I'm having shorts appear on my QFN packages between pins, usually only after a week or so of power-on operation. The shorts are 1K and slowly down to hard shorts over time. Typically a defective part will have failures on multiple pins. I tried having my assembly house switch to a "no-clean" process, however it hasn't eliminated the problem. I'm using Lead-free solder.

Does this problem seem familiar to anyone? I need minimally a test to tell me whether any given assembly is defective, and ideally a defect-free assembly method.