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90 PCB007 MAGAZINE I APRIL 2018 Figure 12: Illustration of the cost benefit of using a continuous controller. It reduces the total chemical requirements and keeps it from reaching dangerous minimum concentrations. Also displayed are actual readings of a copper reducer concentration with and without a controller. • A sensor or transducer sensitive to the concentration of a chemical species. • An analyzer/controller to convert the electrical signal to engineering units and provide outputs to feedback systems. • A replenishment system that refortifies the solution of the chemical species analyzed. Commercial systems are available for control of photoresist developers and strippers based on pH. Some commercial systems use simple panel counting as the control technique. Two examples of commercially available automat- ic controllers are a copper microetch control- lers—one using colorimetric analysis for cop- per, the other a specific gravity (displacer) sen- sor. Standard electroless copper (and MSAP) controllers use three standard sensors: colori- metric for copper, pH for hydroxide concentra- tion after the add of hydrochloric acid, and pH for formaldehyde after the addition of sodium sulfite, as shown in Figure 13. Any PCB process can be automatically con- trolled with the application of a DIY sensor based on: 1) Specific gravity; 2) Colorimeter; 3) pH, ORP or ion-specific probes; 4) CVS sen- sor and available basic stamp-propeller or Ar- duino or Raspberry Pi microprocessor units, as seen in Figure 14. Figure 15 shows a schematic of the high- speed research plating cell for MSAP copper via fill and the automatic chemical control unit. Six basic sensors can be utilized: Specific gravity; specific ion; pH/ORP; CVS; colorime- try and conductivity. Conclusion Chemical control of critical process solutions can be as simple as an indicator light attached to a specific gravity sensor or a continuous ana-