Five basic requirements for reflow soldering

No matter what welding technology is used, it should meet the basic requirements of welding to ensure good welding results. High quality reflow soldering should meet the following five basic requirements.

1. Proper heat means that there must be enough heat energy for the materials on the reflow welding surface to melt and form the intermetallic interface (IMC). Sufficient heat is also one of the basic conditions for providing wetting. On the other hand, the heat must be controlled to a certain extent to ensure that the materials in contact (not only the welding end) will not be damaged by heat, and the formation of IMC layer will not be too thick.

2. Good wetting is not only a symbol of good solderability, but also an important condition for forming the final reflow solder joint shape. Poor wetting usually indicates that the structure of solder joint is not ideal, including incomplete formation of IMC and poor filling of solder joint. These problems will affect the life of reflow solder joints.

3. Proper size and shape of solder joint, in order to have enough service life of reflow solder joint, we must ensure that the shape and size of solder joint meet the requirements of weld end structure. The mechanical strength of too small solder joint is insufficient, and it cannot bear the stress in use, even the internal stress existing after welding. Once fatigue or creep cracking begins to appear in use, its fracture speed is also faster. The bad shape of reflow solder joint will also cause the phenomenon of giving up the heavy and taking the light, and shorten the service life of solder joint.

4. The controlled tin flow direction is also an important part of the reflow soldering process. The molten solder must flow in the required direction to ensure that the formation of solder joints is controlled. The phenomenon of tin absorption in reflow soldering process is the technical details related to the control of tin flow direction.

5. The welding surface does not move during reflow welding. If the welding end moves during welding, it depends on the moving condition and time, which will not only affect the shape and size of the solder joint, but also cause false soldering and inner hole. This will affect the quality and life of solder joints. Therefore, the design and process of the whole product must take into account that the welding end remains stationary during the welding process.

In the reflow soldering process, in addition to the above general welding conditions, there is a special point that the chemical components in the solder paste that have no effect after the printing process must be volatilized in time. This is especially true for the first side of the double-sided welding process.