Alongside the part announcement it released a diagram and its reasons for what it did.
The source-down mosfet is on the right, and the more conventional device on the left.
According to Infineon:
The source-down package (1) is externally the same as the (2) conventional drain-down 3.3 x 3.3mm PQFP package.
Internally, the die in Infineon’s latest drain-down mosfet (3) has less active die area that the new source-down mosfet die (4) as source-down construction removes the need for a gate contact cut-out.
While the drain-down copper clip (5) has to avoid the bond wire, the source-down copper clip (6) can be wider, lowering thermal resistance, as there is no bond wire to avoid.
The source down package (7) also available with ‘centre gate’ footprint (left) which increases source-drain creepage distance and ease layout when paralleling.
Infineon’s source-down product page is here – scroll down for a list of devices.