Due to the continuous innovation of microelectronics technology, capacitors used on both sides of general power circuits and the central processing unit (CPU) of personal computers are required to have excellent electrical properties such as miniaturization, high capacity, and low impedance; tantalum electrolytic capacitors are used as electrolytic capacitors. One of the capacitors has the above characteristics, so tantalum electrolytic capacitors are widely used in communications, aerospace and military industries, submarine telegraphy and advanced electronic devices, civil appliances, televisions and many other aspects.
Definition of tantalum electrolytic capacitors
Tantalum electrolytic capacitors are made of metal tantalum (Ta) as the anode material. According to the different anode structures, they can be divided into two types: foil type and tantalum powder sintering type. Different electrolytes are divided into tantalum capacitors with solid electrolytes and tantalum capacitors with non-solid electrolytes. Among them, the amount of solid tantalum electrolytic capacitors is large, such as CA type, CA42 type, CA45 type and so on.
Compared with aluminum electrolytic capacitors, tantalum electrolytic capacitors have the following advantages
1. Small size. Since tantalum capacitors use tantalum powder with very fine particles, and the dielectric constant ε of the tantalum oxide film is 17 higher than that of the aluminum oxide film, the capacitance per unit volume of the tantalum capacitor is large.
2. Wide operating temperature range. Generally, tantalum electrolytic capacitors can work normally at a temperature of -55°C to 125°C, while aluminum electrolytic capacitors can only work normally at a temperature of -55°C to 105°C. Not as good as tantalum electrolysis.
3. Long life, high insulation resistance and low leakage current. The tantalum oxide film dielectric in tantalum electrolytic capacitors is not only resistant to corrosion, but also maintains good performance for a long time.
4. Good impedance frequency characteristics. For capacitors with poor frequency characteristics, when the operating frequency is high, the capacitance drops significantly, and the loss (tgδ) also rises sharply. But solid electrolytic capacitors can work above 50KHz. As the frequency increases, the capacity of tantalum capacitors also decreases, but the decrease is small. Some data show that the capacity of tantalum capacitors decreases by less than 20% when working at 10kHz, while the capacity of aluminum electrolytic capacitors decreases by 40%.
5. High reliability. The chemical properties of the tantalum oxide film are stable, and because the tantalum anode substrate Ta2O5 is resistant to strong acids and alkalis, it can use solid or acid-containing liquid electrolytes with low resistivity, which makes the loss of tantalum electrolytic capacitors higher than that of aluminum electrolytic capacitors. small and with good temperature stability.
Compared with ceramic capacitors, tantalum electrolytic capacitors have the following advantages
1. Stability characteristics. When the DC bias voltage applied to the capacitor increases or the temperature rises, the capacity will decrease, and the decrease in the capacity will cause the function and stability of the amplifier to deteriorate. However, with the increase of the DC bias voltage, the capacity of the tantalum electrolytic capacitor decreases faster than The capacity of ceramic capacitors decreases much more slowly.
2. AC signal. Due to the small AC signal (20-30%), the capacity decreases, and the ceramic capacitor applies a small AC signal voltage, and the capacity decreases, which will affect the function and poor stability of the amplifier.
3. Filter cleanliness. Ceramic capacitors use barium titanate materials, and there must be a piezoelectric effect, that is, X5R, X7R, Y5V dielectric layers are used in small microphones or speakers, and additional noise will be generated due to the decrease of signal fidelity; while the electrolyte material used in tantalum electrolytic capacitors MnO2 There is a small voltage effect, which ensures the fidelity of the signal.
4. Reliability. According to the military description, the failure rate of ceramic capacitors is more than 30% higher than that of tantalum capacitors.
5. Mechanical properties. High CV ceramic capacitors are easy to break in vertical drop test, while tantalum capacitors have high impact resistance due to their unique plastic-encapsulated structure.
The failure mechanism of tantalum capacitors
In general, there are three main parameters for evaluating the performance of tantalum capacitors: leakage current Ir (μA); capacitance C (μF); loss tangent value tgδ (%); a number of test items are specified in the technical specification to evaluate the three parameters , so as not to fail the components during use.
Due to the characteristics of the solid electrolyte of tantalum electrolytic capacitors, the failure curve of tantalum electrolytic capacitors is in the shape of a bathtub. In fact, in the test, the phenomenon of capacity change and loss tangent failure is very rare, hardly exceeding the requirements of technical specifications, and the failure is mainly concentrated in the deterioration of leakage current.
The leakage current is to apply the rated DC working voltage to the capacitor. The change of the charging current will be observed, which starts to be very large, decreases with time, and reaches a relatively stable state when it reaches a certain final value. This final value current is called Ir as leakage current. current.
