Mathews Malnar and Bailey, Inc. Quality engineering, applied statistical consulting, and training services for R&D, product, process, and manufacturing engineering organizations.

Transistor Secondary Breakdown (Transistor.ppk): The secondary breakdown failure time at 220Vce of a bipolar transistor used in a power supply was a CTQ quality characteristic. A series of 40 units from a small preproduction run was tested. The LSL for time-to-failure was 1 second and the process performance goal was Ppk > 1.33. The original analysis with Ppk Study shown in Figure 1 indicates that the failure times were not normally distributed. Based on historical experience with secondary breakdown times, a log (base 10) transform was applied to the times and the transformed response was reevaluated. (After the log transform, the new LSL is log(1) = 0.) The Ppk Study report of the log-transformed times shown in Figure 2 confirms that they were at least approximately normal and in statistical control. The original 40 observations were not quite sufficient to confirm that the process performance met the  goal (P(1.31< Ppk < 2.09) = 0.95), but a review of the design FMEA determined that the observed performance was sufficient.