Minimum ESR tantalum composite electrode capacitor

Minimum ESR under high voltage – tantalum composite electrode capacitor

power supply and networking applications require tantalum capacitors with a higher voltage range, and the smaller the ESR level, the better. This paper introduces a new technology based on composite electrode design, which can reduce the ESR of 25V and 35V components to the current low level. In order to update the technical level, the electrode design, sintering process, electroforming and manganese treatment must be improved accordingly. In addition, a special electroforming process is used to improve the breakdown resistance under high voltage

introduction

tantalum capacitor technology has many advantages. It is very suitable for the zigzag loading of dc/dc relay cantilever zigzag Charpy samples. It adopts the weight loading situation, the filtering of power supply and other applications. The most important features are as follows:

- low and stable ESR

- high capacity holding capacity at high frequency

- low failure rate

- wide voltage range

- good surge resistance

- good environmental resistance (humidity/temperature)

- low cost

the most influential parameter on the above features is the shape of the electrode. The surface area of tantalum capacitor electrode, especially the ratio of surface area to volume, determines its ESR value. The larger the surface area, the lower the ESR

figure 1 Electrolytic design cross section

due to the reasons of finished products and efficiency, the standard design of capacitors generally adopts single electrode (Fig. 1a). Composite (multiple) electrodes (Fig. 1b) provide the lowest ESR. The flute electrode design (Fig. 1c) is a compromise design between single electrode and composite electrode: that is, low ESR requirements should be considered; Production costs should also be taken into account. See references [1], [2], [3] for further details

compared with the new polymer composite electrode capacitor, the composite electrode design with the traditional MnO2 second electrode system has several advantages, such as "real" lead-free reflux capacity, good stability under wet load, and the high voltage potential of the capacitor. Especially in high voltage fields, such as 25 to 50V multi electrode tantalum design, it can provide the lowest industrial ESR and help electronic designers develop small and efficient power supply products. However, there are some technical limitations that make the preparation of these components difficult. The technology introduced in this paper enables AVX to make 25v-35v capacitors reach the lowest ESR level, and has the potential to develop 50V tantalum SMD composite electrode capacitors

high voltage composite electrode capacitor

development achievements have introduced a new type of low ESR high voltage tantalum capacitor. E housing 33uf 35 V composite electrode capacitor frequency characteristics - see figure 2 The ESR value at 100 kHz is about 40mohms But the way of obtaining is not direct

figure 2 ESR vs frequency, e 33uf/35 engineering measurement specification GB 50026 ⑼ 3V tantalum capacitor with composite electrode

pressing & sintering

it is necessary to sinter the electrode core at extremely high sintering temperature to make high-voltage components with sufficient strength and internal particle coupling. If the thickness of the neck is not enough, the dielectric layer will be disconnected or become too thin when it is formed, which will affect the stability of the component

however, such high-temperature necking is accompanied by large-scale shrinkage of tantalum core. When the structure of electrode 2 and core is analyzed and judged to be uneven or inconsistent, it will shrink and then deform (see Figure 3). Such electrodes are not suitable for composite assembly of electrodes

figure 3 Unqualified electrode shape after sintering to solve this problem, we used a special tantalum powder bonding material to optimize the mixing of tantalum powder. The side pressing method is adopted to obtain uniform pressing effect along the longitudinal length of the electrode. The sintering process should also be improved to reduce nonlinear damage. Finally, the electrode production quality was improved to meet the subsequent composite assembly requirements (Fig. 4)

figure 4 Qualified form of sintered electrode core

electroforming of dielectric layer

electroforming of dielectric layer is the most important process in the production of tantalum capacitors. Tantalum pentaoxide was electroformed into amorphous hydrated glass

however, after sintering precipitation, some seed crystals produced by oxidation always appear on the surface of tantalum. The formation of dielectric crystal layer depends on temperature and time. Therefore, the thermal control of the first molding is very important for the long-term stability of the electrode

since the electrode is subjected to thermal shock in the manganese treatment process, it also needs to be re formed. Therefore, electroforming is usually disturbed by high-temperature baking. However, in addition to mechanical relaxation and medium layer damage (due to the different thermal expansion rates of TA and Ta2O5), dehydration will occur in the medium layer in this process. This is a serious condition, because it will cause oxygen vacancy state and thus participate in dielectric layer breakdown. The vacancy must be intermediated by the oxygen donor through a chemical process. The thicker the dielectric layer, the more serious the problem

