Solder Mask Defined pads vs NSMD

EPC recommends Solder Mask Defined pads. However, what are the disadvantages of NSMD pads?
Related to that, the paste opening with SAC 305 type 3 is bigger than the solder mask opening. Would that no increase the chance of shorts?

In SMD pads, solder mask openings are smaller than copper pads. Solder mask defined pads are suitable for BGAs. The solder mask covers the PCB area between adjacent pads and overlaps on top on pad edges.
Using NSMD pads can result in an asymmetrical solder bump since 100% perfect registration between the copper and mask layers in not likely. It can result in device sitting tilted on PCB. This issue does not arise with SMD pads.

Please refer application note –

Whenever the Solder Paste opening is defined, it’s with respect to the specific Stencil thickness ; simply because it’s the volume of solder paste dispensed, that’s important for proper reflow. Excessive amount of solder paste can lead to formation of solder balls & shorting issues. This is the reason, we strongly recommend referring to the data sheet of the GaN FETs. Data sheet specifies the “Recommended Stencil Drawing” along with the Stencil thickness and type of recommended solder paste. Following these guidelines results in good quality of soldering.

In addition, other potential advantages of solder mask defined (SMD) pads include:

  • Thermal: more copper, including copper planes, are possible under the parts, which conducts heat better. The solder mask takes care of containing the solder to a small portion of the copper.
  • Electrical conduction: for the same reason (more copper on the top layer)
  • Shielding: If wide copper traces or planes are used, especially for ground and DC buses, it may help with EMI shielding. Since the “pad” is just a solder mask opening, there can be a wide copper plane.