Is silicone potting of PCB board beneficial or risky?
What could be the best material for encapsulating a power supply board?
I used 2 part mixed silicone potting compound for encapsulating a microcontroller based (ASIC based, too) power supply, and one of the chip capacitors on the board failed within few hours of regression of device.
Reason is still unknown, looking for some valuable suggestion.
Well, any potting material will put mechanical stress on the board and the components. Any glue or compound, when drying or becoming hard due to its chemical reaction, will do so with the side-effect of changing its volume, even if it happens only ever so slightly. It may well be that one of your MLCCs failed because the compound subjected it to shear stress, maybe worsened by heat.
Also, any chemicals (read: dirt) on the board will remain just there under the potting compound, being trapped forever, and even a bit of moisture might make corrosion effects worse - compared to a board with no potting or coating. This, however, is a long-term effect not likely to be the cause of the particular failure mode you observed on your board.
Try using potting as a last resort.
Ceramic chip capacitors are extremely susceptible to stress fracture, and they almost always fail shorted. Any stress on the PCB can lead to MLCC chip capacitor failure. There are lots of guidelines on chip capacitor placement (away from edges, orienting perpendicular to longest dimension of PCB etc.) Additionally, chip capacitors may even crack due to overheating (as in prolonged soldering). Stress fractures may accumulate over time, and lead to failure.
There is a lot of research underway on how to reduce ceramic chip capacitor failure, including coating them in elastic materials, orienting the pads on the longer edge etc. It is possible that more expensive chip capacitors may be more robust, due to these additional protections.
Standard bathroom silicone is not inert. It reacts long after drying. So it may (or may not) have an effect chemically. I would use only products certified for PCB. Any electrical insulation is also a thermal insulation. Components may heat much faster than expected because there is no contact to air. HTH
