Failure mode of aluminum electrolytic capacitor will the capacitor fail after being discharged for a long time?

Date：2022-05-23 16:41:38 Views：1869

Will the capacitor fail after being discharged for a long time? The electrolytic capacitor cannot be stored for a long time. During the long-term storage process, the electrolytic capacitor needs to be excited by applying the rated voltage regularly to maintain the activity of the electrolyte, otherwise the electrolyte in the capacitor will lose its activity and age. Once the electrolyte loses its activity and age, the consequences will be more serious, and the electrolytic capacitor will be invalid and scrapped. The positive electrode, negative electrode lead-out electrode and shell of aluminum electrolytic capacitor are high-purity aluminum. The medium of aluminum electrolytic capacitor is aluminum oxide film formed on the surface of the positive electrode. The real negative electrode is electrolyte. When working, it is equivalent to an electrolytic cell, but the anodic oxide layer on the surface of the positive electrode has been formed and there is no electrochemical reaction. Theoretically, the current is zero. Due to the existence of impurities in the electrode and electrolyte, it will cause small leakage current. In terms of phenomena, the common failure phenomena and failure modes of aluminum electrolytic capacitors include: dry electrolyte, action of pressure relief device, short circuit, open circuit (no electric capacity), large leakage flow, etc.

1. Concept of aluminum electrolytic capacitor

Aluminum electrolytic capacitor is made of aluminum cylinder as negative electrode, which is filled with liquid electrolyte and inserted with a curved aluminum strip as positive electrode. It also needs to be treated with DC voltage to form a layer of oxide film on the positive electrode as the medium. It is characterized by large capacity, large leakage, poor stability and positive and negative polarity. It is suitable for power filter or low-frequency circuit.

2. Common failure modes of aluminum electrolytic capacitors

When capacitors fail, the materials, structure, manufacturing process, performance and service environment of various capacitors are different, and the failure mechanism is also different. Today, we will reveal the common failure modes of aluminum electrolytic capacitors: liquid leakage, explosion, open circuit, breakdown, deterioration of electrical parameters, etc.

3. Failure mode and cause analysis

(1) Leakage

Liquid leakage is one of the reasons for the failure of capacitors, and aluminum electrolytic capacitors are no exception. The working electrolyte of aluminum electrolytic capacitor is acidic. If it overflows, it will seriously pollute and corrode other components and printed circuit boards around the capacitor. At the same time, the working electrolyte gradually dries up due to liquid leakage inside the electrolytic capacitor, which loses the ability to repair the anodized film medium, resulting in capacitor breakdown or deterioration of electrical parameters. There are many reasons for liquid leakage, such as:

① Poor sealing of aluminum electrolytic capacitor;

② If the rubber plug is used to seal the aluminum electrolytic capacitor, the rubber may be aged and cracked due to long use, resulting in liquid leakage;

③ Problems in mechanical seal process;

④ For the installation problem, the general manufacturers will consider the problem of liquid leakage. They will clearly specify the vertical installation in the enterprise standard, while some enterprises adopt horizontal installation, etc.

(2) Explode

The AC component of aluminum electrolytic capacitor is too large in the working voltage, or there are many defects in the oxide film medium, or there are harmful anions such as chloride and sulfate, so that when the leakage current is large, the gas produced by electrolysis is faster, the longer the working time is, the greater the leakage current is, the more gas in the shell is and the higher the temperature is. The air pressure difference inside and outside the metal shell of the capacitor will increase with the increase of working voltage and working time. If the seal is good and there are no explosion-proof measures, the capacitor will explode when the air pressure increases to a certain extent.

At present, the explosion-proof shell structure has been widely adopted. A pleat seam is added on the upper part of the metal shell. When the air pressure is high, the pleat seam is pushed open to increase the volume in the shell, so as to reduce the air pressure and reduce the explosion risk. In use, such as adding overload voltage, rapidly charging and discharging the capacitor, and applying reverse voltage, the capacitor may explode.

