Analysis Of Precious Metal Brazing Process

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

 

 

Brazing characteristics of precious metals:
As electroshock material, precious metal has the common feature of small brazing area, requiring that brazing metal has good impact resistance, high strength, certain oxidation resistance and can withstand arc attack, but does not change the characteristics of contact material and electric performance of components. Due to the limitation of electro-shock brazing area, no filler metal overflow is allowed. The brazing process parameters should be strictly controlled.

Most heating methods can be used to braze precious metal contacts. Flame brazing is commonly used for larger contact assemblies. Induction brazing is suitable for mass production. Resistance brazing can be carried out with common resistance welder, but smaller current and longer brazing time should be selected, carbon block can be used as electrode. Furnace brazing can be used when a large number of contact components need to be brazed at the same time or when multiple contacts are brazed on one component. Precious metals are brazed in the atmosphere by common methods. The quality of the joints obtained is poor, and vacuum brazing results in high quality joints without any influence on the material itself.

Selection of filler metal for brazing noble metals:
Silver-based and copper-based solders are mainly used for brazing gold and its alloy contacts, which not only ensure the electrical conductivity of the brazing seam, but also make it easy to wet. If the conductivity requirement of the joint can be met, solder containing Ni, Pd, Pt and other elements can be used. It is also possible to use brazing filler metals with nickel and cobalt alloys and good oxidation resistance, such as Ag-cu-Ti filler metal, at a brazing temperature not higher than 1000 C.

The silver oxide generated on the silver surface is not stable and can be easily brazed. Sn-Pb solder with zinc chloride aqueous solution or rosin can be used as soldering agent for silver soldering. Silver solder is often used for brazing, with borax, boric acid or their mixture as the filler metal. Silver-based solders such as B-Ag61CuIn, B-Ag59CuSn and B-Ag72Cu are mainly used for vacuum brazing of silver and its silver alloy contacts.

Brazing palladium contacts can be performed with either gold-based or nickel-based solvents easily formed or silver-based, copper-based or manganese-based solders.

Brazing platinum and its platinum alloy contacts, silver-based, copper-based, gold-based or palladium-based solders are widely used. B-Au70Pt30 solder is selected, which not only does not change the color of platinum, but also can effectively improve the remelting temperature and increase the strength and hardness of the solder joint. B-Ti49Cu49Be2 solder is available if platinum contacts are to be soldered directly on the Kovar alloy. For platinum contacts operating in non-corrosive media at temperatures not exceeding 400 C, pure oxygen-free copper solder with low cost and good process performance should be preferred.

Before brazing, the welding parts, especially the contact components, should be checked and felt. The contacts punched out of the sheet or cut off from the slats must not be deformed by punching or shearing. The brazing surface of the contact formed by upsetting, precision pressing and forging must be straight to ensure good contact with the flat surface of the support. Surfaces of the parts to be welded or of any radius must be matched uniformly to ensure proper capillary action during brazing.

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

For small weldments, use adhesives to pre-position to ensure no displacement during furnace loading, filler metal loading and other handling processes. The adhesives used should not bring harm to brazing. For large weldments or special contacts, assembly positioning must be done by means of clamps with raised or grooves to stabilize the weldment.

Due to the good thermal conductivity of precious metal materials, the heating rate should be determined according to the type of material. The cooling rate should be controlled appropriately so that the brazing joint stress is uniform. The heating method should enable the brazing parts to reach the brazing temperature at the same time. For smaller precious metal contacts, direct heating should be avoided and conductive heating can be performed with other parts. A certain amount of pressure should be applied to the contacts to keep them fixed while the solder melts and flows. In order to maintain the rigidity of the contact mounts or supports, annealing should be avoided and heating may be limited to the area of the brazing surface, e.g. by adjusting the position of flame, induction or resistance brazing. In addition, to avoid dissolving precious metals in the brazing filler metal, measures such as controlling the amount of brazing filler metal, avoiding excessive heating, limiting brazing time at brazing temperature, and uniform distribution of heat can be taken.