If these are related ?
Click images to enlarge.
This is familiar. I'm not aware of anything that drops precipitously like the earlier figure, which very much looks made up, but maybe some manufacturers made something that changes phase very slowly but also rather suddenly (days being slow compared to ~instant changes, and different from the log(time) aging of traditional materials).
A lot of damage was made a few years ago on blog posts, youtube videos and this very forum by "helpful" people teaching around that the issue is mainly with Y5V capacitors and X7R is fine. Some rules of thumbs very pulled out of thin air, such as "Y5V can drop below -80% under full DC bias but X7R only -30%" or "derating voltage helps". These are all totally false.
The best rule of thumb, by far, is simply: if the volumetric energy density is too good to be true, then it isn't. As ejeffrey writes above, if footprint size is a fixed parameter for you, then the height is relevant to look at. And you should really look at joules per volume. And calculate the joules at your DC bias voltage; the rating is meaningless. E / V = CU^2 / (width * length * thickness).
Yeah, while Y5V and Z5U (there's even a Z5V if you want to go looking for true awfulness) may be the worst, there are no exceptions, all type 2's exhibit the characteristic.
I occasionally need to use X5R for the greater density; a modest compromise while temp ratings are respected. I haven't had to use anything worse, at least in quantity. I have heard tale of Z5x being used -- responsibly engineered -- in extreme cost reduction applications.
The volumetric point extends with respect to voltage. Like the 2220 I mentioned earlier, you can get a solid 10 or 22nF in X7R at 250V, and while you can buy 220nF or even more with such a rating, none of them offer much more capacitance
at voltage. The C(V)/C(0) ratio can get much worse than for low-voltage types, because the insulation quality is better -- that is, thicker layers are more reliable so can be pushed closer to limit. (Hm, I haven't tested this, but I'm betting breakdown is closer to ratings -- whereas a 16V part might fail anywhere from 40 to 150V, I'd bet a 250V part might fail 400-600V?) So, not even being able to get a proper 100nF at 250V+ even in large chips like 2220 is disappointing, but works out when you do the numbers -- they're bigger, sure, but they aren't
many times larger than say a 1210.
Y1/Y2 ceramics are also commonly Y5V. IIRC, these tend to drop off by a couple hundred volts, so are functional at mains voltage, but not under surge conditions -- which might well be a feature rather than a bug, the saturation reducing current flow through the capacitor. The temperature characteristics seem the more important property here, something to keep in mind if you're doing EMI filters for wide temperature ranges. (Or conversely, use then if you don't need to pass tests at temp extremes -- shitty, but may happen that that's what a project needs done.)
Tim