1. Weldment must have good weldability
The so-called weldability means that the metal material to be welded can form a good alloy combination with the solder at a suitable temperature. Not all metals have good weldability, some metals such as chromium, molybdenum and tungsten have poor weldability; Some metals, such as copper and brass, have better weldability due to the high welding temperature. The formation of oxide film on the surface affects the weldability of the material. In order to improve solderability, tin and silver plating can be used to prevent surface oxidation.
2. The surface of weldment must be kept clean
In order to make a good combination of solder and weldment, the welding surface must be kept clean. Even for weldments with good weldability, due to storage or contamination, oxide film and oil stains may be produced on the surface of weldments. The dirty film must be removed before welding, otherwise the welding quality cannot be guaranteed. The mild oxidation layer on the metal surface can be removed by the effect of flux, and the metal surface with serious oxidation degree should be removed by physical or chemical methods, such as scraping or pickling.
3. Use appropriate flux
The function of flux is to remove the oxide film on the surface of weldment. For different welding processes, different fluxes should be selected, such as nickel chromium alloy, stainless steel, aluminum and other materials. It is difficult to realize welding without special flux.
4. The weldment shall be heated to a suitable temperature
In the welding process, the role of heating is to melt the solder and heat the object to be welded, so that the tin and lead atoms can get enough energy to penetrate into the lattice of the metal surface to form the alloy. If the welding temperature is too low, it is not conducive to the penetration of solder atoms, can not form alloy, easy to form false welding; If the welding temperature is too high, the solder will be in the non eutectic state, which will accelerate the decomposition and volatilization of the flux and reduce the quality of the solder. It can cause pads on the printed circuit board to fall off. It should be emphasized that not only the solder must be heated and melted, but also the solder must be heated to a temperature that can melt the solder.
5. Suitable welding time
Welding time refers to the time required for physical and chemical changes in the whole welding process. It includes the time when the metal to be welded reaches the welding temperature, the time when the solder melts, the time when the flux acts and the time when the metal alloy is formed. After the welding temperature is determined, the appropriate welding time should be determined according to the shape, nature and characteristics of the welding parts. If the welding time is too long, it is easy to damage the components or weldments; If it is too short, it will not meet the welding requirements. Generally, the longest welding time of each solder joint is not more than 5S.
These are the necessary conditions for PCB automatic soldering machine.