In general, SMT patches are first selected according to the solder paste category, and then selected according to the alloy composition, particle size, viscosity and other indicators.

（1） Classification method:

A. Ordinary rosin cleaning type is divided into RA and RMA: this type of solder paste shows good “tin loading speed” and can ensure good “welding effect” in the welding process; After the welding work is completed, the rosin residue on the PCB surface is relatively large, which can be cleaned with appropriate cleaning agent. After cleaning, the board surface is clean and clean without residue, which ensures that the cleaned board surface has good insulation resistance and can pass the technical test of various electrical properties;

B. Cleaning free solder paste NC: after the solder paste is welded, the PCB surface is relatively clean and less residual. It can be tested by various electrical performance technologies without cleaning again. It shortens the production process and accelerates the production progress while ensuring the welding quality;

C. Water soluble solder paste wma: due to technical reasons, the solder paste produced in the early stage generally has too much residue on the PCB surface, and the electrical performance is not ideal, which seriously affects the product quality; At that time, CFC cleaning agent was mostly used for cleaning. Because CFC was unfavorable to environmental protection, it was banned in many countries; In order to meet the market demand, water-soluble solder paste is produced. After the solder paste is welded, its residue can be cleaned with water, which not only reduces the production cost of customers, but also meets the requirements of environmental protection.

（2） Selection criteria:

1. Alloy composition: generally, the alloy composition of Sn63 / Pb37 solder can meet the welding requirements; For the welding of devices with silver (Ag) or palladium (PD) coating, the solder paste with alloy composition of sn62 / pb36 / ag2 is generally selected; The solder powder containing Bi is selected for PCB welding with heat-resistant impact devices.

2. Viscosity of solder paste: in the working process of SMT, there is a process of moving, placing or transporting PCB from laser stencil printing (or spot injection) and pasting components to reflow heating process; In this process, in order to ensure that the printed (or dotted) solder paste is not deformed and the components pasted on the PCB solder paste are not displaced, it is required that the solder paste should have good viscosity and retention time before the PCB enters reflow heating.

A. The viscosity index (i.e. viscosity) of the solder paste is usually expressed in the unit of “pa · s”; Among them, 200-600pa · s solder paste is more suitable for needle type spot injection system or production process equipment with high degree of automation; The printing process requires relatively high viscosity of the solder paste, so the viscosity of the solder paste used in the printing process is generally about 600-1200 PA · s, which is suitable for manual or mechanical SMT steel screen printing;

B. The high viscosity solder paste has the characteristics of good post forming effect, and is suitable for fine pitch printing; While the low viscosity solder paste has the characteristics of fast falling, tool free brushing and time saving during printing;

C. Another characteristic of solder paste viscosity is that its viscosity will change with the stirring of solder paste, and its viscosity will decrease during stirring; When the stirring is stopped, the viscosity will return to the original state after slightly standing; This is very important for how to choose solder pastes with different viscosities.

In addition, the viscosity of solder paste has a great relationship with temperature. Under normal conditions, its viscosity will gradually decrease with the increase of temperature.

3. Mesh:

In domestic solder paste manufacturers, the “particle size” of solder powder is often used to classify different solder pastes, while many foreign manufacturers or imported solder pastes use the concept of “mesh” to classify different solder pastes. The basic concept of mesh refers to the number of mesh on each square inch of the screen; In the actual production process of tin powder, several layers of screens with different meshes are mostly used to collect tin powder. Because the mesh size of each layer of screens is different, the particle size of tin powder passing through each layer of mesh is also different. The particle size of tin powder particles finally collected is also a regional value;

A. From the above concept, the larger the mesh index of the solder paste, the smaller the particle diameter of the tin powder in the solder paste; The smaller the mesh number, the larger the particles of tin powder in the solder paste; Refer to the following table for comparison:

B. If the solder paste using manufacturer selects the solder paste according to the mesh index of the solder paste, it shall be determined according to the spacing between the solder spots with the smallest distance on the PCB: if there is a large spacing, the solder paste with the smaller mesh can be selected; otherwise, when the spacing between the solder spots is small, the solder paste with the larger mesh should be selected; Generally, the particle size diameter is about 1 / 5 of the opening of SMT steel mesh.

I Requirements for flux for surface mounting

It has certain chemical activity; Good thermal stability; Good wettability; It can promote the expansion of solder; The flux residue remaining on the substrate is not corrosive to the substrate; Good cleaning property; The chlorine content is 0.2% (w / W) or less.

II Function of flux

Welding process: preheat / / the solder starts to melt / / the solder alloy forms / / the solder joint forms / / the solder solidification function: assist in heat transfer / remove the oxide / reduce the surface tension / prevent reoxidation notes: the solvent evaporates / heats, and the flux covers the surface of the substrate and the solder to make the heat transfer uniform / release the activator to react with the ionic oxide on the surface of the substrate to remove the oxide film / reduce the surface tension of the molten solder, Well wetted / covered on high-temperature solder surface to control oxidation and improve solder joint quality

III Physical properties of flux

The physical properties of the flux mainly refer to the melting point, boiling point, softening point, glass transition temperature, vapor pressure, surface tension, viscosity, miscibility, etc. related to the welding performance