It is a normal physical phenomenon for surface mount capacitors (especially the most commonly used multilayer ceramic capacitors MLCC) to have extremely small leakage currents, but the leakage currents must be within the allowable range specified in the manufacturer's specifications. Leakage current exceeding the specifications is considered a fault or abnormal phenomenon, indicating that the capacitor may be damaged or its performance may deteriorate.
Here is a detailed explanation:
Ideal capacitance vs. actual capacitance:
Ideal capacitor: There is a perfect insulating medium between two conductor plates, and direct current cannot pass through at all.
Real capacitance: Any actual insulating medium is not perfect, and when a DC voltage is applied, there will be extremely small currents passing through the medium or leaking along the surface. This small current is called leakage current or insulation resistance current.
Structure and leakage current source of MLCC:
MLCC is composed of alternately stacked ceramic dielectric layers and metal electrode layers.
Main paths of leakage current:
Body leakage current: the current passing through the ceramic dielectric material itself. Ceramic materials have extremely high electrical resistivity (usually ranging from 10 ^ 9 Ω· m to over 10 ^ 14 Ω· m), but not infinite.
Surface leakage current: The current flowing along the surface of the ceramic medium, especially the creepage path between the end electrodes. This is usually related to environmental humidity and surface pollution, but in manufacturing good clean capacitors, this value is relatively small.
Minor defects, impurities, pores, grain boundaries, etc. during the manufacturing process can all become "channels" for leakage current.
Definition in the specification sheet - Insulation resistance:
Manufacturers do not directly label "leakage current", but instead label a more critical parameter: insulation resistance.
Insulation resistance: The resistance value measured after applying a specified DC voltage (usually rated voltage) across the two ends of a capacitor. The unit is usually M Ω (megaohms) or G Ω (gigaohms), sometimes expressed in Ω· F (ohm farads) (representing that the larger the capacitance value, the larger the allowable absolute leakage current).
Calculation formula: 'Leakage current=applied DC voltage/insulation resistance'. The higher the insulation resistance, the smaller the leakage current.
Specification requirements: All qualified MLCCs must have an insulation resistance greater than or equal to the minimum value specified in the specification (e.g. ≥ 1000 M Ω, ≥ 5000 M Ω, ≥ 100 G Ω, etc., depending on the material grade, rated voltage, and capacitance value of the capacitor) when they leave the factory.
The meaning of "normal leakage":
As long as the measured insulation resistance is greater than or equal to the minimum value required by the specification, the corresponding leakage current is a normal physical phenomenon allowed by the design. This current is usually extremely weak (at the nA nanoampere or even pA picoampere level) and does not have a noticeable impact on functionality in the vast majority of circuit applications.
This small leakage current is determined by the physical characteristics of the capacitor itself, which is inevitable but strictly controlled within a safe and acceptable range.
Under what circumstances is electric leakage considered 'abnormal'?
The insulation resistance is lower than the minimum value specified in the specifications: this is the most direct criterion for judgment. The measured leakage current far exceeds the theoretical value calculated based on the insulation resistance according to the specifications.
Reason:
Defects/contamination of dielectric materials: Impurities, pores, cracks, etc. introduced during the manufacturing process lead to a decrease in insulation performance.
Internal cracks: Internal cracks in capacitors that occur during manufacturing (sintering cooling stress), surface mounting (thermal stress), or use (mechanical stress, thermal shock), damaging the insulation layer.
Overvoltage or voltage surge: The applied voltage exceeds the rated voltage or encounters severe surges, causing local breakdown or degradation of the dielectric layer, forming a conductive path.
Electrochemical migration: Under conditions of humidity, bias voltage, and the presence of impurity ions, metal ions may migrate inside or on the surface of the medium, forming conductive dendrites.
High temperature deterioration: Long term operation at high temperatures may lead to deterioration of dielectric materials or interface properties, reducing insulation resistance.
External pollution: Residual flux, moisture, dust, salt spray and other pollutants form conductive paths on the surface of capacitors (increasing surface leakage).
Consequences: Abnormal leakage current will:
Consume battery power (standby current increases).
Affects the accuracy and stability of high impedance circuit nodes, such as sample and hold circuits and precision sensor interfaces.
Causing the capacitor to generate heat on its own, and even leading to thermal runaway (higher risk in high voltage and large capacity capacitors).
In severe cases, it may develop into short circuit failure.
Summary:
Yes: The existence of extremely small leakage currents that comply with the specifications is a normal physical property of MLCC.
No: Leakage current exceeding the specified range in the specification sheet (i.e. insulation resistance lower than the requirements in the specification sheet) is abnormal, indicating that the capacitor has defects, damage, or aging and needs to be replaced.
Key point: The only reliable standard for determining whether the leakage of surface mount capacitors is normal is to measure their insulation resistance and compare it with the minimum value specified in the manufacturer's specifications. In circuit design and troubleshooting, if there is suspicion of abnormal capacitor leakage, a high resistance meter or a digital multimeter with high resistance measurement function should be used for insulation resistance testing.