Failure Analysis Lab examines performs a failure analysis on surface mount glass encapsulated Zener diodes. These diodes were in an appliance control board, and they are surface mounted to the underside a printed circuit board. The entire board is encapsulated in rigid polyurethane. There was a significant yield loss due to cracking of the glass encapsulation around the active assembly. Glass failure analysis determined that the failure origin was at the end of the diode where the glass is in contact with the metal end contact. Examination of new diodes by glass experts found that the glass was cracked in the same location on new diodes. Thus, the failures were initiated before these diodes were assembled onto the printed circuit board.