Capacitor Common Failure Mode and Failure Mechanism

Capacitor common failure mode:
- Short-circuit breakdown; fatal failure
- Open; fatal failure - changes in electrical parameters (including capacitance ultra poor loss tangent value increases, the insulation performance degradation or increased leakage current; Some features failure
- Leakage; Some features lapse
- Fatal failure lead to corrosion or breakage;
- Insulator rupture; fatal failure
- The surface of the insulator flashover; Some features fails
Caused by the failure of the capacitor is varied. The various capacitor materials, structures and manufacturing processes, performance and use of the environment of the same failure mechanism is not the same.
Common failure mode generation mechanism is summarized as follows.
3.1 Failure Modes of failure mechanisms
3.1.1 cause capacitor breakdown failure mechanism
(1) dielectric material of a defect or defects, or contain conductive impurities or the conductive particles;
(2) The dielectric electrical aging and thermal aging;
(3) dielectric internal electrochemical reaction;
(4) silver ion migration;
(5) the dielectric in the capacitor manufacturing process subject to mechanical damage;
(6) dielectric changes in molecular structure;
(7) between the arcing in high humidity or low pressure environment;
(8) under mechanical stress dielectric instantaneous short circuit.
3.1.2 cause the capacitor open the main failure mechanisms
(1) the lead part of a "self-healing", the electrode lead wire insulation;
(2) lead line contact with the electrode surface oxide, resulting in low open circuit;
(3) pinout poor electrode contact;
(4) electrolytic capacitor anode the lead foil corrosion cracking;
(5) liquid electrolyte dry up or freeze;
(6) mechanical stress dielectric instantaneous open.
3.1.3 caused by the deterioration of the electrical parameters of the capacitor failure mechanism
(1) moisture or surface contamination;
(2) silver ion migration;
(3) self-healing effect;
(4) dielectrics aging and thermal aging;
(5) working electrolyte volatile and thickens;
(6) electrode corrosion;
(7) wet electrolytic capacitor dielectric corrosion;
(8) role of impurities and harmful ions;
(9) pinout and the contact resistance of the electrode increases.
3.1.4 The main capacitor leakage caused
(1) Impregnated materials electric field so that the containment pressure rise to the liberation of gas.;
(2) capacitor metal housing with poor sealing cap welding;
(3) insulators with poor housing or lead soldering;
(4) The semi-enclosed capacitor mechanical seal adverse;
(5) half seal capacitor lead surface is not smooth;
(6) working the electrolyte corrosion of solder joints.
3.1.5 main cause capacitor lead to corrosion or fracture
(1) electric field electrochemical corrosion in high temperature environment
(2) electrolyte along lead leakage, the lead subjected to chemical etching;
(3) lead to mechanical damage in the capacitor manufacturing process;
(4) The lead insufficient mechanical strength.
3.1.6 the main capacitor insulators rupture caused by
(1) mechanical damage;
(2) the glass powder insulator sintering process too much residual heat;
(3) welding temperature is too high or uneven heating.
3.1.7 The main cause the surface of the insulator flashover
(1) insulator surface moisture, surface insulation resistance drops;
(2) insulator design is unreasonable;
(3) insulator improper selection;
(4)The environmental pressure is too low;
The rest of the breakdown of the capacitor, open fracture lead insulator rupture the capacitor completely lost the ability to work, failure is a fatal failure failure causes the capacitance can not meet the requirements, and the gradual transition to a fatal failure;
Capacitor under the combined effects of work stress and environmental stress, after working for a period of time, separately or at the same time generate some failure modes. A variety of failure mechanisms in the same failure mode, the same failure mechanism can be generated multiple failure modes. The relationship is not a one-to-one correspondence between the failure modes and failure mechanisms.
