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Abstract: The welding technique between the electrodes and leads of ceramic capacitors used in SF6 high-voltage circuit breakers was studied. By using 62Sn/36Pb/2Ag solder paste along with appropriate processing measures, the problems associated with welding using tin foil—such as difficulty in controlling the welding process, tin accumulation, and silver dissolution—along with the issue of aging of the conductive adhesive used when epoxy resin-based silver conductive adhesives were employed—were resolved. A welding method that is simple to implement and results in significantly improved welding strength was developed. Keywords: ceramic capacitor, lead, welding process. The 550kV class SF6 type high-voltage circuit breakers produced by a domestic high-voltage switch factory were important research projects during China’s Seventh and Eighth Five-Year Plans. The high-voltage ceramic capacitors used in these circuit breakers were previously imported; in order to reduce costs and promote the localization of such capacitors, our factory, through years of research, has successfully developed high-voltage ceramic capacitors suitable for 550kV class SF6 type high-voltage circuit breakers. The structure of this capacitor (see Figure 1 for its appearance) consists of two parallel electrodes to which φ18mm copper electrode leads (referred to simply as leads) are welded; an insulating paint is applied over them. The copper electrode leads serve to facilitate electrical contact and conduction when the capacitors are connected in series, and the strength of the weld between these leads and the capacitors directly affects the performance of the capacitors. During the initial development, the leads were connected to the silver electrodes using soldering; that is, a thin layer of tin foil was placed between the leads and the silver electrodes, and then the mixture was heated to 230°C for 30 minutes to melt the tin, thereby achieving welding. This method is difficult to control in terms of the amount of tin used and the direction of tin melting and diffusion; often, an excess of tin results in tin accumulation on the surface of the silver electrodes, making capacitor assembly difficult. Furthermore, uneven tin diffusion between the leads and electrodes causes poor soldering at the lead areas, reducing the welding strength. Excessive tin, when melted at high temperatures for extended periods, causes silver to dissolve into the tin, a phenomenon known as \"silver dissolution,\" which affects the electrical properties and welding strength of the capacitor. Someone once used an organic epoxy resin combined with conductive silver powder to create a conductive adhesive, in order to bond the leads to the electrodes. Although this method temporarily resolves issues such as tin buildup and silver dissolution, and the bonding strength meets the requirements for now, the organic material epoxy resin loses its strength over time, which increases the risk of the leads coming loose with prolonged use; this can lead to malfunctions in the circuit breaker during operation. To address these issues, we sought a material suitable for connecting flake-shaped leads and electrodes, one that possessed high strength, simple processing requirements, easy control over the amount of material used, an attractive appearance, and did not affect the performance of the capacitor. Through repeated tests, 62Sn/36Pb/2Ag paste solder was selected for welding and positioning, and numerous tests and performance evaluations were carried out. Figure 1: Appearance of ceramic capacitors. 1. Process: Solder paste is primarily used in surface mount technology; by successfully applying solder paste to the welding of ceramic capacitor leads, we have developed a viable process method suitable for this purpose. For welding with 62Sn/36Pb/2Ag solder paste, the welding process curve is shown in Figure 2. The specific steps are as follows: (1) Store the solder paste at room temperature for 2 hours to soften it, then stir it with a rod until it becomes paste-like for use ； (2) Use a squeegee to apply the solder paste evenly onto the leads, controlling the amount used so as to avoid excess ； (3) Press the leads tightly against the silver surface, fix them using a mold, and place them in the oven ； (4) Heat the oven; hold it at 130°C for 3–5 minutes, then raise the temperature to 200°C and maintain it there for 10 minutes. After that, turn off the heating and allow the oven to cool down naturally. Source: High-Voltage Switch Network. Figure 262: Solder paste welding curve for Sn/36Pb/2Ag. Solder paste is composed of ultra-fine particles of lead, tin, and silver along with fluxing organic substances. Due to its fine particles, the specific surface area increases, which reduces the surface free energy; as a result, the melting temperature drops and the melting time shortens. It is held at 130°C for several minutes to promote the volatilization of organic components, after which the temperature is raised to 200°C to melt the Sn-Pb alloy, thereby achieving welding. 