in all final structures of all capacitors, the outer surface and edge of the electrode are the areas that bear the most electric shock. All charges entering the electrode are shunted at the surface. Therefore, when electrical breakdown occurs, it often occurs on the surface. The low rated voltage code is used to protect the outer surface: that is, the so-called "shell structure", that is, the thickness of the dielectric layer is only on the outer surface. It is possible to increase the import of waste paper into waste pulp overseas as a technology

therefore, the development of "shell structure" with high rated voltage code such as e 33uf/35v (see Figure 5) is another important step. When the traditional shell forming method is used, the generated color (the diffraction result of light on the dielectric film) is uneven due to the voltage drop during the electrode shell forming. However, the outer dielectric layer corresponds to a 125 – 130 V molding voltage, for example, compared to a 104 V molding of a "non shell mechanism". This means that the electric field strength of the surface dielectric layer decreases from 190 kv/mm to 150 kv/mm Through further mechanical and chemical improvement, the uniform color effect of the outer dielectric layer can be achieved, and the higher voltage (e.g. 140 V) can be corresponding without reducing the capacitance. The electric field intensity on the surface of the dielectric layer is further reduced to about 135 kv/mm

figure 5 [green] - non shell shaped electrode (104 V)

[red] - traditional shell shaped electrode (V)

[blue] - the effect of reducing the surface electric shock caused by the "shell" method of the shell shaped electrode (140 V)

is very obvious on the breakdown voltage distribution diagram of the capacitor (Fig. 6) Due to the "shell" structure, the breakdown voltage distribution becomes narrower and higher. This means that compared with the traditional forming method, the stability of the components is improved

figure 6 E33uf/35 v

tantalum composite electrode capacitor special shell forming breakdown voltage distribution

manganese treatment

the electrode structure of high rated voltage tantalum capacitor is different from that made of finer tantalum powder. For example, the shape of the structure is different, and the inner granular channel is wide. Therefore, the pyrolysis of manganese nitrate, like a chain reaction, is faster and more intense than that of the fine tantalum powder core observed by us. The internal aggregate channel acts like a "chimney" to collect and accumulate nitrogen oxides to the surface. Nitrogen oxides are dispersed to the surface and form manganese dioxide, creating "spray holes" and "spray mounds". As a result, the outer layer of manganese dioxide is often uneven or layered (Fig. 7)

figure 7 The outer surface of the manganese layer is unqualified. Since the electrode will be composite assembled, such defects will not only affect the smooth assembly of the composite electrode tantalum capacitor, but also affect the final performance of the components. In the orifice area, the possibility of silver diffusion and penetration will be greatly increased

the processing of parts with high rated voltage requires a flat and uniform manganese dioxide coating (Fig. 8). The improved treatment measures include the concentration and application sequence of manganese nitrate, especially the pyrolysis conditions of manganese nitrate

figure 8 Qualified form of high rated voltage electrode after manganese treatment

silver plating and graphite coating

we have specially developed a unique system for flute type low ESR capacitor to ensure the super stability under low ESR and humidity. This system is also applied to the concept of composite electrode. E high stability of 33uf/35 V composite electrode tantalum capacitor under wet conditions refer to figure 9

figure 9 Humidity stability of e33uf/35 V tantalum composite electrode capacitor before (red) and after (blue) [1000 special graphite silver layers, 85% RH after 85 ° C, 250 hours, 20 seconds to measure saturation time]

application

the above process has been applied to capacitors with rated voltage of 25v-35v, and has the potential to increase the voltage to 20V in the near future. The main application fields are telecommunication/base station, power supply and automobile industry, which need miniaturized and efficient power supply. The 25 to 35V tantalum composite electrode will provide the lowest industrial ESR to support the common operating voltage of V. ESR level of E 68uf 5V component is as low as 55mohm; 65mohm for e UF 35V components

summary & CONCLUSION

we have developed the traditional tantalum composite electrode technology to expand the voltage range of the product series. Corresponding technical improvements include:

- electrode design

- pressing & sintering

- dielectric layer forming

- manganese treatment

- silver plating and graphite coating

reference materials

1] Racek at Col. "improved ESR on MnO2 tantalum capacitors at wide voltage range" cars USA 2002, proceeding "ESR improvement of MnO2 tantalum capacitors with wide voltage range"

2] ed, Rshall "18mohms and falling – new ultra low ESR tantalum chip capacitors" carts USA 1999 New Orleans PP "18mohms and failures - new ultra low ESR tantalum core Capacitors"

3] DD, "lowest available ESR conventionally coated multiple anode tantalum capacitors" carts USA 2000 California PP "existing minimum ESR - uniformly coated composite electrode tantalum capacitors"