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78 SMT007 MAGAZINE I SEPTEMBER 2018 On a more serious note, an ideal coating should also be solvent-free, and as I point- ed out in my previous column in this series, there has been some progress in this area. A new range of innovative VOC-free, fast-curing, high-performance two-part conformal coatings are now available for application by selective coating. The two-part (2K) solvent-free selec- tive coating process is an enabling technologi- cal breakthrough that allows all their benefits to be realised. How does the design of the board impact the choice of coating material? The design of the board doesn't directly affect the choice of coating material, although it will determine which application methods can be used to apply the coating. Certain mate- rials are difficult to use in a dip process due to the material curing prematurely (e.g., moisture curing and UV-curable materials), so it could be said that the choice of coating is indirectly related to the board design. The use of single component UV-curable materials has grown significantly over the past decade with rapid cure yielding productivity benefits for many. However, the current sin- gle-part UV materials often suffer cure issues when applied to boards containing tall com- ponents that shadow the exposure to UV light and necessitate out-of-focus cure heights and reduce light intensity and degree of cure. Con- sequently, a secondary cure mechanism must be introduced into UV curing—typically a moisture-activated or heat-activated mecha- nism. What electrical properties are typically required of conformal coatings? Conformal coatings form a protective, insu- lating layer. The most common electrical parameter tested is surface insulation resis- tance (SIR) testing, a measurement often taken before and after coating and subsequent expo- sure to harsh conditions to determine the long- term insulation performance of the coating. The coating should also have high dielectric strength, the minimum level required deter- mined by the inter-track separation and poten- tial difference between adjacent tracks. Dielectric properties are an important con- sideration where the signal integrity of the circuit is concerned. Conformal coatings applied to such circuits must not affect sig - nal integrity and the coating's dielectric con- stant and loss are parameters that must be considered when making a material selection and determining the coating thickness. Oth - er than this, dielectric strength, breakdown, and withstand are commonly measured and reported properties that enable designers to ensure sufficient spacing between compo - nents and determine how much closer com- ponents can be spaced than without coating materials applied. How is the best application method of coating materials defined? There isn't necessarily a best method to apply a conformal coating. The application method chosen for an assembly will depend upon what existing equipment is available to the manufacturer, the coating processes in use, take time (the average time interval between the start of production of one unit and the start of the next), and the design of the assembly. This includes areas of the circuit that must be coated and those that must not (connectors, switches, etc.). The best application method would ensure that each board to be coated receives coat- ing coverage on all required metal surfaces at a sufficient thickness to afford protection against the environment. These requirements will change from each board design and envi- The two-part (2K) solvent-free selective coating process is an enabling technological breakthrough that allows all their benefits to be realised.

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