When I was working as a Field Application Engineer, about once a month I'd get a phone call from a panicked technician working on an assembly line. The call would always go the same. First the technician would explain that during board test, they were finding problems with their X7R and X5R capacitors being out of tolerance. In some cases the cap value would be measuring too high and sometimes it would measure too low. Depending on where the measurement fell, I could tell the issue. Without checking, I was pretty sure it wasn't that we shipped them the wrong capacitor value. At KEMET we measure each ceramic capacitor's value at least twice. Once to determine the tolerance and once right before it goes on the reel. This assures us we didn't make a gross error.
That being the case, why would I get these frequent phone calls then? We have published an application note that helps explain how to correctly measure ceramic capacitors (also attached below).
All capacitors are sold with a capacitance value within a tolerance band. For ceramic capacitors a ±10% tolerance is extremely common. When we, as a manufacturer, set out to build a part our recipe is designed for the nominal, however, that's not always the end result. So every one of the 32 billions capacitors we build per year, have to be measured to know which bin they fall into.
When a ceramic capacitor measures outside of this band, there are two very likely reasons as to why.
When ceramics measure high
If the capacitor measures higher than the expected tolerance band, it is likely related to the aging rate of the ceramic material. Right after exiting the reflow oven, ceramics like X7R and X5R will shift up in capacitance and slowly decay over time. This is called the "Aging Rate." The actual percentage of change varies by dielectric type and by manufacturer. Note that C0G (also known as NPO capacitors do not have this effect.) The first 1,000 hours out of the oven, you can expect an X7R and X5R to exhibit a noticeable shift.
When ceramics measure low
When a ceramic measures lower than tolerance, generally, the mistake is made with the measurement equipment. Understanding how the capacitance meter works is important because if it doesn't compensate for the ceramic's voltage coefficient, it won't measure the capacitance correctly.
Get the application note
Understanding how your measurement equipment works and the technology behind a ceramic capacitor will help in making better measurements. This application note can be downloaded from here: Measure Capacitance of Class-II and Class-III Ceramic Capacitors. Or you can download the attached file.
Feel free to leave questions or comments on the application note below.