An increase in production encouraged ITT Aerospace/Communications Division (A/CD) to search for a way to reduce the test time and overall cost of its testing program. Sampling seemed to be a logical choice to accomplish these goals and still produce a quality product. The company recognized the usefulness of testing as well as the quantity of knowledge gained (e.g., process health via in-circuit testing, materials quality via functional testing, design robustness via environmental testing). The company developed a Test Time Reduction philosophy to ensure that a reduction in testing would not impact the quality and reliability of the product. This philosophy states:
The organization must have a commitment to the program and yield improvements.
Based on MIL-STD-105, ITT A/CD implemented three types of sample testing. These consist of (1) Continuous Sampling which alternates between 100% testing and sample testing; (2) Parameter Sample Testing where each unit receives a particular set of tests, and all other parameters are subjected to sample testing; and (3) No Testing where yields are monitored at the next higher assembly test.
Continuous Sampling involves selecting three units from a lot of ten. If any of the three units fail, then the entire lot is fully tested. If all pass, then the remaining seven are recorded in the system by scanning their internal control codes. ITT A/CD processes five lots in this manner, and the resource manager keeps track of the attribute data. If any two of the last five lots fail, then the system puts testing of that unit into a controlled condition where all units receive 100% testing until five consecutive lots have a 100% yield history.
Parameter Sampling Testing also involves a lot of ten units. Three units receive full-scale testing while the remaining seven only get critical parameter testing. A critical parameter is defined as a parameter that does not receive a test anywhere else in the flow, deemed as part of a reliability critical process, and has a yield problem. Like Continuous Sampling, the resource manager keeps track of the attribute data, and if any failures occur, the same five lot approach is used.
In No Testing, yields and troubleshoot information are monitored at the next higher assembly test. A test engineer monitors the yield data at the radio level, and instructs the system on whether or not to perform testing. Efficiency is based on the skill of the test engineer.
Currently, ITT A/CD uses the Test Time Reduction process on three products. The test time reductions were, respectively, 2.5 hours to less than one hour; 5.8 hours to less than 3.5 hours; and 0.9 hour to 0.4 hour. These results can become significant time savings for large in-house production programs at ITT A/CD. The company estimated one contract as having a potential savings of 16,392 test hours over a 35,000-quantity radio build.
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