SMT007 Magazine

SMT007-Aug2019

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80 SMT007 MAGAZINE I AUGUST 2019 Figure 10 shows an example of how the one- coat coupon should look with no voids and uniform. It was necessary to run the sepa- rate one-coat coupon because it was observed that when spraying the final process for pro- duction—which includes multiple coats—any voids in the coating or changes in the spray pattern were covered up. Additionally, to con- trol pot life, all mixed materials are labeled with the material name, date, time mixed, and pot life so that the operator knows when the pot life will expire. These process controls have been sufficient to ensure the material is not used after the pot life has expired. Maintenance of the machine was also deter- mined to be important for proper coating application. Polyurethane materials are very difficult to clean. This was observed with the hand-spraying process as well; the main- tenance of the hand-spray gun and the auto- spray valve is critical. Any coating remaining in the valve will cure and can either clog the valve or will affect the performance of the next run. The best way to ensure consistent pro- cessing for each new pot of material is to com- pletely remove and deconstruct the valve and clean all of the wet components individually. It was also necessary to inspect the compo- nents under magnifica- tion to ensure they are clean before reassem- bling the valve. Purg- ing the valve with thin- ner is required to ensure all material has been removed from the valve before disassembly and after reassembly. These experiments determined that it is a process require- ment to run at least two coupons at the beginning of each material pot mix- ture to ensure the spray pattern is sufficient and that there are no abnor- malities with the mate- rial before spraying any circuit card assemblies. By following these maintenance requirements, the process has performed consistently. Curing The curing process was also automated for this implementation by using an inline curing oven. Modifications to the oven were required due to the long material cure times. To ensure the oven is reaching the proper temperature for the correct cure times, a thermocouple profiling tool was used to adjust the temper - atures as needed until the cure temperature was achieved. This inline curing oven process differed significantly compared to the cur- rent process of placing the circuit cards into an oven. The process also requires a period for air curing, so a staging area is required. During staging and before entering the inline curing oven, the circuit cards are inspected for any obvious defects and touched up as required before curing. Conclusions The development of an automated process to spray a low-outgassing polyurethane confor- mal coating material was essential for meet- ing an increase in customer demand. Manual processes in manufacturing lead to defects and Figure 10: Example of a uniform one-coat coupon.

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