A comparison of two X7R, 0603 size capacitors from manufacturer data. Both are assumed to have a 5V bias and operate at 70 degrees Celsius. Even though one component has twice the initial capacitance, the final results at 100,000 hours are much closer. All data are based on the manufacturer's datasheet, estimated at 100,000 hours of aging.
The first capacitor is 1µF, 25V, 0603 size, and the second is 2.2µF, 10V, 0603 size. Both are assumed to be biased at 5V and operating at 70°C. The total aging of 100,000 hours is extrapolated from the reference datasheet to be -25% worst-case due to normal aging, DC bias, and operating temperature. Note: the keyword above is "extrapolation" because I do not have my own data to support this.
Even this linear multiplication and addition of terms is misleading because the sum cannot possibly exceed the sum of the possible 80% capacitance decreases. This is because when all the magnetic dipoles are 100% aligned, the material will still have some residual dielectric constant. Therefore, the situation is more complex than the simple linear calculation on the back of the napkin shown in Table 1.
More likely, the following scenario is based solely on data regarding DC bias effects released by a few manufacturers. The image below does indeed show what happens to the capacitor's capacitance as the dielectric material dipole alignment increases from 0% (completely random) to 100% (completely aligned), representing the absolute worst-case combination of DC bias, operating temperature, and aging. A graph was plotted by studying DC bias versus capacitance changes from several manufacturers, and this graph infers the relationship between possible capacitance changes and the dielectric material dipole alignment of the X7R capacitor. 0% is random alignment (left x-axis), and 100% is when the dipoles are aligned (right x-axis), showing approximately 80% of the possible total capacitance loss.
in conclusion
For me, the gain from all of this is:
1) Following the “large capacitor shortage” in 2017, the behavior of X7R parts gave me serious problems as manufacturers scrambled to fulfill orders and substitutes (known and unknown). I found that the capacitance dropped more significantly under DC bias, as well as other parameter issues between capacitor batches produced before and after the shortage of seemingly identical part numbers.
This makes me skeptical of the information released by manufacturers with decades of experience, especially given how rapidly technology changes. Even if you conduct your own reliability studies, you can't be sure when the next capacitor shortage will change all the formulas and wipe everything out.
2) Recent information regarding the increased aging rate with DC bias and rising operating temperature suggests that, after 10 years, designers may wisely increase the expected X7R capacitance by another 25% due to the combined effects of aging, operating temperature, and DC bias. This is due to the initial capacitance decrease caused by tolerances, temperature coefficients, and DC bias.
3) This accelerated DC bias combined with increased operating temperature and capacitance decrease suggests that using high-temperature, accelerated life testing for at least 1000 hours may help understand the expected true capacitance change for products with a longer expected lifespan. Note: You must not exceed 90°C to avoid aging the capacitors during testing.
4) Using high-capacitance X7R capacitors with low rated voltages that operate at high percentages of operating voltage may cause problems with the high-capacitance output filtering of switching power supplies, where capacitors are used to stabilize the control loop, especially if you need to achieve a longer operating life. Test at high temperatures for at least 1000 hours, or use another proven true capacitor technology, such as tantalum or aluminum electrolytic capacitors, to meet your high-capacitance requirements.
5) High-capacitance X7R capacitors operating at low rated voltages and high percentages of operating voltage may be suitable for low-dropout regulator (LDO) output filtering applications. These applications may require a maximum series resistance value and perhaps some minimum capacitance value, but at the opposite extremes of these values, a stable regulator is usually still provided. Verify this by checking the regulatory agency's datasheet.
6) Since the X7R is the best of all other Class 2 dielectric capacitors, it seems strongly recommended that the X5R be used only for high-frequency bypass on multi-MHz digital circuits, where the most important aspect of the capacitor is the series inductance rather than any capacitance value.
I reviewed the capacitance and DC bias data for two common 0603-size X7R capacitor types published by the manufacturers. The first is a common 0.1µF, 50V type for decoupling, and the second is a high-density 1µF, 10V type. It's clear that each manufacturer has a different X7R dielectric formulation, which varies depending on the capacitor's rated voltage. Keep this in mind when you encounter a shortage and choose an alternative "equivalent" part number; it may not be as equivalent as you think!
