Abstract : This paper studies the welding technology between the electrodes and leads of ceramic capacitors used in SF6 high-voltage circuit breakers. Using 62Sn/36Pb/2Ag solder paste and corresponding process measures, the paper solves the problems of difficult process control, easy solder buildup, and silver dissolution associated with previous soldering methods using solder foil, as well as the aging problem of conductive adhesive when using epoxy resin silver conductive adhesive. A simple welding method with significantly improved welding strength has been obtained. Keywords : Ceramic capacitor, Lead welding process. A 550 kV SF6 high-voltage circuit breaker manufactured by a domestic high-voltage switchgear factory is an important research project in China's "Seventh Five-Year Plan" to "Eighth Five-Year Plan". Previously, the high-voltage ceramic capacitors配套 for this circuit breaker were all imported. To reduce costs and promote the localization of this capacitor, our factory, after years of research, has successfully developed a high-voltage ceramic capacitor for 550 kV SF6 high-voltage circuit breakers. The capacitor (see Figure 1 for its appearance) consists of two parallel electrodes welded with φ18mm copper electrode leads (referred to as leads), coated with insulating varnish. The copper electrode leads serve as contact conductors during series assembly of the capacitor. The welding strength between the leads and the capacitor directly affects the capacitor's performance. Initially, the leads and silver electrodes were connected using solder. This involved sandwiching a thin layer of tin foil between the leads and the silver electrodes, then heating to 230℃ and holding for 30 minutes to melt the tin. However, this method was difficult to control in terms of tin usage and the direction of tin melting and diffusion. Often, excessive tin resulted in tin buildup on the silver electrode surface, making capacitor assembly difficult. Furthermore, uneven tin diffusion between the leads and electrodes caused partial cold solder joints in the leads, reducing welding strength. Excessive tin, when melted at high temperatures for extended periods, caused silver to dissolve into the tin, a phenomenon known as "silver dissolution," affecting the capacitor's electrical performance and welding strength. Some researchers have used organic epoxy resin mixed with conductive silver powder (conductive adhesive) to bond the leads and electrodes. While this method temporarily solves problems such as solder buildup and silver dissolution, and the bonding strength temporarily meets the requirements, the organic epoxy resin material ages over time, reducing the bonding strength and potentially causing the leads to detach during long-term use, which could lead to circuit breaker failure during operation. To address these issues, we sought a material suitable for connecting sheet leads and electrodes, possessing high strength, simple processing, easy control of material usage, aesthetically pleasing appearance, and no impact on capacitor performance. Through repeated experiments, we selected 62Sn/36Pb/2Ag paste solder paste for soldering and positioning, and conducted multiple tests and performance evaluations. Figure 1 shows the appearance of the ceramic capacitor. Solder paste is mainly used in surface mount technology. By successfully applying solder paste to the soldering of ceramic capacitor leads, we have explored a feasible process method suitable for capacitor lead soldering. The soldering process curve for positioning with 62Sn/36Pb/2Ag solder paste is shown in Figure 2. The specific operation is as follows: (1) Store the solder paste at room temperature for 2 hours to soften it, and then stir it into a paste with a stick for later use; (2) Use a scraper to evenly scrape the solder paste onto the lead wire, controlling the amount used, and not using too much; (3) Attach the lead wire to the silver surface, fix it with a mold and put it into the oven; (4) Heat the oven, stop at 130℃ for 3-5 minutes, then continue to heat to 200℃ and hold for 10 minutes, stop heating and let it cool down naturally. Figure 2 Soldering curve of 62Sn/36Pb/2Ag solder paste Solder paste is made of lead, tin and silver ultrafine particles with added flux organic matter. Due to its fine particles, the specific surface area is increased and the surface free energy is reduced, which lowers the melting temperature and shortens the melting time. Hold at 130℃ for several minutes to promote the volatilization of organic components, and then heat to 200℃ to melt the Sn-Pb alloy to achieve the purpose of soldering. 2. Experimental Results and Discussion A comparative analysis was conducted on capacitors positioned with solder paste and conductive adhesive, respectively. The strength of the lead terminals after soldering was measured using a RESTRVMENTT5K tensile testing machine (USA), and the influence of the two materials on the capacitor's electrical performance was also investigated. 2.1 Terminal Strength Test The test method for the capacitor lead terminal strength is shown in Figure 3. We conducted lead terminal strength tests according to relevant standards, and the specific results are shown in Table 1. From the data comparison in Table 1, it can be seen that the lead terminal strength of solder paste positioning is greater than that of conductive adhesive positioning. This is because the Sn-Pb alloy in the solder paste has strong wettability to the silver electrode and the lead of the lead, resulting in good soldering performance. Conductive adhesive positioning, on the other hand, is a mechanical adhesion, and its bonding strength is somewhat limited. Solder does not age, therefore, lead detachment during capacitor use is not a problem. Table 1: Lead Terminal Strength Test Results After soldering and positioning, a uniform solder ring is formed at the edge of the capacitor lead, which increases the soldering strength between the lead and the silver electrode. This phenomenon was previously observed during the analysis of similar Japanese capacitors, but the cause remained unclear. Using solder paste for soldering and positioning successfully solved the problem, resulting in high mechanical strength and a smooth, aesthetically pleasing surface after soldering. Figure 3 shows the lead terminal strength test method. 2.2 Influence on Electrical Performance Conventional electrical performance tests and corona tests were conducted on capacitors with solder paste-soldered leads before and after soldering. The results showed that solder paste soldering had no adverse effect on capacitor electrical performance, such as capacitance, dielectric loss, insulation resistance, withstand voltage, and partial discharge. The test results are shown in Table 2. 2.3 When using tin foil for positioning, it is difficult to control the amount of foil used, often resulting in tin buildup on the electrode surface, affecting the appearance and use of the capacitor. Furthermore, the high welding temperature and long time cause silver dissolution, sometimes leading to cold solder joints and reducing weld strength. Using a new material, 62Sn/36Pb/2Ag solder paste, for positioning reduces the welding temperature and time. Simultaneously, 2% silver is dissolved in the Sn-Pb alloy, making the amount of paste easier to control and significantly reducing silver dissolution. Solder paste positioning is simple and easy to master, eliminating the need for flux during soldering, preventing surface contamination of the capacitor, and resulting in a more aesthetically pleasing appearance after soldering. While solder paste positioning increases material costs compared to tin foil, it offers simpler operation, better appearance, and higher yield. Compared to conductive adhesive positioning, solder paste is lower in cost, simpler in process, and significantly enhances the strength of the lead terminals. Table 2. Influence of Solder Paste on Capacitor Electrical Performance 3. Conclusion: The use of paste-like solder paste for electrode lead soldering and positioning in SF6 high-voltage circuit breaker ceramic capacitors improves upon the difficult-to-control process of solder foil soldering, which easily leads to surface solder buildup and silver dissolution. It significantly improves lead adhesion strength compared to epoxy resin silver conductive adhesive, solving the problem of solder strength degradation due to aging of organic materials over long-term use. Solder paste positioning has low cost, high terminal strength, and simple operation, and has been widely applied in production. The products have received positive feedback from users. 62Sn/36Pb/2Ag solder paste is suitable for soldering capacitor leads.