(3) Breakdown

The breakdown between the anode and the aluminum electrolyte is caused by the direct contact between the anode and the aluminum electrolyte. Alumina membrane may be locally damaged due to various materials, processes or environmental conditions. Under the action of external electric field, the oxygen ions provided by the working electrolyte can re form the oxide film at the damaged part, so that the anodic oxide film can be filled and repaired. However, if there are impurity ions or other defects at the damaged part, so that the filling and repair work cannot be improved, micropores will be left on the anodic oxide film, and may even become through holes, causing the breakdown of aluminum electrolytic capacitors.

In addition, with the increase of use and storage time, the solvent in the electrolyte is gradually consumed and volatilized, which increases the acid value of the solution and corrodes the oxide film during storage. At the same time, due to the aging and drying of electrolyte, it is impossible to provide oxygen ions to repair the oxide film under the action of electric field, thus losing the self-healing effect. Once the oxide film is damaged, it will lead to capacitor breakdown.

Process defects are also a major cause of breakdown of aluminum electrolytic capacitors. If the riveting process is poor, the burr on the lead out foil will seriously stab the oxide film, the puncture part will have a large leakage current, and local overheating will cause thermal breakdown of the capacitor. In use, over temperature, over ripple current or over mechanical stress may cause capacitor breakdown failure.

(4) Burn

The burning of aluminum electrolytic capacitor is generally caused by the following reasons:

① Aluminum electrolytic capacitor with positive and negative electrodes connected reversely is a kind of capacitor with positive and negative electrodes. If the positive and negative electrodes are connected incorrectly when installing aluminum electrolytic capacitor, the capacitor will be burned out;

② The withstand voltage is not enough. When the voltage exceeds the withstand voltage value of the aluminum electrolytic capacitor itself, the capacitor will also be burned;

③ The aluminum electrolytic capacitors produced by some manufacturers are unqualified, which may also lead to the burning of capacitors.

(5) Open circuit

The open circuit failure of aluminum electrolytic capacitor may occur when it works in high temperature or hot humid environment for a long time. The reason is that the anode lead out foil is broken due to electrochemical corrosion. For high voltage and large capacity capacitors, there are many failure modes.

In addition, if the anode lead out foil and the anode foil are not fully leveled after riveting, the poor contact will cause intermittent open circuit of the capacitor. In use, excessive mechanical stress may open the capacitor.

(6) Deterioration of electrical parameters

① The capacitance decreases and the loss increases

The capacitance of aluminum electrolytic capacitor decreases slowly in the early stage of operation, which is due to the continuous repair of working electrolyte and thickening of anodic oxide film during load. In the later stage of use of aluminum electrolytic capacitor, due to the large loss of electrolyte, the thickening of solution and the increase of resistivity due to the increase of viscosity, the equivalent series resistance of working electrolyte increases, resulting in the obvious increase of capacitor loss.

At the same time, the electrolyte with increased viscosity is difficult to fully contact the oxide film on the surface of the uneven aluminum foil after corrosion treatment, which reduces the effective area of the electrode plate of the aluminum electrolytic capacitor and causes a sharp decrease in the capacitance. This is also the performance that the service life of the capacitor is approaching the end.

In addition, if the viscosity of working electrolyte increases too much at low temperature, it will also lead to the increase of loss and the sharp decrease of capacitance. In use, over temperature and over ripple current may reduce the capacitance and increase the loss.

② Leakage current increase

The increase of leakage current often leads to the failure of aluminum electrolytic capacitor. Low process level, serious damage and contamination of oxide film, poor formulation of working electrolyte, low purity of raw materials, difficult long-term stability of chemical and electrochemical properties of electrolyte, low purity of aluminum foil, high content of impurities and so on may cause leakage current out of tolerance and failure.

Chloride ion contamination in aluminum electrolytic capacitor is serious. Leakage current leads to the decomposition of oxide film at the contaminated part, causing perforation and further increasing the current. In short, the existence of metal impurities in aluminum foil will increase the leakage current of aluminum electrolytic capacitor and shorten the service life of capacitor. In use, overvoltage may increase the leakage current of the capacitor.

The above is related to the failure mode of aluminum electrolytic capacitor compiled by Chuangxin detection team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing organization for electronic components in China. It has three standardized laboratories with an area of more than 1000 square meters. The scope of testing services covers: testing and verification of electronic components, identification of IC authenticity, product design, material selection, failure analysis, function testing, factory incoming material inspection, X-ray testing of components, tape braiding and other testing items.