Due to the continuous miniaturization of PCB boards of electronic products, chip components appear, and the traditional solder paste soldering method can no longer meet the demand. Most of the components assembled and welded are chip capacitors, chip inductors, mount transistors and diodes. With the development of the entire SMT technology is becoming more and more perfect, a variety of chip components (SMC) and placement equipment (SMD) appear, and the reflow soldering process technology and equipment as part of the placement technology have also been developed accordingly, and its application is becoming more and more widespread, almost It has been used in all electronic product fields. Reflow soldering is to remelt the paste solder that is pre-distributed on the pad of the printed board to complete the soldering of the mechanical and electrical connection between the solder end of the surface assembly component or the lead and the pad of the printed board. Reflow soldering is to solder components to PCB boards, and reflow soldering is to mount devices on the outside. Reflow soldering is based on the action of hot air flow on solder joints, and the colloidal flux undergoes physical reactions under a certain high-temperature air flow to achieve SMD welding; so it is called "reflow soldering" because the gas circulates in the welding machine to generate high temperature to achieve the purpose of welding .

2. The principle of reflow soldering machine is divided into several descriptions:

A. When the PCB enters the heating zone, the solvent and gas in the solder paste evaporate, and together, the flux in the solder paste wets the pad, component terminals and pins, and the solder paste softens, collapses, and covers the pad , to block the pads, component pins and oxygen.

B. When the PCB enters the heat preservation area, the PCB and components are fully preheated to prevent the PCB from suddenly entering the welding high temperature zone and damaging the PCB and components.

C. When the PCB enters the welding area, the temperature rises rapidly to make the solder paste reach the melting state, and the liquid solder wets, diffuses, diffuses or reflows the solder pads, component ends and pins of the PCB to form solder joints.

D. The PCB enters the cooling zone to condense the solder joints; the reflow soldering is completed.

3. Process requirements of reflow soldering machine

The advantage of this process is that the temperature is easy to control, oxidation can be prevented during the welding process, and the manufacturing cost is easier to control. There is a heating circuit inside this device, which heats the nitrogen gas to a high enough temperature and blows it to the circuit board where the components have been pasted, so that the solder on both sides of the component melts and bonds with the main board.

1. It is necessary to set a reasonable reflow soldering temperature curve and do a real-time test of the temperature curve regularly.

2. Solder according to the welding direction of PCB design.

3. Be careful not to vibrate the conveyor belt during welding.

4. It is necessary to check the welding function of the printed board.

5. Whether the welding is sufficient, whether the surface of the solder joint is smooth, whether the shape of the solder joint is half-moon, the condition of solder balls and residues, the condition of continuous welding and virtual welding. And adjust the temperature curve according to the inspection results. During the whole batch production process, the welding quality should be checked regularly.

4. Factors affecting the reflow soldering process:

1. Generally, PLCC, QFP and a discrete chip component have a larger heat capacity, and it is more difficult to weld a large-area component than a small component.

2. In the reflow soldering furnace, the conveyor belt is repeatedly conveying the products for reflow soldering, and it also becomes a heat dissipation system. In addition, the heat dissipation conditions are different between the edge and the center of the heating part. The edge temperature is generally low, and the temperature of each temperature zone in the furnace is In addition to different requirements, the temperature of the same loading surface is also different.

3. The impact of different product loadings. The adjustment of the temperature curve of reflow soldering should be considered to obtain excellent repeatability under the conditions of no load, load and different load factors. The load factor is defined as: LF=L/(L+S); where L=the length of the assembled substrate, and S=the distance between the assembled substrates. To obtain good repeatable results in the reflow soldering process, the larger the load factor, the more difficult it is. The maximum load factor of a general reflow oven ranges from 0.5 to 0.9. This depends on the product status (component soldering density, different substrates) and the different types of reflow furnaces. Practical experience is essential for excellent welding action and repeatability.