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78 SMT007 MAGAZINE I AUGUST 2019 Process Controls Once the process was established and val- idated, the next step was to set up process controls for the critical parameters. A prop- erly documented process and properly trained operators are important factors for process control. It is important that all of the critical parameters are documented in a process con - trol document. Most parameters in the auto- mated-spraying process are values stored in the machine programming, such as the noz- zle distance from the substrate, nozzle speed, number of coats, and curing profile. These values cannot be changed on the equipment and are also documented for reference in the process control document. Some parameters are manually adjustable—such as atomizing air pressure and material syringe pressure— but are monitored by the machine and set up to have machine alarms if they are out of spec - ification. Some critical parameters—such as thinner to material ratio measurements and maintaining the material pot life—while still documented, are left to the operator training and discipline. One of the critical parameters that required well-documented controls was material pot life. As it was discovered in the FTIR testing of the coating coupon samples, a difference in the material was observed when the pot life had expired. In addition, poly- urethane materials have noto- riously short pot lives, which makes them difficult can- didates for automated pro- cesses due to the maintenance requirements. Setting up pro- cess controls to ensure mate- rials are used within their pot life can help eliminate issues that could arise due to expired material. It is important that the operators are properly trained and understand the strict pot life requirements and that the process is properly documented. It was expected that through- out the pot life the material would begin to thicken, which would cause the flow rate to decrease. This theory was tested by processing circuit card assemblies and cou- pons continuously through the automated process for the entire pot life to observe how the material was expected to change. It was observed that the material did begin to thicken causing the flow rate to slightly decrease. This resulted in the spray pattern slightly chang- ing after about three-quarters of the pot life. This was consistent with the current hand- spray process that the operators had observed as well. Reduction in flow rate may cause a decrease in final coating thickness. Therefore, when the mixed material reached three-quar- ters of the pot life, it was necessary to purge all remaining material from the machine and completely remove and clean the valve. To control this requirement, the process has been set up to spray a coupon with one coat and a coupon with the final process for produc- tion, which includes multiple coats with each set of circuit card assemblies. These coupons allow for visualization of the process, and if any abnormalities in the coating or changes in the spray pattern are observed, the visual dif- ference signals the operator that it is time to change the material. Figure 9 is an example of how this coupon may look as the spray pat- tern begins to change; small voids will appear. Figure 9: Example of a one-coat coupon as spray pattern changes.