Electrical Engineer 1: “Of course I would never use a Y5V capacitor in an application like this.”
Electrical Engineer 2: “Neither would I. That would be folly!”
Mechanical Engineer: “Why?”
If you think that you are at risk of finding yourself in a conversation similar to the above, I hope that this article will help. Just about everyone who has designed a circuit board is familiar with the three-character codes that accompany a capacitor’s description, and I think that most engineers have a general idea of which types should be used—or at least which types should not be used—in a given circuit.
But what do these codes actually mean? Why do app notes seem to almost always recommend X7R or X5R? Why does Y5V even exist? If you search Digi-Key for a 0.1 µF 0805 ceramic cap, why are there over 400 results for X7R and zero for C0G (aka NP0)?
The three-character code with the letter-number-letter format is used for capacitors with Class 2 and Class 3 dielectrics. C0G is a Class 1 dielectric, so it’s not included (more on this later). X5R and X7R are in Class 2, and Y5V is in Class 3.
- The first character indicates the lowest temperature that the capacitor can handle. The letter X (as in X7R, X5R) corresponds to –55°C.
- The second character indicates the maximum temperature. The theoretical range is from 45°C to 200°C; 5 (as in X5R) corresponds to 85°C, and 7 (as in X7R) corresponds to 125°C.
- The third character indicates the maximum amount of capacitance change over the part’s temperature range. The spec for --R capacitors (such as X5R and X7R) is ±15%. The capacitance of parts with a code ending in V can actually decrease by as much as 82%! This probably explains why Y5V capacitors are not so popular.
The following graphic gives you a good visual representation of how unstable Y5V and Z5U are compared to X5R and X7R.
Courtesy of Kemet.
This chart also helps us to answer the question “why does Y5V even exist?” Because it’s adequate for devices that always operate at or near room temperature.
Class 1 Caps
As you may have noticed in the chart, C0G is extremely stable (note that C0G and NP0 both have a zero, not an uppercase “O”). C0G is a Class 1 dielectric and an all-around capacitor superstar: the capacitance is not significantly affected by temperature, applied voltage, or aging.
It does, however, have one disadvantage that has become particularly relevant in this age of relentless miniaturization: it is not efficient with respect to volume. For example, if you go onto Digi-Key and search for a 0.1 µF C0G cap, the smallest in-stock part is a 1206. In contrast, you can find a 0.1 µF X7R cap in the 0306 package, and with a voltage rating (10 V) high enough for 3.3 V or even 5 V circuitry.
The 0306 package. They can actually fit an X7R decoupling cap in this tiny form factor. Image courtesy of Digi-Key.
If you design audio devices, or if you simply prefer quiet PCBs, you have another reason to choose C0G over X7R or X5R: Class 2 caps exhibit piezoelectric behavior that can cause them to function as both microphones (that will convert sound into electrical noise) and buzzers (that will convert AC signals into audible noise). Class 1 capacitors don’t have this problem.
Diagram taken from this TDK document on "singing capacitors."
I’m sure that you can find much more information on capacitor types and dielectrics from manufacturers such as Kemet, AVX, and TDK. If you want to see the entire chart for the three-character codes, click here.