Overview
The soldermask needs to be adjusted to compensate for possible misalignments of the soldermask. We use an automated system to detect whether a soldermask opening has already been adjusted or not.
Why is it required to adjust the soldermask opening?
Every process has imprecisions to a certain degree, this also applies to the soldermask of PCBs. A misalignment of the soldermask might cover up parts of a pad, resulting in less space to mount a component. A soldermask opening around a Pad compensates for some misregistration. The opening is typically 20~125µm larger per side than the copper pad, the value depends on the component. Reputable component manufacturers will often provide an example landpattern with a recommended soldermask opening.
Automated Soldermask Opening Adjustment
Some designs may not have adjusted soldermask openings, to provide an optimal PCB we use an automated system to adjust the opening. In this paragraph, we explain how the system works and what its capabilities are.
Our System assumes that optimized soldermask openings are larger than the copper pad. Square, Rectangular and Round openings can be optimized, irregular shapes will be left as is.
When a design is uploaded, our system will create a test opening/area which is 10 µm smaller than the soldermask openings in the design. The system will now create test points along the outline of the test opening/area and probe if the test shape touches copper. If all test points touch copper, we assume the opening was not optimized. We will then enlarge the original opening by 20µm.
If some points do not touch copper, we assume the soldermask opening was already adjusted.
Limitations
Gerber files do not contain any information about pads, so we can only detect whether copper is present or not. This means that the automatic system cannot correctly detect Pads placed in copper fills, as a result, they will not be adjusted. We do not categorize this as problematic, as small pads where a soldermask misalignment might be suboptimal are typically not connected to copper planes.