Issue link: https://iconnect007.uberflip.com/i/1428512
NOVEMBER 2021 I PCB007 MAGAZINE 55 bination provides electrical rules analysis, test point analysis, test strategy optimization, and test cost modeling based purely on schematic information. is, in turn, provides valuable layout guidelines that can be used to optimize the printed circuit board layout. Once the PCBA layout is completed, a mechanical DFT analysis must be conducted to confirm the nets that require test access are not compromised by solder mask, component outline, adjacent probes constraints, etc. Production Test Model e test coverage is the percentage of defects that can be captured by a combination of inspec- tion and test machines. First pass yield (FPY ) is the percentage of boards that pass the test. It can no longer be considered a good measure of the production quality. is is easily dem- onstrated by a test coverage of 0% which will result in a first pass yield of 100%. is leads to the question: Is a board good because it passes the test? From practical experience, we can ask, "Are all failing products really faulty?" For the same reason we may ask, "Are all products that are shipped good products?" e answer to both questions is no. We cannot assume this with- out knowing about the extent of the test cov- erage. "Slip" is the escape rate. is is a key met- ric and represents the faulty products that will be shipped to the end customer and can be returned as field failures. Ultimately, the "slip" is how the end-users will measure the final quality. If a PCBA is failing at system test it is because it fell into the escape rate (or slip), and this is usually much higher than expected. ere are two possible reasons why this situ- ation occurs: • e DPMO (defect parts per million opportunities) figures are higher than expected • e combined coverage is lower than optimal Incorrect DPMO figures are probably due to limited defect traceability or incorrect root cause analysis. e unexpected low coverage could also be due to the use of inadequate cov- erage metrics, such as the confusion between test accessibility and testability. e key to reducing the slip rate is hav- ing a good understanding of the test cover- age. By using tools to gain an understanding of the test coverage, and identifying test cov- erage gaps that need to be filled, the user will be able to reduce the slip as much as possible and, in doing so, reduce the amount of field returns, or bad products, going out the factory door. Sometimes companies need to have return on investment (ROI) justification on the pur- chase of tools to help achieve this. To respond to that question, it isn't hard to show the jus- tification based on reducing the bad product leaving the factory floor. Retuned product is costly to everyone in time, materials, repu- tation, etc., so reducing product return is a large cost savings, and easily justifies the cost of tools to help reduce slip and therefore bad products. As mentioned, the key to understanding how good the process is, and from shipping bad product, is having a good measure of the test coverage. We must be able to measure the effec- tiveness of the test to a relatable metric from any test strategy whether it be electrical, opti- cal, X-ray, or functional test, etc. We need to consider all the manufacturing defects within the defect universe, including missing compo- Figure 5: Production model.