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“Explosive decap” is a tongue-in-cheek term referring to blowing a semiconductor device to bits (usually by accident). While this is rarely intentional, the remaining fragments can be interesting to study.
The primary mechanism of explosive decap is the same as “popcorning” in BGAs: trapped water vapor in the packaging boils, has nowhere to escape, and ruptures the packaging with a small steam explosion. There is, however, one key difference: popcorning is induced by the soldering process (and typically is less dramatic in appearance due to the slower thermal gradients). Explosive decap, however, is much more rapid: common malfunctions such as short circuits or connecting power backwards will heat the device from room temperature to >Tjmax in a fraction of a second, causing much more vigorous fragmentation.
Small pieces of chip shrapnel may be thrown several feet in extreme cases. The example described below involved connecting a 7805 linear regulator to a miswired (unprotected and polarity reversed) 11.2V Li-ion battery pack. The extremely low ESR battery voltage being dumped into what was effectively a forward-biased zener diode caused a massive thermal spike which blew the front half of the TO-220 across the room and exposed most of the die.