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38 The PCB Magazine • November 2015 lenges. The following techniques should be used for controlling the wet process operations: • Statistical process control on chemistry • Control of process parameters • Preventative maintenance • Monitoring of process outputs Chemistry: Statistical process control tech- niques are frequently used for controlling criti- cal chemistry parameters. Consistent output is maintained by tightly controlling chemical bath compositions. In a high product-mix envi- ronment, copper thicknesses with varying treat- ment types can change several times during a shift. This will necessitate adjustments in pa- rameters such as conveyor speed, but chemistry and chemical properties are monitored to stay constant within a controlled range, allowing for a more statistically valid control system. Process parameters: Process parameters may include dwell time, conveyor speed, bath temperature, spray pressure, voltage and cur- rent. Each of these parameters is controlled within a set range, and the exact parameter set- tings are based on a recipe card for each prod- uct family. Parameter settings for recipes are best when statistically determined by designed experiments; recommendations from material suppliers are a good starting point and should be combined with data gathered during process development. Preventative maintenance: A total pre- ventative maintenance program (TPM) should be developed based on analysis of equipment performance and designed to keep machines ProCeSS ConTroLS For FLexIBLe CIrCUIT FABrICATIon All AbouT Flex Figure 2: Flying probe electrical tester.