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76 SMT007 MAGAZINE I AUGUST 2019 test sample circuit card assemblies to ensure the chosen process will work on the prod- uct. Once the sample circuit card assembly was cured, it was inspected per IPC-A-610 and J-STD-001 requirements. Higher magnification was utilized during this process development to analyze the conformal coating quality and inspect more thoroughly for bubbles or other defects. The first process that was developed and tested during this experiment resulted in excessive bubbles around leaded components; the bubbles were bridging conductors and a defect per IPC-A-610 requirements, as seen in Figure 5 [2]. These bubbles could be caused by the material and process, but bubbles could also be caused by contamination or improper baking of the assemblies before application. When reevaluating the process to begin addi- tional experiments, it was determined that the material was sprayed on too thick, which cre- ated bubbles when the entrapped air under- neath components could not escape. The next set of experiments included a reduction in material flow and an increase in the number of coats applied as well as ensur- ing the assemblies were properly cleaned and baked before coating. By decreasing material flow, less material is applied at one time and applying multiple coats allows the material a chance to settle before the next coat is applied. More sample circuit cards were sprayed with the new process to validate that the bubble defect was eliminated. Once this was con- firmed, the new process with a lower material flow and multiple coats became the final pro- cess selected for use on production. The final automated process for produc- tion—with a lower material flow rate and mul- tiple coats—has eliminated approximately one hour of coating processing time, including 10 minutes of manual spraying time per CCA. This improvement to the process has helped to increase production throughput capabili- ties and reduce cost by reducing the amount of time the operator is required to manually spray the circuit card. Process Validation Once the final process for production was established, it was necessary to perform a pro- cess comparison to ensure there was no degra- dation of quality from the manual hand-spray process compared to the new automated pro- cess. The parameters that were validated dur- ing this experiment were the thickness per J-STD-001 requirements, uniformity, and an adhesion by tape test per ASTM D3359 Test Method B (cross-cut tape test). To perform the adhesion by tape test, 10 sample coupons were used for hand spray and 10 were used for auto spray; stainless steel coupons and some sam- ple unpopulated printed circuit boards were used. The coupons were conformally coated per the established process and cured per the datasheet requirements then after curing the adhesion test per ASTM D3359 Test Method B was performed. The adhesion test consists of scoring the conformal coating using a clean sharp knife to cut the coating as the test requirements specify (Figure 6). After the cuts are made, the debris is removed, and tape is applied and smoothed over the cut area before the tape is pulled off. Once the tape is removed, the grid area is inspected and rated per the Classification of Adhesion Test Results listed in ASTM D3359 [4] . Figure 6 shows an example of a test before and after the tape is applied and removed. The goal of this test was to see no change in the adhesion to the same substrate type between the hand-spray and automated-spraying pro- cess. In this experiment, all results yielded a Figure 5: Example of the bubble defect.