Life Stories #7 NPF Reduction

After the HTS application was used throughout manufacturing, we begun to notice a series of periodic test failures that could not be traced back to a specific component failure. As part of the Lean manufacturing initiative, these failures were labeled “No Problem Found” (NPF), and they were set aside for investigation into the root cause of the failures. I lead the team that was responsible for investigating the root cause of all of the NPF failures, and returning the inventory back to production after the root cause was solved. The largest obstacle to this project was the small numbers of failures and the difficulty of reproducing them. This project became part of the CAPA process for manufacturing.

To dig down to the root cause, first we collected all of the failure information from the HTS database. This was ranked, in order by the number of occurrences. Then, we selected the NPF with the highest number of occurrences and began the investigation into the root cause of the test failure.

Most of the changes to the HTS test system were to improve test timing, work around a product software flaw, or to implement new test requirements. When changing a test requirement, we completed statistical analysis of the test results for the biological parameter in question, designed new limits for the parameters, and reviewed the changes with the design engineers before making the changes.

When we made updates to the HTS test system, we discovered that the failures were usually due to a race condition in the test software, and that we could make slight timing changes to the timing of the tests to resolve the problem. We would do this after analyzing the statistical distribution of the timing of the hardware system under test.

Over a period of two years, the team determined the root cause of 28 different NPF issues. We returned over 1000 units back from the NPF area back into production. The rate of NPF issues was reduces to less that 1% in the Dash 3000 HLA area.

This project increased the productivity for High-Level Assembly area, and we saved ~$64K in cost avoidance in 2003 and $46,000 in cost avoidance in 2004, by moving the solutions to other Acquisition products. The cost reduction comes from the removal of unnecessary rework and retest costs in manufacturing.