Cause of diode failure in step-down switching mode power supply
After repairing a returned unit, take a very careful oscillogram across the Schottky with a short 10:1 scope probe (directly connect the tip and use a very short ground wire) and no bandwidth limiting on your good quality (100MHz or better, digital storage preferred) scope.

An example of a short probe, borrowed from elsewhere on EE.SE:



What you're looking for is high voltage (a spike, or ringing) on the leading edge of the voltage waveform. Any time you switch an inductive load, there's the potential for high voltage - unless you carefully look for it (i.e. with a short probe and a good scope) you won't see it. Your layout will have a great influence on whether or not there will be nastyness across the Schottky, so measure to be sure.

If you do end up seeing something ugly, you may need to introduce an RC snubber across the Schottky to keep the diode from avalanching.