Issue link: https://iconnect007.uberflip.com/i/1428512
56 PCB007 MAGAZINE I NOVEMBER 2021 nents, wrong value, misalignment, incorrect polarity, damaged components, open circuits, short circuits, insufficient solder, and excessive solder. We must have test strategies in place that can detect these defects. e ability to detect defects can be expressed by a coverage facet, so that each defect category is aligned with coverage metrics. Table 1 details indus- try standard metrics that have been defined by Philips Research (MPS), ASTER Technolo- gies (PPVSF), Keysight (PCOLA/SOQ), and iNEMI PCOLA/SOQ/FAM. ese metrics allow the estimation of the theoretical coverage, or measurement of the real coverage, for each unique test strategy or combination of test strategies. No single test strategy can detect all the board defects. It is a combination of complementary test strategies that provide the necessary proper overall test coverage. When calculating test coverage, it is impor- tant to consider the DPMO that reflects the current manufacturing process. is way, the test coverage can be aligned so that better coverage is provided where there is a greater opportunity for defects occurring during man- ufacturing process. Industry 4.0: Closing the Loop e Industry 4.0 philosophy focuses on pro- viding a closed loop to identify where prob- lems exist and facilitate remedial action. is is done by measuring the actual test coverage and comparing it to the theoretical model to ensure compliance and understand gaps. An example of where disparity can occur between the expected test coverage and the achieved test coverage is where the test development and PCBA manufacturing is outsourced. It is imperative that the OEM has complete visibil- ity of what is achieved by their supplier. Oth- erwise, there is a good chance that an inferior product could be manufactured and shipped to the end customer. High escape rates also have a direct impact on the "no fault found" bone pile. e completed post debug test program should reflect the estimated coverage require- ments defined by the OEM. A product is passed through a test line which, step by step, detects specific types of defects. e production information is stored in a centralized database for traceability pur- poses. With Industry 4.0, raw data from var- ious machines are aggregated to build and visualize comprehensive information that allows humans to understand the defect uni- verse. Both defect per million (DPM) and defect per million opportunities (DPMO) are used for determining the overall quality of the unit under test (UUT), produced from the sample quantity inspected. DPM is a measure of manu- facturing throughput: how many bad parts slip through the manufacturing process. DPMO is a measure of performance: how many times a manufacturing defect class occurs. DPMO is also an indicator of which manufacturing pro- cess needs improvement. Test strategy and defect occurrences should be linked so that improved test coverage can be targeted toward defects that occur fre- quently. A lack of coverage on defects that never occur has no real bearing on the final product quality. Table 1: Test coverage metrics.