SMT007 Magazine

SMT007-Aug2019

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AUGUST 2019 I SMT007 MAGAZINE 69 circuit card assemblies, which includes clean- ing, baking, and masking any areas that are required to be free of conformal coating per the customer drawing requirements. The coating process also includes preparation of the mate- rial used for spraying; this may be a mixture of two or more parts that must be weighed and mixed properly. The next step is the spraying and curing of the material onto the circuit cards and coupons for thickness measurements. The final steps of the conformal coating process are the removal of the masking materials and tou- chup of the coating. Preparation of the sub- strates, correct mixture of the material, proper curing, and accurate thickness measurements are important variables that could affect the output of the conformal coat spraying process. Therefore, it was necessary to ensure proper controls had been established for all of these aspects of the conformal coating process. Materials and Tools Preparation of materials and circuit card assemblies was a key factor to being success- ful in the transition from hand spraying to automated spraying of circuit card assemblies. Ensuring coupon substrates and CCAs were properly cleaned and baked before the coat- ing was necessary for proper application of the coating material. If any IPC-A-610 defects were observed, it was important to know that the samples were properly prepared to eliminate cleanliness or moisture as a root cause for the defects. It was crucial to understand the require- ments and properties of the two-component, low-outgassing polyurethane material before beginning to research equipment to automate the spray coat process. For this experiment, the datasheet for the low-outgassing polyure- thane material was the starting point to under- standing the material pot life and cure times as well as the mix ratios, thinner ratios, and general material properties. An evaluation of the material showed that there is a very short pot life that needed to be taken into consider- ation when purchasing an automated machine. If the material ages while in the machine, it will create difficulties with the maintenance and could result in damage to the machine or defects when spraying the material. It was also important to understand which material parameters can be changed and which cannot. For example, the ratio of a two-com- ponent material cannot be changed; however, additional thinner may be added to the mix- ture to change the viscosity to improve mate- rial flow. Additionally, the cure times cannot be changed without sufficient process evalua- tion to determine that the material properties are consistent. Another factor that was con- sidered involved the number of coats being sprayed, which is another variable that may be changed. For example, it could be beneficial to spray multiple thin coats instead of one thick coat or a thin coat and cure to a tack-free state before applying a second coat. Validating the measurement system in place was important when developing this new pro- cess. Thickness is a critical parameter for con- formal coating and is required to be 0.03–0.13 mm (0.001–0.005 in.) per J-STD-001 for poly- urethane materials [1] . Before this experiment, fiberglass coupons were being used for thick- ness measurements and were being measured with a digital positioner micrometer measuring tool. This tool was difficult to use and showed to have significant operator-to-operator varia- tion when multiple operators were measuring the same coupon. A new measurement system using steel coupons, instead of fiberglass, mea- sured with an eddy current probe, was evalu- ated. To evaluate and validate the new measure- ment tool with the new steel coupons, 20 sample coupons were conformal coated and measured by four different operators. The vari- Validating the measurement system in place was important when developing this new process.

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