3.2 capacitor failure mechanism analysis
3.2.1 wet deteriorating electrical parameters
When the humidity in the air is too high, the water film condensed on the surface of the capacitor case, enables the surface insulation resistance of the capacitor is decreased. Here, for the semi-sealed structure capacitor, moisture may also penetrate into the internal of the capacitor dielectric, the insulating ability of the insulation resistance of the capacitor dielectric decline. Therefore, the high temperature, high humidity environment capacitor parameters deteriorate very significantly. Drying dehumidification capacitor electrical properties can be improved, but the consequences of the electrolysis of water molecules can not be eradicated. For example: the capacitor to work under high temperature conditions, water molecules in the electric field electrolysis into hydrogen ions (H +) and hydroxide ion (OH-), the root of lead wire electrochemical corrosion. Even if drying dehumidification, it is impossible to lead recovery.
3.2.2 silver ion migration consequences
Inorganic dielectric capacitor is applied to most of the silver electrodes, the semi-seal the capacitor to work under high temperature conditions, water molecules penetrate the inside of the capacitor electrolysis. The oxidation reaction is generated at the anode, the binding of the silver ions and hydroxide ions to produce silver hydroxide. Reduction reaction of silver hydroxide and hydrogen ions produced by the reaction of silver and water generated at the cathode. Since the electrode reaction, the silver ion of the anode is reduced to keep the cathode not continuous metallic silver particles, into a tree connected by a water film to the anode extension. Silver ion migration occurs not only in the surface of the inorganic medium, the silver ions can diffuse into the inside of the inorganic dielectric, causing leakage current increases, completely short-circuit between two silver electrodes can be severe, leading to capacitor breakdown.
Silver ion migration can be serious damage to the silver layer of the positive electrode surface, and the wire bond pads and the electrode surface of the silver layer, the interval between the silver oxide has a semiconductor nature, so that the inorganic dielectric capacitor's equivalent series resistance increases, the metal portion loss increases, the capacitor The loss tangent values ​​increased significantly.
Since the effective area of ​​the positive electrode is reduced, the electrical capacity of the capacitor will therefore decrease. Surface insulation resistance due to the inorganic dielectric capacitor between the two electrodes on the surface of the medium the presence of a silver oxide semiconductor decreases. Severe silver ion migration, between two electrodes erected dendritic SBG, so that the insulation resistance of the capacitor is decreased significantly.
In summary, the silver ion migration not only causes deterioration of electrical properties of the unsealed inorganic dielectric capacitor, and may cause dielectric breakdown field strength decline, finally led to the capacitor breakdown.
Worth mentioning: the silver electrode low frequency ceramic monolithic capacitors due to the migration of silver ions caused by the failure of the phenomenon is much more serious than other types of ceramic dielectric capacitor, because this capacitor firing process with multi-layer laminated structure. A sintering process of the silver electrode and the ceramic dielectric, silver involved in the solid phase reaction of the surface of the ceramic dielectric, and infiltrated ceramic - silver contact at the formation of the interface layer. If enough dense ceramic dielectric, after the penetration of moisture, the migration of silver ions can not only occur in the surface of the ceramic dielectric, and may also penetrate the ceramic dielectric layer. More the gap of the multi-layer laminate structure, and between the electrode position is not easy to precisely, the surface of the medium leaving the edge of a small amount, when the external electrodes of the both ends of the lamination layer coated silver paste to seep through the cracks and reduce the insulation resistance of the surface of the medium, and the electrode the path is shortened, easy to produce a short circuit when the silver ion migration phenomenon.
3.2.3 Up under high humidity conditions ceramic capacitor breakdown mechanism
Breakdown failure is more common serious problems occurred when the semi-hermetic ceramic capacitors (such as: chip capacitors) working under conditions of high humidity environment. Breakdown phenomenon can be divided into approximately dielectric breakdown and surface electrode arc flashover two categories. Dielectric breakdown occurred sooner or later, can be divided into early breakdown and aging breakdown two. Exposure to the early breakdown of the capacitor dielectric material and production process, the presence of defects, these defects cause the ceramic dielectric electric strength decreased significantly, so that in a high humidity environment, under the action of the electric field, the capacitor in the withstand voltage test process, or early work, to produce electrical breakdown. Aging breakdown mostly belonging the electrochemical breakdown areas. Due to the migration of silver ceramic capacitors, ceramic capacitors, electrolytic aging breakdown has become a fairly common problem. Dendrimers was formed conductive silver migration, a local increase of the leakage current, which can cause thermal breakdown, so that the capacitor is broken or burned. Thermal breakdown phenomenon occurred in a small ceramic capacitor tube-shaped or wafer-shaped, because the local heat breakdown powerful, easy to burn or fracture of the thin wall or small porcelain body.
