one explicit statement: the widths in the supplied Gerber or ODB++ are either design target values open to CAM compensation, or already compensated and not to be altered. The same note should say whether the target refers to top, bottom or mid-height width. The absence of that single line is the most common and most avoidable root cause of heavy copper first-article deviations. General package structure is covered in the Gerber preparation checklist, but the compensation basis must be declared specifically for heavy copper rather than inherited from standard-copper habits.
4. Bind width tolerance to a measurement location
A tolerance value and a measurement method form a pair; either one alone is not an acceptable requirement. SKPCB publishes a conductor width tolerance of ±20%, with applicable conditions per the tolerance page and the project documents. Tighter bands on heavy copper builds are case-by-case items requiring engineering review, not an extension of the standard figure.
An executable requirement states which width definition the tolerance applies to, how the measurement is taken (optical measurement, microsection, automated measurement), how points are selected (critical current-carrying conductors, narrowest conductor, dense areas), the sampling rule, and which document governs the verdict. If a customer writes only "width tolerance ±10%" without a measurement plane, the manufacturer and the customer can measure different numbers and both believe they are compliant.
5. Spacing, solder mask and panelization move with compensation
Compensation added to conductors takes the same amount away from spacing. In heavy copper, width and spacing are coupled variables: enlarging compensation to protect the finished width can push minimum spacing below process capability or leave incomplete etching in dense areas. Review the compensated spacing, not only the compensated width.
Solder mask belongs in the same review. Heavy copper conductors stand taller, so mask coverage over steep sidewalls is inherently thinner, and dam widths and openings must be assessed against the real copper height rather than thin-copper minimums. Position within the panel, copper distribution uniformity and border copper also affect local etch rate, so identical features can measure differently across one panel. Sampling should be designed to expose that systematic variation.
6. Close acceptance with microsections and a stated verdict basis
Surface measurement yields top width only and says nothing about the cross-section. Where the cross-section governs, schedule microsections and state in the data: section locations, the items measured at each (top width, bottom width, conductor height), quantity, and the drawing callout the verdict is judged against. Locations should target engineering concerns, one at minimum spacing, one at the highest current-carrying conductor, one each in dense and sparse areas, rather than random sampling.
For acceptance level, SKPCB can organize inspection to IPC-6012 Class II and Class III, with the class, revision and additional requirements set by project documents. An industry standard supplies a framework; project-specific cross-section requirements still have to appear on the drawing or in the specification, and a standard number is not a commitment to a particular width value. The broader inspection organization is described on the quality page.
7. Release checklist
Confirm before production: copper value per layer identified as base or finished; width definition specified as top, bottom or mid-height; manufacturing data declares whether compensation is included; compensation applied by one party only; width tolerance bound to a measurement method and point-selection rule; compensated minimum spacing still within capability; mask dams and openings assessed at actual copper height; microsection locations and verdict basis documented; copper weights beyond standard range reviewed by engineering with the applicable scope recorded.
With the checklist complete, submit the controlled package and measurement requirements through the quote entry, obtain written agreement on the width definition and acceptance method, then proceed to first article. Rework on heavy copper costs more than review, and stating the definition is the cheapest step available.
FAQ
Q1: Will the manufacturer add etch compensation automatically? Do not assume it. Whether compensation is applied must be declared in the data notes, together with whether the target is top, bottom or mid-height width. Both sides compensating produces oversized conductors; neither compensating produces undersized ones.
Q2: Where exactly is a heavy copper conductor measured? It depends on the drawing definition. Top, bottom and mid-height widths are three different numbers because the etched sidewall makes them unequal, and a tolerance without a stated measurement plane is not verifiable.
Q3: Does the ±20% width tolerance apply to heavy copper? The published conductor width tolerance is ±20%, with conditions per the tolerance page and project files. Tighter bands on heavy copper are engineering-review items, not an automatic extension of the standard value.
Q4: Why is a microsection more informative than surface measurement? Surface measurement reports top width only, while current-carrying behaviour depends on effective cross-section. A microsection gives top width, bottom width and conductor height together, which is the direct evidence for the design assumption.
Q5: Does compensation affect spacing? Yes. Widening conductors narrows the gaps, which can reach the minimum spacing limit or cause incomplete etching in dense areas, so width and spacing must be reviewed together.