Proper pad design is critical to solder your components onto your board efficiently. There are 2 common types of soldering methods for an exposed pad package - Solder Mask Defined (SMD) and Non-Solder Mask Defined (NSMD). Each has its own characteristics and advantages.
Solder Mask Defined (SMD)
SMD is to define the pad area to which the solder ball will be soldered. This method reduces the likelihood of the pad lifting during the soldering or desoldering process. The disadvantage, however, is that this method reduces the amount of copper surface area available for the solder ball connection, and reduces the space between adjacent pads. This limits the thickness of the traces between pads and may affect the use of vias.
Non-Solder Mask Definied (NSMD)
NSMD is to use copper to define the pad area to which the solder bump will be soldered. This method provides a larger surface area for the solder ball connection and provides more clearance (compared to SMD) between pads, allowing for wider trace widths and more flexibility in the use of vias, but NSMD is more susceptible to pad lift during soldering and desoldering. In this way, NSMD helps for better solder connections and allows solder joints to package pads.