SEM Lab, Inc. performed failure analysis on a diode bridge. Electrical tests results suggested that one of four internal diodes had short circuited.
The module was chemically decapsulated and the shorted die was isolated for SEM/EDS analysis. This is a BSE SEM image of the shorted die.
The suspected short site was located near a corner of the die.
An elemental map confirms that the suspected short is a silicon melt pipe.
Higher magnification images of the site also confirmed it is the breakdown site.
The size of the breakdown site is related to the melt-thru current , which in this case is estimated as ~14.3 amperes. The location of the melt through suggests that the breakdown was caused by a high voltage transient.
 J. T. May, “Limiting Phenomena in Power Transistors and the Interpretation of EOS Damage”, in Microelectronics Failure Analysis Desk Reference, 3rd Edition, ASM International Press, 1993, pp. 321-328.