Furthermore, in the titania-based ceramic dielectric, the load conditions may produce a reduction reaction of the titanium dioxide, the titanium ions by the tetravalent to trivalent. The aging of the ceramic dielectric, significantly reduces the dielectric strength of the capacitor, and may cause the capacitor breakdown. Accordingly, such ceramic capacitors electrolytic breakdown phenomenon is more serious than Excluded titania ceramic dielectric capacitors.
Silver ion migration so that the capacitor electrode between the edge field generating serious distortion, high-humidity environment because the surface of the ceramic dielectric condensate water film, the corona discharge voltage of the capacitor edge surface significantly decreased, generated under the working conditions of the surface of the inter-electrode arcing phenomena. In severe cases lead to the surface of the capacitor electrode arc flashover. The surface breakdown hydrophobicity and moisture permeability and other factors related with the capacitor structure, distances between electrodes and the load voltage, the protective layer. Mainly is the edge surface of inter-electrode arcing breakdown, because the medium leaving the edge quantity is smaller, in a humid environment when working silver ion migration and surface water film is formed so that the edge surface of the capacitor insulation resistance decreased significantly, causing a corona discharge, and ultimately lead to breakdown. Particularly serious in the high humidity environment. Generation of silver ion migration and development will take some time, so early in the withstand voltage test, the failure mode is mainly to dielectric breakdown until 500h after the test, the main failure mode transition for the edge of the surface electrode arc flashover.
For failure caused by high humidity, can be used in the practical application of the the spraying three anti-paint the entire board after the installation of the device.
3.2.4 The high frequency precision capacitor low level failure mechanism
Mica is an ideal capacitor dielectric material having a high insulation performance, high temperature, the dielectric loss is small, the thickness can be as thin as 25 microns. The main advantage of the mica capacitor is a small loss, frequency stability is good, the distributed inductance is small, the insulation resistance, particularly suitable for use in high frequency communication circuit using precision capacitors. However, mica resources are limited, it is difficult to promote the use of. In recent decades, organic thin-film capacitor was a rapid development, including polystyrene film capacitors with low loss, high insulation resistance, good stability, high dielectric strength advantages. Precision polystyrene capacitors instead of mica capacitors for high frequency circuit. It is noted that: the precision polystyrene capacitors used in the high frequency circuit, generally use a metal foil plate, and to improve the insulation resistance and reduce losses.
The low level failure of the capacitor is the new problems that have emerged since the 1960s. Low level failure is a the capacitor work under the conditions of low-voltage capacitor open or capacity decline ultra poor failure phenomenon. Semiconductor devices widely used since the 1960s, the semiconductor circuit voltage is much lower than the tube circuit, so that the actual operating voltage of the capacitor in some circuits, only a few millivolts, cause capacitor low level failure, specific performance is the capacitor completely lost capacitance or partial loss of power capacity. For low impact, so that the electrical capacity of the capacitor is returned to normal.
Generate a low failure main reason is that the capacitor pinout poor contact with the plates of a capacitor, the contact resistance increases, resulting in the capacitor completely open or capacitance declines.
