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74 SMT007 MAGAZINE I AUGUST 2019 automated conformal coating machines are equipped with a large pressurized material pot and long fluid delivery lines to the spray valve. With this low-outgassing polyurethane material having such a limited pot life, it was determined that long fluid delivery lines and a large material pot would cause significant maintenance issues in the long run. During the machine evaluation process, a more direct material delivery system would be required. One option was to have the material directly fed to the spray valve by syringe; this is the best way to reduce maintenance issues that could have occurred if long fluid delivery lines were used. Also, it was necessary to evaluate machines that could mimic the dexterity of the operators; therefore, machines with X-Y-Z rota- tions, fourth-axis motion, and selective spray valves would have to be evaluated. Machine Evaluation and Selection After reviewing the current process and the material and customer requirements, the machine evaluation process began. At this point, preliminary research was done on auto- mated conformal coating machine manufactur- ers. Manufacturers were contacted to determine their level of experience with the polyurethane material to be used in the machine. When eval- uating the machines, it was important to thor- oughly review the machine specifications and limitations. It was also important to review the machine software and determine how easy it is to program. Working with the manufacturer helped narrow down the machine selection before testing them. After selecting machines, some were tested by the manufacturer using their recommended settings on coupons and with sample cir- cuit card assemblies. It is best to perform the machine evaluation on-site with the manufac- turer to validate the process being used. It was important when evaluating machines with the manufacturer to understand all of the machine and spray-valve parameters that can be manip- ulated. Parameters—such as valve distance from the substrate, atomizing air pressure, material syringe pressure, valve speed, and material flow rate—will need to be adjusted during the process development. While choos- ing the automated conformal coating machine, it was very important to understand the pro- cess, material, and customer requirements to ensure the proper machine and features are selected. Process Development Once the machine was installed, process development began. Using the machine manu- facturer's recommendations as a starting point, multiple parameters were adjusted over several weeks of performing experiments to develop the process. To begin, before spraying any cir- cuit cards, the process was tested on the flat steel coupons. Starting with the manufactur- er's recommendations, individual parameters were increased or decreased and tested to see the effects on the coupons. For each coupon processed, the thickness data was measured. It was at this time that the fogginess, as described previously, was observed and eval- uated. Once the fogginess was deemed to be acceptable, process development continued with additional experiments to begin auto- coating on assemblies. Some of the parameters that were adjusted were the material-to-thin- ner ratio, number of coats, valve distance from the substrate, material syringe pressure, atom- izing air pressure, valve speed, and material flow rate. These parameters were adjusted and tested to develop a proper spray pattern. The spray pattern is important to guarantee compo- nent and surface coverage are sufficient. The most important parameter for proper spray pattern was the material flow rate. If there are gaps in the spray pattern caused by the width of the spray, then there will be voids in the coating that will require a touch-up. Once an ideal set of parameters were established, and the thickness was validated, sample circuit card assemblies were coated using the estab- lished process. It was observed that the material moved dif- ferently on a circuit card while it appeared flat and smooth on the unencumbered steel cou- pons. Due to this, more experiments and adjust- ments were made to optimize the process after spraying populated boards. It is important to