Yes, solder masks can overlap, although it's generally undesirable and can lead to manufacturing issues.
When designing a printed circuit board (PCB), you typically want the solder mask to cover the copper traces and pads everywhere except where you need to solder components. Overlapping solder masks can create problems like:
- Exposed Copper: Overlap on the same side of the board between the copper layer and the solder mask layer creates exposed copper. These areas will either be solder-coated or gold-plated, depending on the board's surface finish. While intentional exposure can be fine (e.g., for test points), unintended exposures are problematic.
- Solder Bridging: When solder mask is too close to a pad, or is inaccurately applied, it can cause solder to flow between pads during reflow, leading to short circuits. Overlapping solder masks exacerbate this issue.
- Adhesion Problems: If the solder mask overlaps in unintended areas, especially component footprints, it can interfere with the proper adhesion of solder and components during assembly.
- Manufacturing Defects: Overlapping solder masks can cause issues during the PCB manufacturing process, such as uneven coating or bubbling.
- Thin Solder Mask: Intentionally designing overlapping solder mask is generally poor design practice and will result in a thinned solder mask around the pad.
Instead of relying on overlaps, design practices should prioritize proper clearances between solder mask and copper features. This includes:
- Solder Mask Clearance: Ensuring a sufficient gap between the solder mask and the pads to be soldered. This clearance value will depend on the manufacturer's capabilities and the design requirements.
- Solder Mask Expansion: Specifying an expansion value for the solder mask openings to ensure the pads are properly exposed.
While there might be niche cases where overlapping solder masks are used deliberately, these are rare and require careful consideration of manufacturing constraints. For example, a small overlap might be acceptable in very specific situations involving tightly spaced components and a highly controlled manufacturing process, but even then, it's preferable to achieve the desired result with proper clearances.