Precision polystyrene film capacitors commonly used aluminum foil as a plate, copper lead wires with aluminum foil plate spot-welded. Aluminum foil in the air easily oxidized; electrode surface to generate a layer of semiconductor thin film of alumina, the voltage on the oxide film is low under conditions insufficient to breakdown, and thus the aluminum foil is formed between the gap electric capacity equivalent series the smaller capacity gap capacitance, the series equivalent capacity is also smaller. Therefore, the low-capacity depending on the thickness of the alumina layer of the electrode surface, the thicker the alumina layer, the smaller the capacitance of the capacitor under the conditions of the low. In addition, when working in an AC circuit, the capacitor, the effective capacitance due to the contact resistance is too large decline, the contact resistance is large effective capacitance can be reduced to the extent of open. Even if the plate a lead does not exist between the poor conductivity of the spacer layer, will produce this effect.
Failure caused Precision polystyrene capacitors low specific factors are summarized as follows:
(1) the wire surface oxidation or the soiled layer is too thin, so that welding is not strong;
(2) lead and aluminum foil poor spot welding, does not eliminate the alumina coating layer of the spot welding at the surface of the aluminum foil;
(3) single lead structure too little solder points, to increase the probability of failure of low-level;
(4) crude lead roots hit the flat part of the contact area although larger, but after spot solder joints at stress, heat treatment or thermal cycling process may damage the contact area, worsening the contact situation;
(5) moisture into the capacitor slug, oxidation corrosion of joints, so that the contact resistance increases.
Specific factors causing the mica capacitors low level failure is summarized as follows:
(1) silver electrode and a lead film of the thin layer between the copper foil and the copper foil and the lead card to December. Low under the conditions, the applied voltage is insufficient to breakdown the layer insulating film, a gap capacitor and the contact resistance is increased;
(2) silver electrode and the copper foil by the erosion of the harmful gases, so that the contact resistance increases. Wet sulfur gas environment silver and copper easily vulcanization, so that the increase in the contact resistance between the electrode plate and the lead.
3.2.5 metallized paper dielectric capacitor failure mechanism
Metallized paper dielectric capacitor plate is vacuum-evaporated on the surface of the metal film of the capacitor paper
A, the deterioration of the electrical parameters of failure
"Self-healing" is a unique advantage of metallized capacitor, but the healing process is quite complex, self-healing are able to avoid the capacitor immediate medium short circuit breakdown, but the self-healing site will definitely migration of metal particles and dielectric materials heated in cracking phenomenon. Capacitor paper is composed of fibers, the cellulose is a polymer of the carbohydrate material. Capacitor at high temperatures the cellulose solution into a free state in a carbon atom or carbon ions, an increase in the surface conduction capacity of the healing site, resulting in a capacitor resistance decreases, and the loss increases and the capacitance decreases. Severe enables capacitor failure due to the degree of deterioration of the electrical parameters exceed the scope of technical conditions permit.
Metallized paper dielectric capacitors work below the rated working voltage conditions, lack of self-healing energy, the capacitor paper in the presence of the conductive impurities in the electric field in the formation of a low resistance path, can also lead to the reduction and loss of insulation resistance of the capacitor increases .
Capacitor paper is a polar organic porous dielectric materials, and can easily absorb moisture. Capacitor core impregnation treatment, but if the process is improper or impregnated impure, or as impregnated aging considerable time working in the electric field, the insulation resistance of the capacitor will therefore reduce the loss will therefore increase.
Capacitance ultra poor a failure in the form of producing metallized paper dielectric capacitor failure. Metallized paper dielectric capacitor storage under high temperature conditions may increase due to the capacitance excessive failure in high temperature conditions plus voltage work time and may reduce the excessive and failure due to capacitance. High temperature storage, semi-enclosed metal paper dielectric capacitors inevitably absorb moisture, the water is strongly polar substances, its dielectric constant is nearly 20 times the impregnation capacitor dielectric constant. Therefore, a small amount of moisture intrusion capacitor core, can cause the power capacity was significantly increased. The capacitance was baking dewetting declined. If the capacitor is to work in a high-temperature environment, the common role of moisture and the electric field causes the metal film electrodes to produce electrolytic corrosion, so that the plate reduces the effective area of ​​the electrode plate resistance increases, resulting in a substantial decline in electric capacity. If the contact portion of the lead and the metal film layer cause corrosion, then the contact resistance increases, and the effective capacity of the capacitor will be further reduced. Individual capacitance of the capacitor manufacturer can be reduced to close to the open degree.
