About Soldering Tin

Soldering tin is an important industrial raw material for welding electronic components. It is a low melting point metal solder, that mainly refers to the solder made of tin-based alloy.

1. Function and Moisture

Solder is a metal material used to connect two or more metal surfaces and act as a metallurgical bridge between the surfaces of the joined metal. It is the most commonly used traditional solder material in circuit assembly. Commonly used solder is a fusible alloy, usually consisting of two basic metals (tin, lead) and several metal doping with melting point below 425 C. Because tin accounts for the majority of its constituents, it is called "soldering tin". The reason why solders can reliably connect two metals is that they can wet the two metal surfaces and form intermetallic compounds between them. Wetting is a necessary condition for welding

The wettability of solder and metal surface is generally described by wetting angle. The wetting angle is the angle between the two formed by the wetting spreading of molten solder along the jointed metal surface. The wetting angle is smaller, the better the wettability of solder and the welded metal surface is. It is generally believed that the wetting angle is greater than 90 degrees, the metal surface can not be wetted (not welded).

2. Composition and Temperature Characteristics

There are many types of solder alloys. The composition and proportion of alloys in solders have significant effects on the melting point, density, mechanical properties, thermal properties and electrical properties of solders. In tin-lead solder, the lowest melting point can be reduced to about 150 C by adding bismuth. The melting point of tin-lead solder can be raised to about 300 C when the content of tin in the solder decreases to less than 10% or when silver and other metals are added into the solder.

Solders with different compositions have different characteristics. For example, Sn-Pb alloy is the most common solder alloy in electronic assembly applications. It has suitable strength and wettability, but because it can form brittle metal compounds with silver or gold (which can cause corrosion), it is not suitable for welding silver, silver alloy and gold.

3. Shape and Characteristic Requirements

Soldering tin is often processed into various shapes according to the specified size to meet different production processes. The main shapes are rod, filament and preform (sheet, ring, paste). Welding wire is mainly used for manual soldering of soldering iron, generally filling rosin flux in hollow; welding rod is mainly used for wave soldering and dipping soldering; preforming is mainly used for laser reflow soldering, generally flaky, circular and spherical; solder paste is mainly used for general reflow soldering.

The main function of solder in electronic products is to connect electronic components and pads on printed circuit boards to form a circuit path. It must have the following characteristics:

(1) The melting point is lower than that of the welded workpiece. It is easy to integrate with the welded workpiece and does not produce brittle metal compounds when combined with the welded workpiece. It has good mechanical properties.

(2) It has good affinity with most metals and can wet the surface of the welded workpiece. The oxide generated will not be the cause of poor wetting.

(3) It is necessary to have good conductivity, fast crystallization speed and certain thermal stress absorption capacity, and the win-win situation is suitable for automatic production.

4. Notices in Application

(1) Correct selection of temperature range. Solder is a temperature sensitive material. For example, Sn-based solder is prone to change its isotropy and brittleness at low temperatures, and its change speed is the fastest at - 45 C, so it is not suitable for use at low temperatures. At high temperatures, the creep characteristics are remarkable, and the strength decreases greatly at 100 C.

(2) Applicability of mechanical properties. Sn-Pb solder belongs to soft solder, and its mechanical stress is not high. The welding parts are prone to plastic deformation, and eventually lead to weld cracks.

(3) A variety of intermetallic compounds are formed by the combination of welded metal and solder composition. These compounds have hard and brittle properties, which may be the cause of solder cracking.

(4) Melting point. Most SMC/SMD can adapt to the general welding process of surface assembly, but some of the heat-sensitive components can not withstand the eutectic temperature of Sn-Pb (183 C).

(5) Preventing corrosion. Under welding conditions, the welded metal will dissolve in the molten solder alloy. This phenomenon is called dissolution.

(6) Prevent solder oxidation and deposition. In dipping and wave soldering processes, the longer the solder holding time is, the more scum will be caused by liquid level oxidation during the stopping of soldering. Therefore, appropriate anti-oxidation additives should be added to the solder tank to reduce oxidation rate and improve wettability.

(7) Lead-free solder. As lead and its compounds are toxic substances that pollute the environment, the use of lead-free solders is an inevitable trend of development.