In one of my PCB design, the microprocessor soldering leg pad size are .254mm x 1.4mm while the ground(GND) routing trace is .4mm thick. I'm trying to figure out is it safe to route the .4mm thick trace to the .254mm soldering leg pad of the microprocessor?
You should be OK with the large copper area under the pad. However, you should use solder resist to define the pad area (the purple lines on pads 3 and 5) if soldering with solder paste, to stop the solder running away from the joint. This is less important if you are hand soldering, as you can just add more solder during soldering.