Why do 100 nF ceramic disc capacitors from multiple sources, measured with two instruments, all appear to be below tolerance?

Question

I have a collection of 100 nF ceramic disk capacitors, which came from three completely different sources. They are all through-hole packages comprising brown disks and two leads, and marked only with the code 104.

My understanding is that, since there is no tolerance code, I should treat them as having a capacitance in the range 80–180 nF (-20% to +80%).

I have two instruments for measuring capacitance. One is a cheap digital multimeter, and the other is a cheap general purpose component tester that I bought as a DIY kit and soldered together. They give capacitance readings within about 10% of one another.

No matter what capacitor or instrument I try, the measured capacitance is about 45–75 nF, which is well below the tolerance range.

Are there any "obvious" (not obvious to me!) explanations for this phenomenon? Bad capacitors? Bad instruments? Some kind of resonance effect?

Answer 1

Perhaps you should buy one or two parts with known provenance, such as a 100nF BFC241641004 +/-2% film cap from Vishay-Dale, which are less than $1.50 ea.

Keep them in a cool dry place and never do anything with the capacitors but remove them from the storage location and measure with your instrument.

To answer your question, there are/were some really horrible ceramic dielectrics such as Z5U and Y5V that age and and change with temperature greatly. Y5V will typically decrease in value by ~7%/decade hour after manufacture, so if those parts are 20-30 years old it's conceivable. Most modern parts use somewhat better dielectrics such as X7R in that capacitance range. Even class I (NP0) dielectric is available in values that high now.

Also, if your cheap meters put a DC voltage across the capacitors that may decrease the value significantly.

More likely there is some problem in your measurement technique, the meters are faulty, need a new battery or something of that ilk, but getting a single known-good reference would allow you to nail it down.

If you feel like experimenting you can 'cook' your capacitors for a couple of hours at 150°C which would restore the aging, if it was aging. This won't do much for the solderability.

Especially with the class III (Z5U/Y5V) caps, the tolerance applies only to the value 1000 hours after manufacture or 'cooking', at a reference temperature and with 0VDC applied across the capacitor. The value may have already decreased out of tolerance before you receive it from a distributor. Or it could be a bit out of tolerance high if it was shipped quickly.

Answer 2

You should use LCR meter. That is proper instrument to measure capacitance.
So it is possible that the problem is in use of wrong instrument. Multimeter (DMM) does not use proper method to measure capacitance. (not to mention that you used cheap multimeter)

UPDATE:
LCR meters use signal with frequency 100Hz, 120Hz, 1kHz, 10kHz, or 100kHz, and measure impedance. It is the same method as manufacturers use.
Multimeters use completely different method. They measure time constant. So they charge capacitor by constant current and measure voltage and time. Then they calculate capacitance.
According to my experiece this method gives values that are tens of % different from proper measurig.
Capacitance is heavily dependant on frequency, so the only proper method is the one used in LCR meters and you have to use the same frequency as manufacturer. This frequency is mentioned in datasheet of capacitor.