Electronic – Why do ceramic insulators have a stacked disc structure

This is a bit of a controversial subject. Some people seem to feel that a capacitor beyond resonance has no benefit for bypassing. Others point out that even past resonance the part is basically just a very small inductor short-circuited to ground and it still has fairly low impedance.

This plot from Murata shows the (magnitude of the) impedance vs. frequency for three different capacitor values in the same package (0402):

This shows that after a high value part (like 0.1 uF) passes resonance, a lower-value part (like 0.01 uF) just barely achieve a lower impedance before it too hits resonance and its behavior becomes dominated by its inductive parasitic, which is basically the same as in the high-value part.

That said, as others have pointed out, the more capacitors you can put in parallel, the more you reduce the series resistance and inductance of the ensemble of parts; so adding more parts is going to help at least a little bit.

Edit:

I should also point out that if you start considering larger values in larger pacakges, say 1 uF in 0805 and 10 uF in an electrolytic A-size package, you can definitely improve the impedance at lower frequencies (below 10 MHz).

If by monolithic, you mean the multi-layer chip caps (sometimes labled MLCC), that's what all the high density ceramic caps are.

The traditional disc caps are basically just a slab of ceramic with plate on each side, radial leads attached, and dipped in epoxy or maybe ceramic for coating. These are low capacitance devices (100 or so pF) but can be very high voltage. Sometimes they are also called safety caps.

Also, there are disc or cylindrical shaped caps that are really MLCC structures with an electrode around the outside edge and the other electrode in the middle. These are used as feed through caps in housings of EMI filters, or maybe even as part of a transmission line termination.

Wikipedia has a pretty good write-up of capacitors here.

Edit:

The question of the disk cap and MLCC having the same voltage rating and capacitance and having to choose between the two occupies a very small space in the Venn diagram. MLCCs were derived from disc technology to overcome limitations in capacitance and allow wider use of ceramic dielectrics which are comparatively easy to fab. The main things the disk has going for it are the high voltages and rugged construction. A single wafer of ceramic is going to take more abuse than a stack of thin layers of ceramic. The choice would come down to things like that.

If you needed a rugged part, or a part that could not be surface mounted you would choose the disc (I know you can get leaded multilayer ceramics, but its a disappearing market). If you needed a part with low ESL and more compact size you would choose the MLCC. It's pretty clear which is usually more desirable.

The uses for disc type caps really are those for which it has an advantage, like high voltages from 3kV to 6kV where you might need a low dissipation coefficient or NPO type stability. MLCCs don't offer much competition at those higher voltages.