Hi,
Semiconductors fail because of:
* overvoltage
* overcurrent
* overtemperature
One of these may cause your device to fail.
* It's unlikly that you overheat a microcontroller in a way that it gets VCC and GND short. --> but check on overtemperature.
* I'd say it's unlikely that overcurrent will damage your microcontroller. --> but check on overcurrent
* overvoltage. Means voltage beyond specification, on any pin, even for a very short time, like nanoseconds. It could be positive or negative overvoltage.
Some possible sources of overvoltage:
* ESD. During manufacturing, storing, transport, assembling, or even on your board
* VCC overvoltage: Not properly decoupled voltage regulator, bad PCB layout, transients on power supply
* overvoltage on any pin: EMI, ESD, bad layout, (switching) inductive loads
There are standards to follow.
* use a solid GND plane on your board, without cuts, without other traces.
* use (multiple) vias and very short traces to connect the nodes to GND.
* place a power supply ceramics capacitor on every supply pin of every device with short traces to GND.
* place ESD protection on every signal that enters/leaves your PCB.
Most likely:
Bad PCB layout. --> show us your PCB layout.
Klaus