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PCBD-May2017

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48 The PCB Design Magazine • May 2017 by Alistair Little ELECTROLUBE To Encapsulate or Coat: That is the Question SENSIBLE DESIGN If I had a pound for every time our customers have approached me about this one, I'd be a rich man: When is it appropriate to use a confor- mal coating rather than an encapsula- tion resin and vice versa? Well, the short answer is "it depends." But as a general rule of thumb, your choice will be guided by how an electronic circuit is to be housed within an assembly and the nature of the environment in which it must operate. Examine the design of the housing within which the PCB will be enclosed. If this is intended to provide primary environmental protection, then a con- formal coating is probably the best choice as it will be the second line of defence should the housing seals fail in any way. However, where a separate hous- ing is not an option or where it is unable to provide that primary environmental protect ion, then a re in is likely to be the better choice But before I pursue the "when" and "why," it might be helpful to de- fine the key differences between an encapsulation resin and a conformal coating, and what they share in com- mon with one another. Both are generally organic poly- mers—acrylic, epoxy, polyurethane and silicone being the most widely used— which cure to form an elec- trically insulating layer that offers varying degrees of chemi- cal resistance and thermal protection, de- pending upon their for- mulations. C o n - f o r m a l c oatings are ap- plied in their liquid state as thin films, which cure to provide a dry film thickness of between 25 and 100 microns, adding minimal weight increase to the assem- bly. These coatings are usually clear and allow rework, so coated com- ponents are visible and easy to re- place. The level of chemical resis- tance and thermal protection that conformal coatings provide is gener- ally good for short exposures. The majority of these coatings are sin- gle-part systems that are easy to apply and quick to cure with little temperature rise. On the down side, single-part coatings are solvent-based in order to modify their viscosity for appli- cation purposes, which can have environmental repercussions. Conformal coatings can be ap- plied manually using a paint brush, a spray gun or simply by dipping the cir-

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