Why are ceramic disc capacitors sometimes coated in wax?

[Ranayna]

The other day i soldered a cheap kit that contained a few small ceramic disk capacitors. When i populated them to the board, i did not really notice the wax coating, but after i soldered them, i immediated saw that they made a mess on my esd mat. I had one that was not yet soldered, and with taht one i could confirm that they were coated in some kind of wax. Not all of them were, just all of a certain value.

Now in hindsight, i remember from the past, especially when i tore down electronics as a kid, that a lot of those disk capacitors were covered in wax. Sometimes whole boards were covered. For the whole boards i figured it was some kind of cheap conformal coating or vibration dampener, but what is the reason to coat only capacitors?

[Gyro]

The simplistic answer is, to keep moisture out. Ceramic disks are covered with some kind of cement rather than resin / epoxy, so impregnating it with wax achieves the moistureproofing function. I suspect that without it, there would be issues with the internal silver metalization degrading.

[SeanB]

Ceramic disks are generally wax impregnated as the wax keeps the silver used to coat the plates from oxidising away very quickly. The ones that are dipped in epoxy ( the nice shiny hard ones) are not wax impregnated, as the epoxy does this job, and chip ceramics have a clear coating on them to do the same, keep water vapour out.

Most capacitors need environmental protection, in many cases by dipping in epoxy, potting in resin or coating in wax, and electrolytics use a rubber bung so the liquid electrolyte does not evaporate.