Non-Solder-Mask-Defined Pads

Copper Adhesion

The bond between the copper pad and the fiberglass substrate is the primary source of strength. Because the solder mask does not overlap the copper, the pad is free to expand and contract during thermal cycles. This freedom reduces the strain at the interface between the mask and the metal.

Improved mechanical reliability is a common result of this design choice.

Stress Relief

Solder joints are less likely to crack under mechanical shock when they have a consistent shape. The solder wetting covers the vertical walls of the copper pad. This creates a strong anchor point.

In contrast, mask defined pads often suffer from stress concentrations at the mask edge.

Manufacturing Tolerance

Registration errors between the mask and the copper layer have less impact on the final pad size. The copper etching process determines the landing area.