Gerber Files: The Key to Correct PCB Solder Mask Coverage
After a PCB is delivered, manufacturers sometimes receive questions or complaints about incorrect solder mask coverage, especially regarding whether vias should be exposed or covered with solder mask.
Some engineers assume that selecting an option such as “Via Open” or “Via Tenting” when placing an order is enough to determine the final result. In practice, however, such order-level selections may not override the manufacturing data.
The key principle is simple: Gerber Files are the primary manufacturing data used to define the actual PCB artwork, including the solder mask pattern. Order notes and production options serve as supporting information, but they should not contradict the supplied manufacturing files.
Understanding this relationship is essential for preventing PCB Solder Mask errors.
What Are Gerber Files?
Gerber files are standardized manufacturing files generated from the original PCB design data. Unlike the original design source files, Gerber files represent the manufacturing output and contain the graphical information required to fabricate the PCB.
They precisely define the position and shape of features on individual PCB layers, including:
- Copper traces
- Pads
- Copper areas
- Solder mask
- Silkscreen
- Other manufacturing features
For engineers preparing files for PCB Manufacturing, it is important to understand that each layer has a specific manufacturing purpose.
What Is a Solder Mask Layer?
The Solder Mask Layer is one of the most important Gerber layers for determining which areas of the PCB receive solder mask and which areas remain exposed.
A typical Gerber package contains dedicated solder mask files, such as:
- Top Solder Mask
- Bottom Solder Mask
These files define the areas where solder mask openings are required.
In simple terms, the solder mask artwork determines where the PCB surface is protected by solder mask and where copper or pads are intentionally exposed.
For vias, the relationship can generally be understood as follows:
- If the corresponding via location contains a solder mask opening in the Gerber solder mask layer, the via is exposed.
- If there is no corresponding opening, the via is covered by solder mask.
Therefore, the final solder mask pattern should be verified directly in the manufacturing files rather than relying only on an order-page selection.
Why Can’t the Manufacturer Simply Modify the Solder Mask?
Engineers sometimes ask why a manufacturer cannot simply change the solder mask when the order requirement does not match the Gerber files.
For example, an engineer may select “Via Tenting” during ordering while the supplied Gerber files clearly define openings around certain vias.
It may seem reasonable for the manufacturer to remove those openings and make the vias covered.
However, doing so without confirmation from the design side can create a different problem.
1. The Manufacturer Cannot Reliably Determine Design Intent
A manufacturer can see what the Gerber files specify, but cannot always determine why a particular opening was included.
A via with an opening may be intentionally designed as:
- A test point
- A thermal or heat-transfer feature
- A solderable connection
- An exposed via for inspection
- A specific assembly requirement
Alternatively, the opening could simply be the result of an incorrect design setting.
Without confirmation from the engineer, the manufacturer cannot reliably distinguish between these situations.
Changing the Gerber artwork based solely on an order option could therefore cause an intentional design feature to be removed.
2. Manufacturing Data Should Not Be Changed Without Approval
Manufacturers generally should not modify critical design data without authorization from the customer.
This includes parameters such as:
- Solder mask openings
- Copper patterns
- Hole sizes
- Trace widths
- Pad dimensions
- Board outlines
- Layer structures
This principle protects both the designer and the manufacturer.
It also reduces the risk of manufacturing a board based on an undocumented modification.
If a customer requires a change, the preferred approach is to update the original PCB design, regenerate the manufacturing files, and submit the revised files for production.
This creates a clear and traceable relationship between the design intent and the manufacturing data.
How to Prevent PCB Solder Mask Errors
The most effective way to prevent solder mask problems is to ensure that the design requirements and manufacturing files are completely consistent.
The following workflow can significantly reduce the risk of errors.
1. Define Via Requirements During PCB Design
Before generating the manufacturing files, determine how each type of via should be treated.
For example:
- Standard electrical vias may be tented with solder mask when appropriate.
- Test-point vias may require solder mask openings.
- Thermal vias may require specific solder mask treatment depending on the application.
- Vias used for soldering or inspection may need to remain exposed.
These requirements should be defined directly in the PCB design software.
For example, in Altium Designer, the appropriate via solder mask properties can be configured during PCB layout.
For projects that require professional design review, engineers can also use PCB Design & Layout services to identify potential manufacturing issues before production.
2. Check the Solder Mask Layer After Export
Do not assume that the Gerber files are correct simply because the original PCB design looks correct.
After exporting the Gerber package, open the files using a Gerber viewer and inspect the solder mask layers.
Pay particular attention to vias and other areas where solder mask coverage matters.
Check the following:
- Do all vias that should be exposed have the correct solder mask openings?
- Are the opening sizes correct?
- Are vias that should be tented free of unintended openings?
- Are there any unexpected solder mask openings?
- Does the solder mask pattern correspond to the PCB design?
This verification step is especially important before sending files for PCB Manufacturing.
3. Make the Order Information Match the Manufacturing Files
Order-page selections such as “Via Open” or “Via Tenting” should be consistent with the actual Gerber files.
If the manufacturing requirement does not match the Gerber output, do not attempt to correct the discrepancy by simply changing the order option.
Instead:
- Return to the original PCB design.
- Modify the relevant via or solder mask properties.
- Regenerate the Gerber files.
- Review the new solder mask layers.
- Submit the updated files for manufacturing.
This workflow provides a clear and traceable manufacturing instruction.
4. Verify the Files Before PCB Prototyping
For prototype projects, file verification is particularly important because the prototype is often used to validate both electrical performance and manufacturing feasibility.
A proper Rapid PCBA Prototyping workflow can help identify manufacturing problems before a design moves into larger production quantities.
For prototype orders, engineers should make sure that the Gerber files, drill files, BOM, assembly drawings, and other production documents are consistent before manufacturing begins.
Why Gerber Verification Matters Before PCB Assembly
Solder mask errors are not limited to bare PCB fabrication. They can also affect subsequent assembly processes.
An incorrect solder mask opening may influence:
- Solder paste application
- Pad exposure
- Solder bridging risk
- Component soldering
- Inspection
- Test-point accessibility
- Final assembly quality
For this reason, solder mask verification should be completed before the PCB enters assembly.
For prototype projects, a reliable Prototype PCB Assembly process should begin with accurate and fully reviewed manufacturing data.
After assembly, inspection and testing can further verify whether the completed PCBA meets the required electrical and functional specifications.
Gerber Files Are the Critical Manufacturing Reference
The fundamental principle of PCB manufacturing is “build according to the supplied manufacturing data.”
Whether a via is exposed or covered by solder mask should not be treated simply as an order-page selection. It should be clearly defined during PCB design and correctly reflected in the final Gerber output.
The complete workflow should therefore be:
PCB Design → Via and Solder Mask Definition → Gerber Export → Gerber Verification → PCB Manufacturing → PCB Assembly → Testing
If the design requirement and Gerber output are consistent, the risk of solder mask errors can be significantly reduced.
Ultimately, the best way to prevent PCB Solder Mask problems is not to rely on a last-minute manufacturing request, but to ensure that the design intent is correctly translated into the final Gerber files before production begins.
