Two types of land patterns are used for surface mount (SM) pads, non-solder mask defined pads (NSMD) and solder mask defined pads (SMD). The difference between the two is shown below.
NSMD pads have the solder mask opening larger than the pads, whereas SMD pads have the solder mask opening smaller than the copper pad. Most vendors suggest NMSD be used. The reason is it provides tighter control of copper artwork registration compared to the positional tolerance of the solder masking process.
Moreover, SMD pad definition may introduce stress concentration points that may result in solder joint cracking under extreme fatigue conditions. For NMSD, the pad size is typically reduced by 15 percent over the BGA diameter to achieve balanced stress on the solder joints. This method can also help to increase the routing channel on the primary layer.