Brazing of precious metal contacts

Precious metals mainly refer to Au, Ag, PD, Pt and other materials, which have good conductivity, thermal conductivity, corrosion resistance and high melting temperature. They are widely used in electrical equipment to manufacture open and closed circuit components.

(1) Brazing characteristics as contact materials, precious metals have the common characteristics of small brazing area, which requires that the brazing seam metal has good impact resistance, high strength, certain oxidation resistance, and can withstand arc attack, but does not change the characteristics of contact materials and electrical properties of components. Since the contact brazing area is limited, solder overflow is not allowed, and the brazing process parameters should be strictly controlled.

Most heating methods can be used to braze precious metals and their precious metal contacts. Flame brazing is often used for larger contact components; Induction brazing is suitable for mass production. Resistance brazing can be carried out with ordinary resistance welding machine, but smaller current and longer brazing time should be selected. Carbon block can be used as electrode. When it is necessary to braze a large number of contact components at the same time or braze multiple contacts on one component, furnace brazing can be used. When noble metals are brazed by common methods in the atmosphere, the quality of joints is poor, while vacuum brazing can obtain high-quality joints, and the properties of the materials themselves will not be affected.

(2) Brazing gold and its alloy are selected as brazing filler metals. Silver based and copper based filler metals are mainly used for the contact, which not only ensures the conductivity of the brazing joint, but also is easy to wet. If the joint conductivity requirements can be met, the brazing filler metal containing Ni, PD, Pt and other elements can be used, and the brazing filler metal with brazing nickel, diamond alloy and good oxidation resistance can also be used. If Ag Cu Ti brazing filler metal is selected, the brazing temperature shall not be higher than 1000 ℃

The silver oxide formed on the silver surface is not stable and is easy to braze. The soldering of silver can use tin lead filler metal with zinc chloride aqueous solution or rosin as flux. When brazing, silver filler metal is often used, and borax, boric acid or their mixtures are used as brazing flux. When vacuum brazing silver and silver alloy contacts, silver based brazing filler metals are mainly used, such as b-ag61culn, b-ag59cu5n, b-ag72cu, etc.

For brazing palladium contacts, gold based and nickel based solders that are easy to form solid solutions, or silver based, copper based or manganese based solders can be used. Silver base is widely used for brazing platinum and platinum alloy contacts. Copper based, gold based or palladium based solder. Selecting b-an70pt30 brazing filler metal can not only not change the color of platinum, but also effectively improve the remelting temperature of brazing joint and increase the strength and hardness of brazing joint. If the platinum contact is to be brazed directly on kovar alloy, b-ti49cu49be2 solder can be selected. For platinum contacts with working temperature not exceeding 400 ℃ in non corrosive medium, oxygen free pure copper solder with low cost and good process performance shall be preferred.

(3) Before brazing, the weldment, especially the contact assembly, shall be checked. The contacts punched out from the thin plate or cut from the strip shall not be deformed due to punching and cutting. The brazing surface of the contact formed by upsetting, fine pressing and forging must be straight to ensure good contact with the flat surface of the support. The curved surface of the part to be welded or the surface of any radius must be consistent to ensure proper capillary effect during brazing.

Before brazing of various contacts, the oxide film on the surface of the weldment shall be removed by chemical or mechanical methods, and the surface of the weldment shall be carefully cleaned with gasoline or alcohol to remove oil, grease, dust and dirt that hinder wetting and flow.

For small weldments, the adhesive shall be used for pre positioning to ensure that it will not shift during the handling process of furnace charging and filler metal charging, and the adhesive used shall not cause harm to the brazing. For large weldment or special contact, the assembly and positioning must be through the fixture with boss or groove to make the weldment in a stable state.

Due to the good thermal conductivity of precious metal materials, the heating rate should be determined according to the type of material. During cooling, the rate should be properly controlled to make the brazing joint stress uniform; The heating method shall enable the welded parts to reach the brazing temperature at the same time. For small precious metal contacts, direct heating should be avoided and other parts can be used for conduction heating. A certain pressure shall be applied to the contact to make the contact fixed when the solder melts and flows. In order to maintain the rigidity of the contact support or support, annealing shall be avoided. The heating can be limited to the brazing surface area, such as adjusting the position during flame brazing, induction brazing or resistance brazing. In addition, in order to prevent the solder from dissolving precious metals, measures such as controlling the amount of solder, avoiding excessive heating, limiting the brazing time at the brazing temperature, and making the heat evenly distributed can be taken.

Post time: Jun-13-2022