Solder mask is typically designed as a negative image.
This means that the solder mask layer defines the areas where solder should not be applied. The areas where solder is required are left exposed, allowing solder paste to be applied during the manufacturing process.
Here's a breakdown:
- Negative Image: The solder mask defines the areas where the mask will be present (covering the board). Solder is applied to the areas not covered by the mask.
- Positive Image (Less Common): In a positive image system, the solder mask would define the areas where solder should be applied. This is less common because it's usually easier to define the smaller areas of exposed copper pads.
Why Negative Image is Preferred:
- Precision: It is generally easier to precisely define the relatively small areas that need to be soldered (pads and vias) compared to the larger areas that don't.
- Manufacturing Tolerances: A negative image approach is more forgiving of slight misalignments during the solder mask application process. Small errors are less likely to expose unwanted copper.
In summary, while the concept of positive or negative is applicable to solder mask, the negative image approach is the industry standard because it offers better precision and more reliable manufacturing.
