Basic knowledge of electrode materials

The inner and outer electrodes are important components of capacitors. The inner electrode is mainly used to store charge. The size of its effective area and the continuity of the electrode layer are two major factors affecting the quality of the capacitor. The outer electrode is mainly used to connect the inner electrodes of each layer parallel to each other in parallel and connect them to the peripheral circuit. The outer electrode of the chip capacitor is the chip terminal.
The materials used to make the inner and outer electrodes are generally metal materials.

1. Internal electrode material
As we all know, the internal electrode of chip capacitors is made by printing. Therefore, the internal electrode material exists in the form of a fluid metal or metal alloy slurry before sintering, so it is called internal electrode slurry, or internal slurry for short. Since chip multilayer ceramic capacitors use BaTiO3 series ceramics as dielectrics, this series of ceramic materials are generally sintered at around 950℃~1300℃; therefore, the internal electrodes are generally made of high melting point precious metals such as Pt, Pd, Au, etc., and are required to be sintered at a high temperature of about 1400℃ without oxidation, melting, volatilization, loss, etc.
Melting points of several metals

At present, the commonly used pastes in the world are Ni, Ag/Pd, and pure Pd pastes. Ag/Pd and pure Pd are both precious metal materials and are expensive. The inner electrode of pure Ag has a relatively low sintering temperature, and the reliability of the manufactured products is relatively poor. Therefore, it is rarely used now. In view of the low melting point and high temperature instability of silver, metal Pd and Ag alloys are generally used to increase the melting point of the inner electrode and Pd is used to suppress the fluidity of Ag. The ratio of Pd to Ag in the commonly used inner paste is 3/7, 6/4, and 7/3 (the numerator is metal Pd and the denominator is metal Ag), and the inner electrode of pure Pd is rarely used due to its high price.
For chip capacitors, the cost of its inner electrode accounts for 30% to 80% of the capacitor, so using cheap metal as the inner electrode is an effective measure to reduce the cost of monolithic capacitors. Therefore, in Japan and some other countries, electronic pastes with base metals as inner and outer electrodes were developed as early as the 1960s. At present, the process of using Ni as the inner electrode and Cu as the outer electrode is very mature. In this way, high firing and high reliability and the use of base metals can reduce costs make their chip capacitors very competitive in the world. Three Japanese companies, Taiyo Yuden, Murata Manufacturing, and TDK, have already put Ni electrode products into large-scale production and put them on the market. Murata's GRM600 series temperature compensation monolithic capacitors use Cu as the inner electrode, with a monthly production volume of 100 million.
Metal nickel is a very ideal base metal as an inner electrode and has good high-temperature performance. Its characteristics as an electrode are: (1) The electron migration speed of Ni atoms or atomic groups is lower than that of Ag and Pd-Ag. (2) High mechanical strength. (3) Good electrode wettability and resistance to welding heat. However, it is easily oxidized to green nickel oxide at high temperatures, so the quality of the inner electrode layer cannot be guaranteed. Therefore, it must be fired in a reducing atmosphere. However, on the contrary, if titanium-containing ceramics are sintered in a reducing atmosphere, Ti4+ will be reduced to low-valent ions and the insulation of the ceramic will decrease. Therefore, in order to ensure the quality of the Ni electrode and the dielectric properties of the BaTiO3 titanium-containing ceramics at the same time, sintering in a protective atmosphere is generally adopted.

2. Terminal electrode material
The terminal electrode serves to connect the multi-layer internal electrodes of the ceramic body with the peripheral circuits. Its greatest impact on the chip capacitor is mainly reflected in the solderability and solder resistance of the chip. We currently have two basic forms:
(1) Pure silver terminal electrode. Because the melting points of silver and tin are quite different, this terminal electrode is generally only suitable for manual soldering. The advantage of this welding method is that during welding, only the two ends of the chip are subjected to the high temperature thermal shock of the soldering iron, while the thermal shock of the ceramic body is relatively small. Therefore, the capacitor is less affected by the thermal shock, but the efficiency is low and the terminal electrode adhesion is poor.
(2) Three-layer electrode, commonly used are Ag-Ni-Sn, Ag-Ni-Au, Cu-Ni-Sn (BME) with a total of three layers. The first silver layer is prepared through the termination process; the second nickel layer and the third tin layer (or gold) are plated through the electroplating process. Because the outermost layer of this terminal electrode is a tin layer, it is suitable for all welding. The role of the three-layer electrode:
Ag: Good contact with the inner electrode, which directly affects the reliability of the chip, the thickness is generally 50um.
Ni: Thermal barrier layer, its thickness has a direct impact on the chip's resistance to welding heat, thickness 2 to 4um.
Sn: Good contact with the peripheral circuit. Directly affects the chip's solderability, thickness 4 to 7um.
The advantage of this welding method is that it is suitable for large-scale automated production, that is, SMD patch system.
In addition, when soldering chip capacitors on circuit boards, the choice of solder paste is also very important. At present, the most commonly used solder paste is Sn62. Several other solders at home and abroad are listed as follows: