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Sensible Design: Top Tips for Successful Potting

My last column turned to some of the more technically challenging aspects of resins and their applications and was largely based on customer queries concerning thermal conductivity, dielectric constant, and choice of resin chemistries. With important resin considerations aside—which hopefully had you thinking a bit more deeply about these extraordinary materials—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 components 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 formation of eddies in the resin as it is poured, which can lead to voids and air entrapment, which compromise the thermal 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 layers to achieve the desired thickness, rather than attempting the cast in a single pour. This helps control the exotherm potential (remember—two-part resins in large volumes get quite hot when mixed) and reduce void formation.

Void formation (air bubbles) is a particular bugbear of resin application that must be avoided at all costs because it reduces the thermal performance 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 formation 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 decanting of the resin and hardener into the dispensing equipment reservoirs and then leaving the material undisturbed for a couple of hours will allow the bulk of any air that has been trapped in the mix during the decanting process to be released naturally back into the environment. If the reservoir is equipped with a stirrer, then this should be set to a low speed not to incorporate air during the mixing process. Also, make sure that the lines of the dispensing equipment are fully bled of air before commencing production.

It is good practice to check the shot weights of both components of the resin periodically during the day to ensure that the dispensing equipment is operating reliably. If the weights begin to drift significantly, this could be a sign that the pumps are not working correctly, such as air coming into the system from a split pipe or leaking coupling, or the pipe was not properly bled before starting.

To read this entire column, which appeared in the November 2018 issue of Design007 Magazine, click here.