The leakage current reflects the insulation quality of the tantalum capacitor dielectric film Ta2O5. The ideal capacitor dielectric should be a perfect film, and its insulation resistance can reach hundreds of megohms or more. In fact, there are various tiny defects, voids and gaps on the surface of Ta2O5, and the leakage current is composed of the impurity ion current and electron current passing through these defects. If the current is large, under the high stress of the test, the electrical stress will be concentrated and the current density will be large, which will "crystallize" the oxide film around the defect, expand the area of the defect, further deteriorate the dielectric quality, decrease the insulation resistance, and increase the leakage current. When the leakage current increases beyond the specified value of the technical specification, the capacitor fails. Sometimes the leakage current becomes infinite, the actual dielectric film has broken down, and the capacitor is completely useless. Therefore, when discussing the failure mechanism of capacitors, in theory, all three parameters may fail. In fact, when the leakage current increases, it is the fatal failure of the capacitor. If a tantalum capacitor with a large leakage current and failure is used (such as on the whole machine), the whole machine cannot work, and even some lines are burned. In the tantalum capacitor test project, more than 90% of the quality problems reported by users are mainly reflected in the deterioration of the leakage current.
Factors affecting the failure of tantalum capacitors
There are two main factors that affect the failure of tantalum capacitors. On the one hand, it is the influence of the materials that make capacitors - tantalum powder and tantalum wire, and on the other hand, it is the influence of the process of making capacitors.
(1) Influence of tantalum powder and tantalum wire
The chemical properties, physical properties, impurity content, particle shape, size and breakdown voltage of tantalum powder and tantalum wire directly affect the quality of tantalum capacitors. The impurity content in tantalum powder and tantalum wire has a great influence on the quality of the formed oxide film. The anode core of tantalum capacitors needs to be sintered at high temperature and high vacuum at 1500-2050 ℃ when forming. One of the purposes of sintering is to remove impurities in tantalum powder and tantalum wire, and those refractory impurities, such as tungsten, molybdenum, Silicon, iron, copper, etc., are difficult to be completely removed during sintering, and become the "nucleus" of defects when forming an oxide film, and become conductive channels. Therefore, the requirements for the impurity content of tantalum powder are extremely strict, generally less than 10-50PPM. There are many specifications of tantalum powder. According to the working voltage of the capacitor, it is divided into high-voltage powder, medium-pressure powder and low-voltage powder. The specific volume, physical properties and breakdown voltage of various powders are different. The specification of the capacitor and the reasonable and proper selection of tantalum powder can ensure the quality of the capacitor.
(2) Influence of capacitor manufacturing process
The production process of tantalum capacitors also directly affects the performance of tantalum capacitors, especially several key processes will directly affect the leakage current of tantalum capacitors.
First, the anode manufacturing process is to shape the tantalum powder and sinter it in a high temperature vacuum. The purpose is to shape and purify. It needs to be sintered in a high vacuum at 1500-2050 °C to remove impurities and achieve the purpose of purification. If the purification effect is not good, the residual impurities in the tantalum anode core will become the "crystal nucleus" in the dielectric film, which is a hidden danger of leakage current.
Second, the energizing process is to place the tantalum anode in the electrolyte, apply a DC voltage, and the oxygen ions in the electrolyte and the tantalum in the tantalum anode form a Ta2O5 film. In this process, the formulation of the electrolyte, temperature, voltage, boost current density, and constant voltage time must be strictly controlled according to the process requirements. After the energizing process is completed, the energizing effect inspection, especially the capacitance and leakage current, must meet the process requirements. It is hoped that the smaller the leakage current value, the better. During the energizing process, if a certain link is not well mastered, the phenomenon of "crystallization" is easy to occur. Therefore, the energizing process requires the manufacture of a complete dielectric film layer without the phenomenon of "crystallization".
Third, the coating process is the production of capacitor cathode MnO2, and it is also a key process that affects the leakage current. Now the process adopts the "water vapor coating process", which improves the quality of the MnO2 layer, and its resistance value is extremely small, so the loss value of the capacitor is small. The leakage current depends on the process control of the coating and the effect of intermediate energization.
Fourth, in addition, the quality of the assembly process and the effect of the aging process all affect the leakage current performance of the capacitor.
Fifth, the screening process is a process of further strict inspection of the finished tantalum capacitors. Usually, high and low temperature screening, long-term high-temperature aging screening, and X-ray transmission inspection are used.
Conclusion
First, the capacitance per unit volume is particularly large, that is, the specific capacity is very high.
Second, during the working process of the capacitor, it has the performance of automatically repairing or isolating the defects in the oxide film, so that the oxide film can be strengthened and restored to its proper insulating ability at any time, without continuous cumulative damage. That is to say, the failure probability of tantalum electrolytic capacitors is very low.
Third, the specific surface area of tantalum electrolytic capacitors is large, which is a necessary condition to ensure their miniaturization.
Fourth, the leakage current and loss tangent are small, and the temperature and frequency characteristics are good.
Fifth, good use stability and reliability, long service life.