Lead fracture failure
Metallized paper dielectric capacitor is to work in a high humidity environment, the capacitor positive terminal lead roots would be subjected to severe corrosion, electrolytic corrosion leading to reduced lead mechanical strength, lead can cause serious fracture failure
3.2.6 aluminum electrolytic capacitor failure mechanism
The aluminum electrolytic capacitor of the positive electrode is a high-purity aluminum, the dielectric is formed on the metal surface oxide aluminum film, the negative electrode is a viscous electrolyte work quite an electrolyzer. Aluminum electrolytic capacitor common failure mode: leakage, explosion, open, breakdown, deterioration of the electrical parameters of the failure mechanism is analyzed as follows.
A, leakage
Electrolytic capacitor electrolyte leakage is a serious problem. Working electrolyte slightly showed acidic, the leaked electrolyte serious pollution and corrosion capacitors around the other components and printed circuit boards. Electrolytic capacitor internal leaking the leaving work electrolyte gradually dry, the loss of the ability to repair the anodic oxide film media, lead capacitor breakdown or deterioration of the electrical parameters and failure.
Many causes of a leaking sealed aluminum electrolytic capacitors poor. Using aluminum anode foil is sandwiched between the housing edge and the sealing plate a sealing structure is very easy to leakage of the electrolyte solution in the shell side. Rubber stopper sealing the capacitor, may also be due to aging of rubber, cracking caused by the leakage. In addition, the mechanical seal process the product in question is also prone to leakage. In short, the structure of the leakage and the seal, the sealing material and the sealing process of a close relationship.
Especially aqueous capacitor, this capacitor in order to increase the permittivity of the electrolyte and water proportion. The dielectric material of the electrolytic capacitor is very sensitive to temperature, especially aqueous electrolytic capacitor, which is because the aqueous electrolyte of the aqueous electrolytic capacitor with the temperature expansion and contraction, the temperature is too high and high volatiles may even generate a gas, resulting inside the capacitor the pressure is too high, and when the pressure exceeds the critical point, the shell cracked electrolyte overflow. The capacitor capacity formula is as follows:
C [F] = ε0 · ε · S / t
Q: electricity (C)
V: Voltage (V)
C: electric capacity (F)
S: electrode area [m2]
t: medium thickness
ε: relative permittivity
ε0: a dielectric constant of the medium in the vacuum state
Explosion
Aluminum electrolytic capacitor in the working voltage of the AC component is too large, or the oxide film dielectric has many defects, or the presence of chloro-root like harmful sulfate anion, so that the leakage current is large electrolytic action generating gas rate, large part of the gas is used to patch the anodic oxide film, less the portion of the oxygen is stored in the capacitor housing. The working time is longer, the greater the leakage current, the more containment gas, the higher the temperature. The the capacitor metal shell inside and outside air pressure difference increases with the operating voltage and the increase in working hours. If the product seal is poor, it will cause leakage; if the seal is good, no explosion-proof measures the pressure increases to a certain extent, it will cause the capacitor explosion. High-voltage large-capacity capacitor leakage current explosion is more likely. Currently, commonly used by the structure of the explosion-proof enclosure, one folds of increase in the upper part of the metal casing folds of high pressure to open the top, increasing the containment volume, thereby reducing the air pressure, reduce the risk of explosion.
C, the open circuit
The aluminum electrolytic capacitor in the high temperature or hot flashes environment over a long work may appear open circuit failure, the reason is that the anode lead-foil subjected to electrochemical corrosion and fracture. This failure mode is more for high-voltage large-capacity capacitor. In addition, the anode lead foil and anode foil riveting, without fully flat, poor contact causes the capacitor intermittent open circuit.
