Classification and characteristics of welding materials in SMT patch processing

Classification and characteristics of welding materials in SMT patch processing

Solder in SMT chip processing can be divided into tin lead solder, silver solder and copper solder according to their components. According to the ambient humidity, it can be divided into high-temperature solder (solder used at high temperature) and low-temperature solder (solder used at low temperature). In order to ensure the welding quality, it is important to select different solders according to the different objects to be welded. In the assembly of electronic products, tin lead series solder, also known as solder, is generally selected.

Solder has the following characteristics:

1. It has good conductivity: because tin and lead solder are good conductors, 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 melt at 180 ℃, and can be welded with 25W external heat type or 20W internal heat type electric soldering iron.

4. It has certain mechanical strength: because the strength of tin lead alloy is higher than that of pure tin and pure lead. Due to the light weight of electronic components, the strength requirements of solder joints in SMT patches are not very high, so they can meet the strength requirements of solder joints.

5. Good corrosion resistance: the welded printed circuit board can resist atmospheric corrosion without coating any protective layer, so as to reduce the process flow and reduce the cost.

In tin lead solder, the melting point below 450 ℃ is called soft solder. Oxidation resistant solder is used in automatic production line in industrial production, such as wave soldering. When this liquid solder is exposed to the atmosphere, the solder is very easy to oxidize, which will produce false welding and affect the welding quality. Therefore, adding a small amount of active metal to tin lead solder can form a covering layer to protect the solder from further oxidation, so as to improve the welding quality.

Because tin lead solder is composed of more than two metals in different proportions. Therefore, the properties of Sn Pb alloy will change with the change of Sn Pb ratio. Due to different manufacturers, the configuration ratio of tin lead solder is very different. In order to make its solder ratio meet the needs of welding, it is very important to select the appropriate ratio of tin lead solder.

The common solder ratio is as follows:

1. Tin 60%, lead 40%, melting point 182 ℃;

2. Tin 50%, lead 32%, cadmium 18%, melting point 150 ℃;

3. Tin 55%, lead 42%, bismuth 23%, melting point 150 ℃.

The shapes of solder include wafer, strip, ball, solder wire, etc. The commonly used solder wire is sandwiched with solid flux rosin. There are many kinds of solder wire diameters, including 4mm, 3mm, 2mm, 1.5mm, etc.