Reflow soldering process characteristics and basic process requirements

Reflow soldering is to weld components to PCB board, and reflow soldering is to mount components on the surface. Reflow soldering relies on the effect of hot air flow on the solder joint, and the colloidal flux reacts physically under a certain high-temperature air flow to achieve the welding process of SMD; The following editor will share the characteristics and basic process requirements of reflow soldering process.

1、 Reflow soldering process characteristics

1. The size of reflow solder joint is controllable. The desired solder joint size or shape requirements can be obtained through the size design of the pad and the amount of solder paste printed. Reflow soldering only applies solder to the required parts, and the amount can be controlled. The components do not need to be directly immersed in molten solder, so the components are subject to small thermal shock.

2. The application of solder paste generally adopts the method of steel screen printing. In order to simplify the process flow and reduce the production cost, solder paste is usually printed only once on each welding surface. This feature requires that the components on each assembly surface can use a steel mesh (including steel mesh with the same thickness and stepped steel mesh) for solder paste distribution.

3. Reflow furnace is actually a tunnel furnace with multi temperature zone, and its main function is to heat PCBA. The components arranged on the bottom surface (surface B) shall meet the specified mechanical requirements, such as BGA packaging, and the ratio of component mass to pin contact area ≤ 0.05mg/mm2, so as to prevent the components on the top surface from falling down when welding.

4. When there is a certain deviation in the placement position of reflow components, due to the surface tension of molten solder, as long as the solder is placed in the correct position, reflow welding can automatically correct this small deviation during welding, so as to fix the components in the correct position - automatic positioning effect. During reflow soldering, the components are completely floating on the molten solder (solder joint). If the pad size is larger than the pin size, the component layout is heavy and the pin layout is less, it is easy to shift under the surface tension of asymmetric molten solder or the blowing of forced convective hot air in the reflow soldering furnace.

5. The components in the solder will not be mixed with impurities to ensure the composition of the solder. It can be on the same substrate, and wave soldering and reflow soldering can be mixed.

6. The reflow soldering process is simple and the welding quality is high. The formation of reflow solder joint morphology mainly depends on the wettability of molten solder and the effect of surface tension, such as 0.44mmqfp. The printed solder paste pattern is a regular cuboid.

2、 Basic requirements for reflow soldering process

1. Set a reasonable temperature curve. Reflow soldering is a key process in SMT production. Set a reasonable temperature curve according to the working principle of reflow soldering to ensure the quality of reflow soldering.

2. Welding shall be carried out according to the welding direction of circuit board design.

3. During reflow soldering, the circuit board shall be placed gently on the conveyor belt to prevent the conveyor belt from shaking, and pay attention to receiving the circuit board at the reflow soldering outlet to prevent the later circuit board from falling on the first circuit board and damaging the SMD element pins.

4. The first reflow welded circuit board must be inspected to check whether the welding is sufficient, whether there are traces of insufficient melting of solder paste, whether the surface of solder joint is smooth, whether the shape of solder joint is half moon, solder ball and residue, continuous welding and false welding. In addition, check the color change on the surface of the circuit board. After reflow soldering, the circuit board is allowed to change color slightly but evenly. Adjust the temperature curve according to the inspection structure, and regularly check the welding quality.