Aluminum electrolytic capacitor using the electrolyte of DMF (dimethyl formamide) as a solvent, DMF solution is the oxidant, the oxidation ability at a high temperature. Working after some time may be due to anode lead caulking portion of the sheet of foil with the welding to generate an oxide film caused by capacitor open. If using ultrasonic welding machine lead foil and solder together, can reduce such failure phenomenon.
D, breakdown
Aluminum electrolytic capacitor breakdown anodic alumina dielectric membrane rupture, leading to electrolyte direct contact with the anode. The alumina membrane may be partial damage due to reasons of a variety of materials, processes, or environmental conditions. The working electrolyte provided in the applied electric field, oxygen ions can be provided in the site of injury to form an oxide film, the anodic oxide film to fill repair. However, if the presence of the impurity ions or other defects at the site of injury, so filled repair work can not be perfect, will leave microporous anodic oxide film, and may even become a penetrating hole aluminum electrolytic capacitor breakdown.
Moreover, with the growth of the use and storage of the time, gradually consumed and volatilization of the electrolyte solvent, and the solution acid value increased, the corrosive effect of the oxide film occurs during storage. Electrolyte aging and drying up under the electric field has been unable to provide the oxygen ions repair oxide film, so that the loss of the self-healing oxide film once damaged it will cause the capacitor to breakdown. The workmanship is also a major cause of aluminum electrolytic capacitor breakdown. Enabling the process of the formation of the anodic oxide film is not dense and firm enough, the oxide film from serious injury in the follow-up pieces, riveting. This anodic oxide film is difficult to repair in the last of the old-refining process improvement, so that the use of the process of a capacitor leakage current, local self-healing can not save the final breakdown of fate. Another example is the riveting poor leads burrs serious stab wounds oxide film on the chaff, stabbed parts of a large leakage current, local overheating of the capacitor to produce thermal breakdown.
E, the deterioration of the electrical parameters
Capacitance decreased loss increases
The electric capacity of the aluminum electrolytic capacitor is decreased slowly during work, this is due to the working electrolyte in the load during the constant repair and thickening of the anodic oxide film caused. The aluminum electrolytic capacitor in use late, due to the electrolyte depletion more, and the solution thickens, resistivity rises due to viscosity increases the the working electrolyte equivalent series resistance increases, resulting in capacitor loss significantly increased. Meanwhile, the viscosity increases of the electrolyte is difficult to sufficiently contact with the oxide film layer on the uneven surface of the aluminum foil by the etching process, thus making the effective area of ​​the electrode plate of the electrolytic capacitor decreases, causing the electric capacity sharply declines. This capacitor service life near the end of the performance.
In addition, if the working electrolyte at low temperature viscosity increases too much, will cause the consequences of loss increases sharply with the electric capacity. Boric acid glycol systems work electrolyte poor low temperature performance, the viscosity is too large to result in equivalent series resistance surge, so that the loss becomes larger and the effective capacitance is smaller, causing the lapse aluminum electrolytic capacitor is used in a cold environment.
Leakage current is increased.
The increase in leakage current often leads to failure of aluminum electrolytic capacitors. Enabling low level of technology, the formation of oxide film is not dense and solid technology behind the opening piece, the oxide film damage and severe contamination, poor work electrolyte formula, purity of raw materials, the chemical nature of the electrolyte and electrochemical properties difficult long-term stability, not high-purity aluminum foil, impurity content ... these factors could cause leakage current ultra-poor failure.
The aluminum electrolytic capacitors of chloride ion contamination seriously, the leakage current leads to contamination of parts of decomposition of oxide film, resulting in perforation, which causes current to be further increased. In addition, the higher the impurity content of the aluminum foil, generally the size of the particles of iron impurities is greater than the thickness of the anodic oxide film, so that the current is easy to conduction. Copper and silicon, the presence of impurities affect the structure of the aluminum oxide to crystalline transition. Copper and aluminum also micro battery electrolyte, the aluminum foil was corrosion damage. In short, the presence of metal impurities in aluminum foil, make aluminum electrolytic capacitor leakage current is increased, thereby shortening the life of the capacitor.