Circuit board of the issues that need to be from the brazing technology, brazing adopts the liquidus temperature is lower than the base solidus temperature of solder metal materials, components and solder will be heated to melt solder, using liquid solder wetting parent metal, filling and joint clearance with parent metal dissolution and diffusion, and after that, the liquid crystal solidification of solder, so as to realize the connection of parts.
Brazing can be divided into two categories depending on the welding temperature. The welding heating temperature is lower than 450℃ is called soft brazing, above 450℃ is called hard brazing.
It is widely used in the welding of conductive, air-tight and water-tight devices in electronics and food industry. Tin welding with tin lead alloy is the most common solder. Brazing alloys generally require flux to remove oxide film and improve wettability. There are many kinds of flux, rosin alcohol solution soft brazing is widely used in electronic industry. This flux after welding residue on the workpiece without corrosion effect, known as non-corrosive flux.
Flux for welding copper, iron, etc., consisting of zinc chloride, ammonium chloride, vaseline, etc. Welding aluminum needs fluoride and fluorborate as flux, and hydrochloric acid and zinc chloride as flux. These flux welding residue has a corrosive effect, called corrosive flux, must be cleaned after welding.
Brazing joint high strength, some can work under high temperature. Brazing brazing alloys are widely used, with aluminum, silver, copper, manganese and nickel as the most widely used brazing alloys. Aluminum base filler is often used for brazing aluminum products. Silver – based and copper – based brazing alloys are often used for brazing copper and iron parts.
Manganese base and nickel-based brazing alloys are commonly used for welding parts of stainless steel, heat resistant steel and high temperature alloys that work at high temperatures. For welding of refractory metals such as beryllium, titanium, zirconium, graphite and ceramics, palladium base, zirconium base and titanium base solder are commonly used. The characteristics of base metal and the requirement of joint performance should be considered when selecting solder.
Brazing flux is usually composed of alkali metal and heavy metal chloride and fluoride, or borax, boric acid, fluoroborate, etc., can be made into powder, paste and liquid form. Some filler metals are also added with lithium, boron and phosphorus to enhance their ability to remove oxidation film and wetting. After welding, the flux residue is cleaned with warm water, citric acid or oxalic acid.
Compared with the fusion welding method, brazing has the following advantages:
● brazing heating temperature is low, which has little effect on the structure and properties of base metal.
● brazing joint flat and smooth, beautiful appearance.
● welding parts deformation is small, especially the use of uniform heating (such as brazing in the furnace) brazing method, welding parts deformation can be reduced to the minimum, easy to ensure the size accuracy of welding parts.
● some brazing methods can be welded into dozens or hundreds of brazing joints, high productivity.
● may realize the dissimilar metal or the alloy, the metal and the nonmetal connection.
But brazing has its drawbacks:
● brazing joint strength is relatively low, heat resistance is poor.
● the electrochemical corrosion caused by the large difference between the base metal and the solder leads to poor corrosion resistance and higher assembly requirements.