Solder mask opening is smaller than copper pad, what is the problem here?

This is because our engineers found during the review that the opening size of the solder mask on some solder pads is slightly smaller than that of the copper solder pads. This will cause the solder pad edges to be partially covered by the solder mask ink, thereby affecting the soldering quality. This situation is not necessarily a design error, but it needs to be confirmed whether it is your intended design.

In normal design, we recommend that the solder mask layer be expanded approximately 0.05–0.1 mm beyond the copper pads. This ensures that the pads are fully exposed and the solder adheres evenly. If your design indeed requires a reduced opening (such as for special electrical performance or heat dissipation control), please note this when placing the order. We will produce strictly according to your file. If not specified, PCBfast will automatically optimize to the standard solder mask expansion method to ensure reliable soldering and production consistency.