Precious metals mainly refer to materials such as Au, Ag, Pd, Pt, which have excellent conductivity, thermal conductivity, corrosion resistance, and high melting temperature. They are widely used in the manufacturing of circuit components for opening and closing electrical circuits in electrical equipment.
(1) Characteristics of Brazing
As contact materials, precious metals share common characteristics such as small brazing area. The brazed metal should have good resistance to impact, high strength, certain oxidation resistance, and be able to withstand arc erosion without altering the properties of the contact material and the electrical performance of the component.
Due to the limited brazing area for the contacts, overflow of the brazing material is not allowed, and strict control of brazing process parameters is necessary. Various heating methods can be used for brazing precious metals and their contacts.
Flame brazing is commonly used for larger contact components. Induction brazing is suitable for mass production, and resistance brazing can be performed using a conventional resistance welding machine, with smaller current and longer brazing time, and carbon blocks can be used as electrodes.
Furnace brazing can be employed when a large number of contact components need to be brazed simultaneously or when multiple contacts need to be brazed on a single component.
Brazing precious metals using common methods in the atmosphere yields poor joint quality, while vacuum brazing can achieve high-quality joints without affecting the material’s properties.
(2) Selection of Brazing Filler Materials
Silver-based and copper-based brazing fillers are mainly used for brazing gold and its alloy contacts. These fillers ensure good conductivity of the brazed joint and are easily wettable. If the conductivity requirements of the joint are met, fillers containing elements such as Ni, Pd, Pt can be used.
Brazing fillers such as nickel-based alloys and diamond-like alloys, which exhibit good oxidation resistance, can also be used. For example, Ag-Cu-Ti fillers can be used, with a brazing temperature not exceeding 1000℃.
Silver forms unstable silver oxide on its surface, making it easy to braze. Silver soft soldering can be achieved by using tin-lead solder with zinc chloride solution or rosin as flux. In hard soldering, silver solder is commonly used, with borax, boric acid, or their mixtures as flux.
When vacuum brazing silver and its silver alloy contacts, silver-based brazing fillers such as B-Ag61Culn, B-Ag59Cu5n, and B-Ag72Cu are primarily chosen.
For brazing palladium contacts, gold-based or nickel-based fillers that form solid solutions easily can be used, as well as silver-based, copper-based, or manganese-based fillers. When brazing platinum and its platinum alloy contacts, silver-based, copper-based, or palladium-based fillers are widely used.
B-An70Pt30 filler, for example, retains the color of platinum while effectively increasing the brazing temperature and improving the strength and hardness of the joint. When directly brazing platinum contacts onto non-noble alloys, B-Ti49Cu49Be2 filler can be chosen.
For platinum contacts operating in non-corrosive media at temperatures not exceeding 400℃, oxygen-free copper filler with low cost and good process performance should be preferred.
(3) Brazing Process
Before brazing, the workpiece, especially the contact components, should be inspected to ensure that contacts formed by punching or shearing from thin plates do not deform. The brazing surface of contacts formed by forging, precision pressing, or forging must be flat to ensure good contact with the base. Curved or surfaces with arbitrary radii should match consistently to ensure proper capillary action during brazing.
Chemical or mechanical methods should be employed to remove oxide films from the surface of the workpiece before brazing. The workpiece surface should then be thoroughly cleaned with gasoline or alcohol to remove oil stains, grease, dust, and any contaminants that may hinder wetting and flow.
For small-sized workpieces, adhesive agents can be used for pre-positioning to ensure that there is no displacement during furnace loading and brazing material application. The adhesive used should not harm the brazing process.
For large-sized workpieces or dedicated contacts, proper positioning should be achieved through fixtures equipped with protrusions or grooves to stabilize the workpiece.
Due to the good thermal conductivity of precious metal materials, the heating rate should be determined based on the material type, and the cooling rate should be appropriately controlled to ensure even stress distribution in the brazed joint. The heating method should allow the workpieces to reach the brazing temperature simultaneously.
For smaller precious metal contacts, direct heating should be avoided, and heat conduction from other parts can be utilized. A certain pressure should be applied to the contacts to keep them fixed during the melting and flowing of the brazing material.
To maintain the required rigidity of the contact support or backing components, annealing should be avoided. The heating process should be limited to the area of the brazing surface, for example, by adjusting the position during flame brazing, induction brazing, or resistance brazing.
Furthermore, measures such as controlling the amount of brazing filler, avoiding excessive heating, limiting the brazing time at brazing temperatures, and ensuring even heat distribution are taken to prevent the dissolution of precious metals by the brazing filler.
