The phenomenon of low capacitance of surface mount capacitors (especially MLCC) when tested directly with a capacity of over 1uF is usually caused by the following key factors:
The impact of testing frequency (the main factor):
The measurement frequency of nominal values: The capacitance values marked by the manufacturer (especially for MLCCs with larger capacitance values, such as 1uF and above) are usually measured at low frequencies (such as 120Hz or 1kHz). This is because these capacitors are mainly used for power filtering and energy storage, and their operating frequency is relatively low.
The default frequency for commonly used testing instruments: Many common LCR meters, multimeters with capacitance settings, or online testers have a default testing frequency of 1kHz.
Characteristics of dielectric materials: Class II dielectric materials used in MLCC (such as X5R, X7R, Y5V, etc.) have significant frequency dependence on dielectric constant. As the testing frequency increases, the polarization response of the material cannot keep up with the changes in the electric field, resulting in a decrease in its effective dielectric constant.
Result: When you measure a large capacity MLCC with a nominal value at 120Hz or 1kHz (but actual 1kHz is close to its frequency response drop point) at 1kHz, the measured capacity is inevitably lower than the nominal value due to the decrease in dielectric constant at high frequencies. The higher the frequency, the greater the deviation.
DC bias effect:
The piezoelectric/ferroelectric properties of dielectric materials: Class II dielectric materials (X5R, X7R, Y5V, etc.) have piezoelectric or ferroelectric properties, and their crystal structure changes with the magnitude of the applied DC voltage.
Decrease in effective dielectric constant: Applying a direct current bias (even if it is a small bias contained in the test signal itself or the operating voltage across the capacitor) can cause some of the domains inside the material to be "pinned" or difficult to flip, thereby reducing the effective dielectric constant of the material.
Result: When measured with a DC voltage applied (or with a bias in the test signal), the measured capacity will be significantly lower than its nominal value (which is usually measured at 0V or very small AC signals). The larger the capacity (higher dielectric constant), the higher the operating voltage, and this effect becomes more pronounced.
The influence of temperature:
Temperature coefficient: MLCC, especially Class II dielectric materials, have a large temperature coefficient (such as X7R with ± 15%). The nominal value is usually given at+25 ° C.
Test environment temperature: If the test environment temperature deviates from 25 ° C, the capacitance value will deviate from the nominal value. The capacitance value usually decreases at low temperatures (for X7R, etc.). Even if the temperature is within the room temperature range (such as 15 ° C-35 ° C), the variation may reach several percentage points.
The equivalent series inductance and loss of the capacitor itself:
Although ESL mainly affects high-frequency impedance, its impact is relatively small during 1kHz testing. However, for very large capacitors or specific internal structures, if the inductive reactance generated by ESL at the testing frequency cannot be ignored, it may interact with the capacitive reactance, slightly affecting the accuracy of LCR meter reading complex impedance and calculating capacitance value, but this is usually not the main reason for low accuracy.
Accuracy and calibration of testing instruments:
Range selection: When testing large capacitors, the instrument must be in the appropriate range. If the range selection is improper (such as too small), it may lead to measurement errors or exceeding the range.
Probe compensation/calibration: LCR meters need to undergo open and short circuit calibration before use to eliminate the parasitic parameter effects of test fixtures and cables. Incorrect calibration can lead to system errors.
Contact resistance: Poor contact between the test probe or fixture and the capacitor terminal electrode can increase series resistance, which may affect measurement accuracy, especially when testing small-sized capacitors.
The accuracy of the instrument itself: The measurement accuracy and stability of low-end instruments may be limited.
Aging of capacitors:
The MLCC of Class II dielectric materials exhibits aging phenomenon. After experiencing high temperatures (such as welding), the capacitance value of a capacitor will slowly decrease over time (in a logarithmic relationship). The capacitance of the newly welded capacitor is the highest, and after being left for a few days or weeks, the capacity will decrease to a stable level below the initial value (the value after aging). The nominal value usually refers to the stable value after aging, but newly manufactured or newly welded capacitors may have slightly higher values. However, aging usually leads to a decrease in capacity, which is also a factor that may result in lower test values (especially for newer capacitor test values that may be close to the nominal value and will be lower than the nominal value after aging).
How to obtain accurate measurement results (for high-capacity MLCC):
Use the correct testing frequency:
For MLCC with 1uF and above, it is necessary to use 120Hz or 100Hz for measurement. This is the frequency closest to its calibration conditions and usage scenarios. Check if your LCR table supports and is set for low-frequency testing.
Consider DC bias voltage:
If a capacitor is subjected to DC voltage when working in an actual circuit and you want to know its capacity in operation, you need to use an LCR meter that supports DC bias function.
Measure using a low-frequency (120Hz) AC signal at a specified DC bias voltage (such as the rated operating voltage of a capacitor). This will obtain the capacitance value closest to the actual working state (usually much lower than the nominal value at 0V).
Control temperature:
Measure at standard room temperature (25 ° C ± small amount) and pay attention to the temperature coefficient of the capacitor.
Proper use and calibration of instruments:
Choose the appropriate range for capacitance.
Strictly perform open circuit and short circuit calibration of the instrument.
Use appropriate testing fixtures to ensure good and reliable contact. For small-sized capacitors (such as 0402, 0201), high-quality SMD testing fixtures or probes are crucial.
Understand the meaning of nominal values:
Recognizing that nominal values are typical values under specific conditions (low frequency, no bias, 25 ° C, after aging), actual measured values are greatly affected by testing conditions.
Summary:
The main reason for the low direct test capacitance of high-capacity (≥ 1uF) surface mount MLCCs is that at the commonly used 1kHz test frequency, the effective dielectric constant of Class II dielectric materials is significantly lower than the value used for their nominal capacitance at low frequencies (120Hz/1kHz). The DC bias effect is another key factor that causes the measured value to be much lower than the nominal value of 0V. In order to obtain accurate and valuable measurement results, it is necessary to use low-frequency (120Hz) testing and consider applying DC bias as needed, while ensuring instrument calibration and appropriate ambient temperature.