To remove solder mask from a polygon in Altium, you need to use a positive region on the solder mask layer. This is because the solder mask is a negative layer, meaning that anything drawn on the solder mask layer is actually removed from the solder mask.
Understanding Solder Mask and Negative Layers
Solder mask is designed to protect the copper traces on a PCB from accidental solder bridging. It's applied as a coating and has openings where components are soldered. In Altium, the solder mask layers (Top Solder and Bottom Solder) are negative layers. This means that:
- Drawing on a solder mask layer = Removing solder mask. To expose copper, you place objects on these layers.
- Not drawing on a solder mask layer = Applying solder mask. Areas without graphics on the solder mask layer will retain solder mask coating.
Using a Polygon Pour to Remove Solder Mask
The key to removing solder mask from a polygon is to use a polygon pour on the solder mask layer, rather than a polygon cutout.
Steps to Achieve This:
- Select the appropriate Solder Mask layer: Ensure you are working on either the Top Solder or Bottom Solder layer, depending on which side you're modifying.
- Choose Polygon Pour: From the "Place" menu, select "Polygon Pour."
- Draw your Polygon: Draw the polygon shape that you want to remove solder mask from on the selected solder mask layer.
- Repour or Update: If the polygon is part of an existing copper pour (on a copper layer), you may need to repour the copper polygon to see the solder mask removal. You can update the polygon pour using 'Tools' > 'Polygon Pours' > 'Repour All' or by selecting a specific polygon and using the Repour option.
Comparing Polygon Pour and Polygon Cutout
Here's a table summarizing the difference between polygon pours and cutouts on solder mask layers:
| Action | Solder Mask Layer Effect | Use Case |
|---|---|---|
| Polygon Pour | Removes solder mask from area | Exposing copper for soldering or other purposes. |
| Polygon Cutout | Does not remove the soldermask, it removes the underlying copper polygon. | Creating openings in copper polygons, not solder mask. |
Example:
- If you want to expose a large area of copper for a heatsink, draw a polygon pour on the solder mask layer, sized accordingly.
Key Takeaway
In summary, to remove solder mask from a specific polygon area in Altium, draw a polygon pour on the corresponding solder mask layer. Remember, you are creating a positive region on the negative solder mask layer.
