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40 The PCB Magazine • November 2015 running within ideal process parameters, as op- posed to a program that is merely designed to keep a machine from breaking down. Output monitoring: Critical outputs are monitored and recorded at the source. Measure- ments include plating thickness, etch ratio, trace width, metal composition, via wall integrity, and other numerical measurements. Discrepan- cies found should generate a "non-conforming material report." Non-conforming material re- ports should be regularly reviewed, corrective actions identified, and results tracked. This on- going corrective action system becomes a closed loop system of "plan-do-check-act" as part of an effective continuous improvement program. Similar systems for process control have proven effective in achieving desired quality levels at ac- ceptable yields across a wide range of industries. Effective process controls are a critical, but incomplete, method for insuring proper custom- er product quality. The addition of strategic in- spection and monitoring operations also play a role. Within the world of printed circuit boards, several containment methods are common: • Bare board automated optical inspection (AOI): Any discrepancy in conductor width and spacing is flagged by this test. All layers should undergo this 100% in- spection. • Bare board electrical testing: These circuits without components often undergo an electrical test designed to catch all opens and shorts. This can be done with a bed- of-nails test fixture or with a point-to- point flying probe tester. • Populated board electrical testing: Circuits with connectors, passive components and/or active components may also un- dergo a functional electrical test. • Populated board AOI: Optical inspection machines also are used to examine popu- lated circuits to ensure all components are present, orientation is correct, and loca- tion accuracy is proper. • Quality audits: In-process inspection and testing are done to a statistically based sample of a production work order. • Customer specific audits: Unique data gathering is often required by certain customers or within specific industries, especially those involving high-reliability applications. Specific features are verified or test data is generated to insure quality conformance. These requirements can be as varied as the applications within our wonderful world of electronics. Printed circuit fabrication involves process- ing through a long list of sequential operations. Producing a completed part might involve drill- ing, imaging, etching, resist removal, cover film attachment, lamination, surface treatment, and many other individual processes. In many cases, fabrication involves 40 or more unique process- ing steps. A 99% yield at every process would result in a final lot yield of less than 67%, which would likely be too low to sustain the business. Yields of 100% at most processes are required for profitability, and good process controls are required to achieve this level of product robust- ness and predictability of the output. PCB Dave Becker is vice president of sales and marketing at all Flex Flexible circuits llc. to view past columns or to reach Becker, click here. Figure 3: Flex circuit with component assembly. ProCeSS ConTroLS For FLexIBLe CIrCUIT FABrICATIon All AbouT Flex