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The code stamped on a ceramic capacitor — C0G, X7R, X5R, Y5V — is an EIA classification, and it answers three questions at once: the temperature range the part is rated for, how much its capacitance is allowed to move across that range, and how much it may drift with age.
Reading the code. The first character gives the low-temperature limit, the second the high-temperature limit, and the third the permitted capacitance change across the range. C0G is the tight one: no more than 30 ppm per degree Celsius, effectively flat, which is why it is also written NP0. X7R is rated from −55 °C to +125 °C with up to ±15 % change. X5R covers −55 °C to +85 °C with the same ±15 %. Y5V covers only −30 °C to +85 °C and permits a change of +22 % to −82 %, which is the code that surprises people.
Class 1 and Class 2 are different components in practice. C0G is a Class 1 dielectric: low loss, no piezoelectric behaviour, essentially no capacitance change with voltage. X7R, X5R and Y5V are Class 2: higher permittivity, so far more capacitance per volume, at the cost of voltage and temperature dependence. The practical consequence is that a Class 2 part’s nominal value is a small-signal value measured under conditions you rarely operate at.
DC bias is the effect that ruins designs. A Class 2 ceramic loses capacitance as DC voltage across it rises. The loss is worst in the smallest, highest-capacitance parts, and the datasheet curve that describes it is easy to miss because it is a graph, not a table. A part sold as 10 µF in an 0402 body at its rated voltage may retain a small fraction of that. The two standard mitigations are to specify a larger case size than the footprint strictly needs, or to specify a higher voltage rating than the rail requires, because both flatten the bias curve.
Temperature and ageing add to the same problem. Class 2 parts also lose capacitance with age, following a logarithmic decay, and the loss resets when the part is heated above its Curie point during reflow. Net capacitance at the end of a product’s life is therefore lower than at the start, and the datasheet’s tolerance band usually reflects only the initial value.
Where each family belongs. C0G for anything where the value sets a frequency, a filter corner or a timing constant, and for high-current snubbers where low loss matters. X7R for decoupling and general filtering across an automotive or industrial temperature range. X5R where the temperature range is commercial and the capacitance density is worth more than the range. Y5V only where a very loose value is acceptable and cost dominates — and rarely in a design that will be produced for years.
A working rule. For decoupling, decide the effective capacitance you need at the worst-case bias and temperature, then choose the case size and voltage rating that deliver it, rather than choosing the nominal value and hoping. For anything where the value is a design parameter rather than a bypass, use C0G and remove the question.
This article describes general dielectric classifications and behaviour, not the specification of any particular part. Voltage coefficients, temperature curves and ageing rates differ between manufacturers and case sizes; confirm them against the datasheet for the exact part.
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