ATmega48PB/88PB/168PB can function as a drop-in replacement for the 32-pin TQFP package, and the 32-pin VFQFN with the 32-pin MLF package, but there are conditions to follow. See chapter Added/Modified Pin Functionality. For other package types, expect to change the PCB layout when migrating from ATmega48A/88A/168A to ATmega48PB/88PB/168PB. The data sheet includes dimensions and physical drawings of the footprint for each device under "Packaging Information". Below are the different package types compared to the different devices.