What is solder? Solder is a fusible metal whose melting point is lower than the melting point of the soldered metal. It is used for bridging interconnection between circuit board pads and components. During the pcb assembly process, when the solder is melted and the surface of the soldered PCB metal can be immersed if soldered metal is not melted, and an alloy layer is formed at the surface of the contact pad to be joined to the soldered metal. In the general PCBA electronic product assembly, it is commonly known as solder.
In pcb manufacturing, the commonly used pcb solders have the following four conditions:
1) The melting point of the solder is lower than that of the workpiece to be welded.
2) It is easy to be integrated with the object to be welded, and it must have a certain pressure resistance.
3) Have better electrical conductivity.
4) Have a faster crystallization rate.
Solder powder, also known as tin powder, is mainly composed of tin-lead, tin-bismuth, tin-silver-copper alloy. The solder forms commonly used in pcb assembly welding technics are rod-shaped, wire shaped. shaped preformed and paste shape. Different forms of pcb solder should be used in different pcb assembly applications. The following are common pcb solder shapes that I hope to help some novice engineers understand.
(1) Rod solder. Used for dip soldering and wave reflow soldering. The rod solder is shown in Fig. 1. The rod solder should be dissolved in the solder bath during the pcb soldering. There are two problems to be aware of in practical applications, that is oxidation of the liquid surface and unevenness of the solder composition.
(2) Wire solder. As shown in Figure 2, the wire solder is commonly used in manual soldering iron soldering applications for DIP processes. The wire solder is made by wire drawing processing - cold extrusion or hot extrusion, and the hollow portion is filled with flux, and the flux can be uniformly supplied during the welding proces. Filling flux with a number of knack to prevent splashing of the flux. In some cases, solid wire solder can also be used.
(3) Preformed solder. As shown in Figure 3, the preformed solder is mainly used in reflow soldering technics such as lasers, and can also be used in ordinary reflow soldering technics. This type of solder has a wide variety of shapes, generally in the form of gasket, small piece, disk, ring, and ball, which can be selected according to different circuit board standards.
(4) Paste solder. Cream solder, called solder paste, is used in the reflow soldering technics, as shown in Figure 4. Its viscosity and reliability is more important for the most common paste solder used in the usual SMT process.
Different forms of pcb solder should be used in different pcb assembly applications.