Failure analysis and finite-element analysis were used in conjunction to determine the cause of zener diode failures. A mechanical/plasma depot method was developed for the plastic-encapsulated SMB package and used to observe the presence of remelted extruded solder material on the die surface. That material provided a conductive path which manifested electrically as premature breakdown. Transient-thermal finite-element analysis was then used to show that a recent change of in-house surge test parameters could result in part temperatures during surge testing in excess of the solder melting temperature. These efforts lead to a respecification of the in-house surge test duration which resolved the problem.