Original Date: 02/01/1991
Revision Date: 01/18/2007

Litton Guidance and Control Systems Division (GCSD) has implemented comprehensive environmental stress screening (ESS) tailoring techniques on delivered hardware. ESS tailoring has taken the forms of adjusting the number and sequence of tests, and adjusting the stress levels of a given test based on the characteristics of the hardware. Key to the success of the tailoring is accurate hardware characterization of responses (g levels, temperature rate of change, and defect fallout rates). Vibration testing is performed followed by temperature cycling, a change that has proven more effective for detecting failures. Many failures precipitated by vibration are not detected until the temperature cycling is performed. This change in the screening order was made based on the Failure Reporting and Corrective Action Systems analysis of production incidents and field performance. In addition to the system level screening, Litton GCSD implemented 50 cycles of thermal shock (6 degrees/min) from -55 degrees C to +90 degrees C at the card level. By performing the additional screen, they were able to eliminate eight failures for every nine systems populated. An evaluation with screened high reliability components was performed to determine if the screen was still cost effective. The screen eliminated nine failures for every 13 systems populated. By adding additional temperature cycling and random vibration screens during the process, Litton GCSD has further improved the final acceptance test yield. The temperature and vibration screening levels were tailored to be more effective without exerting undue stresses on the assemblies. The screenable defect rate has decreased from 11% to less than 0.5%. The screening effectiveness has improved by a factor of 20 for fielded units. The throughput time was reduced by 30 hours per unit. A substantial savings in warranty costs was achieved in FY86 and FY87. The ESS imposed by Litton GCSD has been proven to be extremely cost effective by saving time and money; however, the most significant benefit is represented by the customers who are satisfied with the reliability and performance of the delivered system.

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