Why is it that when the capacitance value of a surface mount capacitor exceeds 1UF, the direct testing results in a lower capacitance?
Date：2025-07-10
Viewed：65
Root cause and solution for low testing of surface mount capacitors with capacitance values above 1 μ F
1、 Core cause analysis
1. Testing instruments and conditions issues
Insufficient voltage:
When testing large capacity capacitors (such as 1 μ F or above), the actual voltage applied across the capacitor may be lower than the set value due to impedance voltage division inside the instrument. For example:
When using HP4263B to test a 10 μ F capacitor, the actual voltage was only 10% of the set value, resulting in a low capacity reading.
Formula: Actual voltage V=V setting x Z instrument+Z capacitor Z capacitor, where Z capacitor decreases with capacity and frequency.
Improper frequency setting:
Different capacitance values require corresponding testing frequencies for capacitors:
Capacity>10 μ F: 120Hz
1 μ F<Capacity ≤ 10 μ F: 1kHz
Capacity ≤ 1000pF: 1MHz
If the frequency is set too high, the capacitance impedance will decrease, and the instrument will not be able to provide sufficient voltage, resulting in a lower capacity test value.
Instrument impedance mismatch:
Some testers (such as HP4263B) have high output impedance (100 Ω), which does not match the low impedance of large capacity capacitors (such as about 16 Ω at 10 μ F/1kHz), resulting in voltage division and insufficient actual voltage.
2. Environmental factors
Temperature impact:
The capacity of non temperature compensated capacitors (such as X7R/X5R) decreases significantly in high temperature environments. For example:
The capacity of Y5V capacitor is 20% lower at 40 ℃ than at 25 ℃.
Material aging:
Ferroelectric materials (such as MLCC) will age over time and gradually decrease in capacity, but can be restored by high-temperature baking (150 ℃/1 hour).
3. Characteristics of capacitors themselves
Voltage dependence:
The capacitance may decrease under high voltage and needs to be tested at rated voltage.
Frequency effect:
The capacitance decreases at high frequencies, and the testing frequency needs to be adjusted according to the capacitance value.
4. Testing methods and operations
ALC function not enabled:
Some testers (such as HP4284A) need to turn on Automatic Level Control (ALC) to ensure voltage stability, otherwise the actual voltage will be insufficient.
Welding and installation issues:
Welding overheating or unstable installation may cause internal damage to the capacitor, affecting the capacity test results.
2、 Solution
1. Adjust the testing conditions
Using a low impedance tester:
Select a low impedance tester such as HP4278A, or enable the ALC function (such as HP4284A) to ensure that the actual voltage is consistent with the setting.
Correctly set test parameters:
Voltage: 1.0 ± 0.2Vrms
Frequency: Select according to the capacitance value (e.g. 120Hz for 10 μ F and 1kHz for 1 μ F).
Verify actual voltage:
Use a voltmeter to directly measure the voltage across the capacitor to ensure it reaches the set value.
2. Control environmental factors
Temperature control:
Stabilize the capacitor at 20 ℃ before testing, avoiding high temperature or high humidity environments.
Dealing with aging issues:
Perform high-temperature baking (150 ℃/1 hour) or reflow soldering on aging capacitors to restore their capacity.
3. Standardize testing operations
Check instrument settings:
Ensure that the ALC function is enabled and the impedance level matches the capacitance impedance.
Optimize welding and installation:
Avoid welding overheating, ensure stable installation of capacitors, and reduce mechanical stress.
4. Capacitor selection and quality control
Choose high reliability capacitors:
For example, Murata GRM31CC72A475KE11L and other X7S dielectric capacitors improve the anti-aging ability.
Supplier audit:
Ensure strict control of the capacitor production process and reduce capacity deviation.
3、 Summary
The core reason for the low testing of surface mount capacitors with a capacitance value above 1 μ F is due to improper testing conditions, environmental interference, and the characteristics of the capacitor itself. By optimizing testing parameters, controlling environmental conditions, handling material aging, and standardizing operations, this problem can be effectively solved to ensure the accuracy of capacity testing.