2 Test Results and Discussion A comparative analysis was conducted on capacitors using solder paste and conductive adhesive for positioning purposes. A RESTRVMENTT5K tensile tester from the United States was used to measure the strength of the lead terminals after welding, as well as the impact of these two materials on the electrical performance of the capacitors. 2.1 Terminal strength test: The method for testing the strength of capacitor lead terminals is shown in Figure 3. We conducted strength tests on the lead terminals in accordance with relevant standard requirements; the specific results are shown in Table 1. As can be seen from the comparison in Table 1, the strength of the lead terminals secured using solder paste is greater than that of the terminals secured using conductive adhesive. This is because the Sn-Pb alloy in solder paste has strong wettability for the silver in the electrodes and the lead in the leads, resulting in good welding quality; whereas the bonding achieved by conductive adhesive is a mechanical adhesion, and its bonding strength is limited to a certain extent. Tin does not age, and therefore there is no risk of the leads coming loose in capacitors during use. Table 1 shows the results of the lead terminal strength test. After welding positioning, a uniform tin ring is formed at the edge of the capacitor lead, and this tin ring enhances the welding strength between the lead and the silver electrode. This phenomenon was observed previously during the analysis of similar capacitors in Japan, but it was unclear how it occurred. By using solder paste as a material for welding and positioning, this problem was successfully resolved; the resulting products exhibit high mechanical strength, as well as a beautiful and smooth surface. Figure 3: Test method for the strength of lead terminals 2.2 Impact on electrical properties: Capacitors with leads soldered using solder paste were subjected to routine electrical property tests and corona tests before and after soldering. The results showed that soldering the leads with solder paste had no adverse effect on the electrical properties of the capacitors, such as capacitance, dielectric loss, insulation resistance, voltage tolerance, and partial discharge. The test results are shown in Table 2. 2.3 Others: When using tin foil for welding and positioning, it is difficult to control the amount of foil used, which often results in an accumulation of tin on the surface of the electrodes, affecting the appearance and performance of the capacitors. Additionally, due to the high welding temperature and long duration, silver dissolution occurs, sometimes leading to poor welds that reduce the welding strength. By using a new type of tin paste composed of 62Sn/36Pb/2Ag, the welding temperature is reduced and the time required for welding is shortened. The inclusion of 2% silver in the Sn-Pb alloy makes it easier to control the amount of tin paste used, thereby significantly reducing silver dissolution. Using this tin paste for welding and positioning simplifies the manufacturing process and is easy to master. No flux is needed during welding, which prevents contamination of the capacitor’s surface; moreover, the resulting appearance after welding is attractive. Positioning is achieved using solder paste; although the material cost is higher compared to tin foil, the solder paste positioning process is simple to operate, results in better product shapes, and increases the yield rate ； Compared to conductive adhesives for positioning, solder paste has a lower cost and simpler manufacturing process, while also significantly enhancing the strength of the lead terminals. http://img.hc360.com/electric/info/images/200805/15_143511.gif Table 2: Effect of solder paste welding of leads on the electrical properties of capacitors. 3 Conclusions: The use of paste-like solder paste for welding and positioning the electrode leads in ceramic capacitors used in SF6-type high-voltage circuit breakers improves upon the difficulties associated with the foil soldering method, such as poor control over the welding process, easy accumulation of solder on the surface, and silver dissolution ； Its adhesion strength is **higher** than that of epoxy resin-based silver conductive adhesive wires; it solves the problem of reduced welding strength due to aging of organic materials over time. It features low costs for solder paste application, high strength of the terminals, and simple processing procedures. It has been widely used in production, and the products supplied to users have received positive feedback. 62Sn/36Pb/2Ag solder paste is suitable for soldering capacitor leads.