When is it a bad idea to use ceramic capacitors?

[tubesrule]

You need to be careful when referring to ceramic caps as there are different types. Class 1 ceramic caps can be used where film capacitors are used but are usually expensive. Class 1 ceramics (typically NPO/COG) are extremely stable, non-microphonic/piezoelectric with very low ESR and dissipation factors. In modern electronics I routinely design in Class 1 ceramics in sensitive areas such as coupling, timing/oscillators and PLL loop filters as surface mount film capacitors are expensive. The down side to Class 1 ceramics is they have very low volumetric efficiency so they are limited as to value/voltage rating, however values up to and beyond 0.01uF and 1KV are not unheard of. This still doesn't get us to the vertical coupling caps on an electrostatic set without multiple parts which is why films make sense in this application and why these parts work fine in the horizontal section.

Class 2 and 3 ceramics contain barium titanate which has a much higher volumetric efficiency which is why it is used, but is also what gives what people think of as ceramic capacitors a bad name.
Class 2 and 3 ceramics have very poor to extremely poor temperature stability, poor to very poor dissipation factors, poor aging and because barium titanate is piezoelectric, microphonics. You can take a typical Class 2 or 3 ceramic capacitor connected to a scope and by just tapping it, observe the voltage it generates.
Class 2 caps can be used in some coupling and filtering applications, but Class 3 are strictly designed for power supply filtering applications where they can replace tantalum and aluminum electrolytic caps.

There was at one time Class 4 ceramics with even worse parameters, but these are pretty much gone.

Darryl