Welding materials classification characteristics in SMT processing
Solder in SMT processing can be divided into tin-lead solder, silver solder, and copper solder according to its components. According to the used environment humidity , it can be divided into high temperature solder (solder used at high temperature) and low temperature solder (solder used in low temperature environment). In order to ensure the quality of the solder in the SMT processing, it is important to select different solders depending on the welded objects. In the assembly of electronic products, tin-lead series solders, also called solders, are generally used.
Tin has the following characteristics:
1, Has good electrical conductivity: because tin, lead solder are good conductors, so its resistance is very small.
2, It has strong adhesion to component leads and other wires and is not easy to fall off.
3, low melting point: it can be melted at 180 ° C, using 25W external heating or 20W internal heating soldering iron can be welded.
4, Has a certain mechanical strength: because the strength of tin-lead alloy is higher than the strength of pure tin and pure lead. Moreover, due to the light weight of the electronic components themselves, thestrength requirements of the solder joints in the SMT process are not very high, so the strength requirements of the solder joints can be met.
5, Good corrosion resistance: welded printed circuit boards do not need to apply any protective layer to resist atmospheric corrosion, thereby reducing the craft process and reducing costs.
Among tin-lead solders, a melting point below 450 ° C is called a soft solder. Anti-oxidation solder is a solder used in automated production lines in industrial production, such as wave soldering. When the liquid solder is exposed to the atmosphere, the solder is easily oxidized, which will result in a virtual weld which will affect the quality of the welding. Therefore, by adding a small amount of active metal to the tin-lead solder, a cover layer can be formed to protect the solder from further oxidation, thereby improving the quality of the soldering.
Because tin-lead solder is composed of two or more metals in different proportions. Therefore, the performance of tin-lead alloys will change with the ratio of tin-lead. Due to the different manufacturers, the tin-lead solder configuration ratio is very different. In order to make the tin-lead ratio meet the needs of welding, it is important to choose a suitable ratio of tin-lead solder.
Commonly used solders ratio are as follows:
1, Tin 60%, lead 40%, melting point 182 ° C;
2, Tin 50%, lead 32%, cadmium 18%, melting point 150 ° C;
3, Tin 55%, lead 42%, Bi 23%, melting point 150 ° C.
The shape of the solder is disk, ribbon, sphere, solder wire. A commonly used solder wire has a solid flux rosin inside it. There are many types of solder wire diameters, commonly used 4mm, 3mm, 2mm, 1.5mm and so on.
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