2026-09-12
SMD Pad (Solder Mask Defined / Coverlay-Over-Pad Design)
Solder Mask Defined Land Pattern (SMD). The solder mask opening is smaller than the copper pad. In FPC manufacturing, it is commonly known as "Coverlay-Over-Pad" design.
Finished effect diagram of Coverlay-Over-Pad design: Equal pad length, not easy to break.
Finished effect diagram of Coverlay-Over-Pad design: Equal pad length, avoids SMT tombstoning issues.
NSMD Pad (Non-Coverlay-Over-Pad Design)
Non-Solder Mask Defined Land Pattern or Copper Defined Land Pattern (NSMD). The solder mask opening is larger than the copper pad. In FPC manufacturing, it is commonly known as "Non-Coverlay-Over-Pad" design.
Finished effect diagram of Non-Coverlay-Over-Pad design: Unequal pad length, easy to break.
Finished effect diagram of Non-Coverlay-Over-Pad design: Unequal pad length, prone to SMT tombstoning issues.
Solder joint pulling force refers to the bonding force between the solder paste and the pad.
(SMD Diagram / NSMD Diagram)
Although the copper area of an SMD pad is large, its perimeter is covered by solder mask, so only one surface participates in soldering. Under the same pad size, because the four sides of the NSMD copper foil also participate in soldering, it is equivalent to having three sides participating in soldering. The soldering area is larger than that of SMD, so the solder joint strength is higher.
However, during the PCB or FPC etching process, NSMD pads are relatively isolated and easy to be over-etched or side-etched. Reasonable compensation for the pad needs to be considered; otherwise, it will lead to undersized pads, which also affects solder joint strength and pad pulling force.
Pad peel strength refers to the bonding force between the component and the PCB or FPC substrate after soldering.
(SMD Pad Design / NSMD Pad Design)
The copper area of the SMD pad itself is large. Although the exposed area is the same as that of the NSMD pad, the actual contact area with the substrate is much larger. Therefore, the bonding force between the SMD pad and the substrate is much better than that of the NSMD pad. The pad and the substrate are relatively firm, and the pad is not easy to fall off.
FPC uses coverlay as solder mask. If an SMD pad design is used, the coverlay can press down the perimeter of the pad (commonly known as Coverlay-Over-Pad design), which will make the pad and the substrate even firmer.
a. Regular pad shape, unaffected by trace routing, suitable for small component pads such as 0402, 0201, etc.;
b. Not easy to tear or fall off during rework/repair, because the actual copper size of the SMD solder joint is relatively larger than NSMD, and the solder mask covers part of the pad.
a. Because the solder mask covers the pad, the stencil is higher than NSMD by the thickness of the solder mask, increasing the solder volume, which easily causes short circuits during production;
b. Reduces the copper surface area available for solder joint connections and reduces the spacing between adjacent pads;
c. The soldering strength of SMD pads is relatively weaker.
a. Larger soldering area and relatively higher soldering strength;
b. Larger spacing between pads, allowing wider trace widths and more vias.
a. Pad shape is affected by trace routing;
b. NSMD is an isolated pad, which easily falls off during the rework/repair process.
Advantages: The pad is firmly bonded to the substrate and not easy to fall off; no tombstoning effect (in cases of inconsistent pad sizes).
Disadvantages: Large pad area makes routing between pads difficult; fast heat dissipation, slightly worse tinning effect (cross-relief pads can be considered), and the stencil thickness requires extra compensation for solder mask thickness to prevent insufficient solder paste.
Advantages: Good bonding force between pad and solder joint; easy trace routing on pads; good tinning effect.
Disadvantages: Pads are easy to fall off, especially obvious on small BGA or FPC boards; if part of the pad is on thick traces or copper pour, it easily causes tombstoning effect, resulting in uneven soldering; pad edges are more prone to micro-cracks during temperature cycling from -55°C to 125°C.
PCB: Prioritize using NSMD pads. However, for BGAs and micro pads (such as packages smaller than 0201), SMD pads are recommended to prevent pads from peeling and falling off the substrate during rework.
FPC: Prioritize using SMD pads. The solder mask can press down around the perimeter of the pad, playing a role in supporting pad strength and preventing pad detachment.