Yesterday, I noted that we recommend NSMD (Non Solder Mask Defined), or pad defined, pad for BGAs. With the BGAs, the NSMD pads will allow the BGA to sag down just a bit more and adhere to both the top and the sides of the pad, resulting in a better mechanical connection.
Not all parts work that way though. International Rectifier has a package called "DirectFET" which is designed to use solder-mask-defined layouts. In this package, the FET source and gate connections are directly on the FET die. The drain connection is a plated copper can directly bonded to the drain side of the silicon die. This system gives a very low-loss capable part with great thermal conduction properties.
Unlike BGAs, though Internal Rectifier recommends solder-mask-defined pad layouts. Take a look at their application note 1035 for complete details on designing with this package.
Drain that dust spec. Drain that dust spec