A Gerber preview that looks like the negative of the expected layer can result from several different issues. The viewer may be displaying clear and dark regions differently, the file’s polarity information may be interpreted incorrectly, or the user may be treating mask openings as mask material. Do not invert the image by hand until the cause is understood.
Ucamco’s Gerber specification downloads provide the authoritative definitions for layer image construction and polarity attributes. Use a viewer that interprets the format, then compare the rendered result with the design intent. Screen color by itself is not a manufacturing definition.
Choose features with known meaning
For copper, identify a known isolated pad, its connection, and a surrounding clearance. For solder mask, identify a pad that should be exposed and nearby board area that should remain covered. For paste, use a component with an unmistakable aperture pattern.
Write down what the displayed dark region represents in the current viewer. A mask file commonly communicates openings, so a large uncolored area need not mean that the board lacks solder mask. Confirm the layer function before evaluating whether its image is correct. Include the board boundary in the view so material regions are not interpreted against an arbitrary viewer background extending beyond the circuit.
Compare independent evidence
| Comparison | Question answered |
|---|---|
| Source CAD layer and exported render | Did export preserve the intended geometry? |
| Same file in a second compatible viewer | Is the discrepancy specific to one import? |
| Layer identity and polarity metadata | Does the file declare the intended interpretation? |
| Known pad and clearance features | Does the rendered image have physical meaning? |
Preserve the original files and viewer settings during diagnosis. If a supplier reports an inversion, request an image that includes a known feature and the interpreted layer identity. This is more useful than a general statement that the layer looks wrong.
Fix the export or interpretation
- Confirm that the correct source layer was exported.
- Review format and polarity options in the export configuration.
- Regenerate the file from the controlled design when needed.
- Repeat the known-feature comparison.
- Have the receiving workflow confirm the corrected interpretation.
A manual image inversion can produce an apparently plausible picture while breaking the relationship between geometry and metadata. Keep any conversion traceable and verify it as a manufacturing-data transformation, not a cosmetic edit.
Finish with a consistent rendered package whose layer functions are explicit. The Gerber layer-function review addresses identification; polarity diagnosis addresses how the identified layer’s image is interpreted.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist