Leakage of surface mount capacitors (i.e., a decrease in insulation resistance causing unexpected current to pass through the dielectric) is a common failure mode, typically caused by multiple factors acting alone or in combination. Its essence lies in the destruction of the insulation performance of the dielectric:
Defects and deterioration of the material itself:
Inherent defects of dielectric materials: During the manufacturing process, there may be small pores, impurities, or uneven grain boundaries inside the ceramic dielectric (MLCC). These defects will become weak points for electric field concentration, and are more prone to local breakdown or increased leakage current under high electric field strength. For electrolytic capacitors such as tantalum capacitors and aluminum electrolysis, the purity, density, and thickness uniformity of the oxide layer (dielectric) are crucial, as any defects can reduce insulation resistance.
Aging of dielectric materials: Some types of ceramic dielectrics (such as Class II X7R, Y5V and other ferroelectric materials) themselves exhibit aging phenomena. Over time, the internal domain structure will slowly change, resulting in a decrease in dielectric constant, and the insulation resistance may also slowly decrease (although it is usually still within the specification range, it may exceed the standard during extreme aging or critical initial values). The oxide layer of electrolytic capacitors will slowly degrade over time, especially under high temperature and no pressure conditions.
Silver ion migration (mainly for MLCCs with silver containing end electrodes): In high temperature and high humidity (HTHH) environments, especially in the presence of DC bias, silver ions (Ag ⁺) in the end electrodes may migrate towards the opposite electrode through moist dielectric surfaces or microcracks, forming conductive dendrites. Once the dendrite bridges two electrodes, it can cause serious leakage or even short circuit. The use of nickel barrier layer or palladium silver electrode in MLCC can significantly suppress this phenomenon.
Manufacturing process defects:
Laminated Defect (MLCC): During the stacking process of multi-layer ceramic capacitors, if there are bubbles, foreign objects, or poor bonding between layers, internal gaps or weak areas may form, which can easily generate leakage channels under high field strength.
Sintering defects: Insufficient sintering or uneven temperature of ceramic bodies can lead to an low-density grain structure, uneven distribution of grain boundaries or residual pores, significantly reducing insulation performance.
Internal/edge cracks: Microcracks generated during the manufacturing process (such as cutting, burning ends), or cracks caused by improper release of packaging stress, can damage the integrity of the medium. Cracks not only directly provide a leakage path, but may also absorb moisture and accelerate degradation.
Terminal electrode defects: Poor adhesion between the terminal electrode and the ceramic body (poor wettability), presence of voids or contamination, may cause abnormal contact resistance or local electric field concentration, leading to leakage.
Oxide layer defects (electrolytic capacitors): When forming an oxide film (such as Ta ₂ O ₅ in tantalum), improper process control (such as formation voltage, temperature, time) can lead to uneven oxide layer thickness, pinholes or impurities, directly reducing its insulation strength.
External stress damage:
Mechanical stress:
PCB bending/twisting: When surface mount capacitors (especially large-sized MLCCs) are subjected to excessive bending or twisting stress during circuit board assembly (such as splitting) or use, the ceramic body may crack (often manifested as cracks perpendicular to the terminal electrodes). These cracks become direct leakage channels.
Shock/vibration: Severe physical shock or sustained strong vibration may also cause microcracks inside the capacitor.
Improper operation: Excessive manual welding temperature, prolonged time, direct impact of soldering iron tip on capacitors, or use of tin absorbers during maintenance can cause local stress concentration, all of which may damage capacitors.
Thermal stress:
Temperature shock: Rapid and drastic temperature changes (such as wave soldering, rework, and switching between hot and cold environments) can cause stress due to differences in coefficient of thermal expansion (CTE) between ceramic bodies, electrode materials, and PCB materials, which may lead to cracking.
Local overheating: There are heating elements or poor welding near the capacitor that cause abnormal heating, and high temperatures can accelerate the deterioration of the dielectric material (such as the decomposition of the electrolytic capacitor oxide layer and the accelerated aging of the ceramic dielectric), reducing the insulation resistance.
Electrical stress:
Overvoltage: Applying voltage exceeding the rated voltage of the capacitor (especially surge voltage or repeated instantaneous overvoltage) can directly penetrate the dielectric or seriously damage the dielectric structure, resulting in permanent leakage increase or even short circuit. Even if it is not immediately broken down, repeated overvoltage can cause the medium to "fatigue" and the insulation performance gradually decreases.
Reverse voltage (electrolytic capacitor): Aluminum electrolysis and tantalum capacitors (especially MnO ₂ cathode tantalum capacitors) are extremely sensitive to reverse voltage. A small reverse voltage may damage the oxide layer medium, leading to a sharp increase in leakage and even explosion and fire.
Environmental stress:
High temperature and high humidity (HTHH): This is a key environmental factor that induces electrical leakage, especially silver migration and electrochemical corrosion. Moisture infiltrates into the interior of capacitors (through packaging materials, microcracks, or edges) and participates in electrochemical reactions (such as electrolysis, electrode corrosion) or promotes ion migration under the action of an electric field.
Contaminants/Chemical Corrosion: Pollutants such as solder residue, salt spray, hydrogen sulfide (H ₂ S), sulfur dioxide (SO ₂), etc. on PCBs may corrode the terminal electrodes or internal structures of capacitors in humid environments, forming conductive pathways or damaging media.
Radiation: High energy radiation (such as in space or nuclear environments) may damage the molecular structure of dielectrics, leading to a permanent decrease in insulation performance.
Circuit design and usage factors:
Uneven voltage distribution: When multiple capacitors are used in series (such as for high voltage), if effective voltage sharing measures are not taken (such as parallel voltage sharing resistors), it may cause one capacitor to withstand voltage exceeding its rated value and cause damage and leakage.
Inappropriate testing methods: Using inappropriate instruments (such as a regular multimeter ohm range) or applying excessive testing voltage to measure insulation resistance may cause damage to the capacitor or result in incorrect readings (contact resistance impact).
Summarize key points:
The core of leakage in surface mount capacitors is the failure of dielectric insulation performance. This failure may stem from:
Congenital deficiencies: inherent defects in materials and manufacturing processes.
Post natal damage: Physical damage (cracks) or chemical/electrochemical degradation (silver migration, corrosion, medium aging and decomposition) caused by external stresses such as mechanical/thermal/electrical/environmental factors.
Improper use: overvoltage, back pressure, long-term operation in harsh environments, circuit design defects.
Understanding these reasons is crucial for preventing capacitor failure, improving circuit reliability, and quickly locating problems in fault analysis. Just as cracks in a reservoir inevitably lead to leakage, once a "crack" (in a physical or chemical sense) appears in a capacitive medium due to the above reasons, leakage occurs.