Failure Mechanism Analysis of Capacitors
Time：2022.01.12
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1 Overview
Capacitors are important components in electronic devices. There are many types of capacitors, and their main failure modes and mechanisms are different. Common failure modes include breakdown short circuit, open circuit, electrical parameter degradation (capacity change, loss tangent increase, leakage current increase and insulation resistance decrease), electrolyte leakage and lead corrosion fracture.
Failures such as open circuit and short circuit that occur suddenly and completely lose their function are called fatal failures (complete failures), and failures that gradually lose their functions due to excessive electrical parameters are called degenerate failures (partial failures). Among them, fatal failure is extremely harmful, but degradation failure is often the cause and omen of fatal failure.
Capacitors will occur separately or simultaneously under the combined action of working stress (voltage, current, pulse voltage, high-frequency current, pulsating voltage) and environmental stress (temperature, relative humidity, sunlight, time, vibration, shock, mold and harmful gases). For certain failure modes and failure mechanisms, over time one failure mechanism can lead to another failure mechanism. Even under one stress, more than two failure mechanisms can be induced simultaneously. Due to the difference in the material structure, manufacturing process, performance and use environment and conditions of various capacitors, their failure modes and mechanisms are also different. Combined with the characteristics of capacitors, according to practical experience, there are several stresses that are particularly effective in exciting internal defects in electronic components. Therefore, screening is usually performed with only a few typical stresses.
1.1 Temperature shock
The purpose of temperature shock is to measure the ability of components to withstand extremely high and low temperatures, and the effect of alternating high and low temperatures on the device. Changes in electrical properties and shape damage that occur during temperature shocks, mainly caused by changes in size and other physical properties.
1.2 High temperature load
High temperature aging is a static process in which components are continuously operated at specified high temperatures to force early failures. The screening mechanism is to force the development of defects by providing additional thermal action. High-temperature aging screening is an effective method for precipitating defects in electronic components, and is widely used in the screening of components.
2 Failure mode and mechanism analysis
Now I will make a simple analysis of the main failure modes and failure mechanisms of various capacitors combined with my actual work.
2.1 Failure mechanism of solid tantalum electrolytic capacitors
The anode of solid tantalum electrolytic capacitor is a tantalum column (or block) formed by die casting and high temperature sintering of tantalum powder, the dielectric is a tantalum pentoxide (Ta2O5) film on the surface of the tantalum column, and the electrolyte is manganese dioxide. The failure mechanism of solid tantalum capacitors is closely related to the structure, material, manufacturing process and use conditions. The main failure modes during reliability testing and use are instantaneous short circuit, sudden breakdown, increased leakage current and increased loss tangent.
2.1.1 Instantaneous short circuit
Instantaneous short-circuit is a common failure mode of solid tantalum electrolytic capacitors. It is extremely harmful to electronic products. The short-circuit time is between several milliseconds to tens of microseconds, and the leakage current suddenly rises from microamps to milliamps or even amperes. class. Although this instantaneous short circuit will not cause fatal failure of the capacitor. But it caused serious interference to the work of the electronic equipment, which caused the electronic equipment to malfunction.
The reason for the instantaneous short circuit is the presence of blemishes or defects in the Ta2O5 film on the surface of the tantalum core. The appearance of defects or defects is caused by the introduction of impurities due to impure tantalum material or improper process, or defects such as cracks and holes.
For the instantaneous short circuit, we adopt the means of high temperature load, add high temperature and multiple voltages under stress conditions, and eliminate the instantaneous short circuit. At high temperature, the remaining impurities form a solution with tantalum metal, or exist in the form of grain boundary inclusions. Since the current is concentrated in the part where the impurities exist, local heat is generated, causing cracks and cracks in the oxide film, and manganese oxide particles will take advantage of the opportunity. into the gap, resulting in increased leakage current.
2.1.2 Sudden breakdown
The reason for the sudden breakdown is that the crystallization point on the tantalum pentoxide film gradually increases. When the amorphous film is burst, avalanche thermal breakdown occurs immediately. Due to the large short-circuit area, the local self-healing effect of manganese oxide cannot be repaired, resulting in fatal failure of the capacitor. By means of high temperature load, it is also possible to eliminate those short-circuited but not self-healing devices that are damaged by sudden breakdown.
2.1.3 Failure due to increased loss angle
The deterioration and failure of solid tantalum electrolytic capacitors is mainly caused by the increase of the loss tangent value. The increase of loss is extremely harmful to the high-frequency characteristics. The loss includes the dielectric loss of the tantalum pentoxide film, and the contact resistance loss between the welding part of the metal wire and the particle layer.
When there is a bad contact inside the capacitor, the loss tangent value will jump or drift. In view of the problem of loss angle, we adopt the means of temperature shock, through repeated shocks of corresponding high temperature and low temperature, to eliminate the poor contact and Not binding.
2.2 Failure mechanism of aluminum electrolytic capacitors
The positive base of aluminum electrolytic capacitors is high-purity aluminum foil, the dielectric is the aluminum oxide film formed on the surface of the aluminum foil, and the cathode is a solution electrolyte with a certain viscosity. Aluminum electrolytic capacitors are widely used in electronic equipment. The failure mode and mechanism are described below.
2.2.1 Breakdown
The reason for the breakdown is that the raw materials have impurities or manufacturing defects, so some extremely tiny holes or even perforations are generated, resulting in the breakdown of the electrolyte in direct contact with the anode. Consistent with the method of tantalum electrolytic capacitors, for breakdown, we use high-temperature load methods to increase high temperature and multiple voltages under stress conditions. If there are impurity ions or other defects on the surface of the oxide film, some extremely tiny holes or even become perforations, resulting in breakdown.
2.2.2 Liquid leakage
The operation of aluminum electrolytic capacitors is similar to that of an electrolytic cell. The higher the working voltage, the longer the time, the more gas is generated on the positive and negative electrodes. Most of the gas is used to repair the defects of the dielectric oxide film, and a small amount of gas is released and stored. in the cavity inside the capacitor. If the oxide film has defects such as gaps, holes and defects, it will cause the capacitor to generate more gas and heat due to the increased leakage current, thus causing a higher air pressure inside the capacitor. When the air pressure increases to a certain extent, it will carry The electrolyte was squeezed out of the casing and leaked.
Liquid leakage not only causes corrosion of the components and printed boards around the capacitor, but also causes the electrolyte of the capacitor to dry up, causing the capacitor to lose its self-healing ability, decrease the capacitance value, and even cause open-circuit failure in severe cases. We usually test the airtightness of the aluminum electrolytic capacitor shell through temperature shock, and solve the problem of liquid leakage through high temperature load.
