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Design007-Nov2018

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80 DESIGN007 MAGAZINE I NOVEMBER 2018 My last column turned to some of the more tech- nically challenging aspects of resins and their applica- tions and was largely based on customer queries concern- ing thermal conductivity, dielectric constant, and choice of resin chemistries. With important resin con- siderations aside—which hopefully had you think- ing a bit more deeply about these extraordinary materi- als—I thought it appropriate to return to the nitty gritty of resin encapsulation and offer my top tips for successful potting. For effective potting, ideally, the layout of the circuit com- ponents should be such that the material can flow smoothly around them without too much turbulence. When possible, it is always good practice to space components in a regular pattern. Irregular spacing—particularly bunching of components in discrete areas of the PCB—causes the forma- tion of eddies in the resin as it is poured, which can lead to voids and air entrapment, which compromise the ther- mal performance of the resin. Where large volumes of resin are required, particularly where the encapsulated volume is deep, it is far better to cast the resin in lay- ers to achieve the desired thickness, rather than attempting the cast in a single pour. This helps con- trol the exotherm poten- tial (remember—two-part resins in large volumes get quite hot when mixed) and reduce void formation. Void formation (air bub- bles) is a particular bugbear of resin application that must be avoided at all costs because it reduces the thermal per - formance of the cured resin encapsulation and may entrap areas of high moisture content at the surface of the PCB, which can lead to corrosion issues. The best method to prevent void for - mation is vacuum potting, but this may not always be possible due to the geometry of the device to be potted, production volumes, or cost. If vacuum potting is not an option, then the best approach is to handle your materials with great care to minimise air entrainment at every stage of the application process. For example, careful decant- ing of the resin and hardener Top Tips for Successful Potting Sensible Design by Alistair Little, ELECTROLUBE

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