Quality Requirements and Formation Process of PCB Reflow Oven Joints

 

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The reflow solder joint is formed by the solder paste melted in the SMT reflow oven and then cooled and solidified. The joints connected by reflow soldering are called reflow solder joints. Generally, a high-quality reflow solder joint not only needs good electrical Performance and certain mechanical strength, but also a certain gloss and clean surface. Guangshengde reflow soldering here to share the quality requirements of reflow solder joints and the five processes of formation.

 

1. Quality requirements for reflow solder joints

1. Reflow solder joints first need to have good electrical conductivity. Generally speaking, a good reflow solder joint should be a metal compound formed by the mutual expansion of the solder and the surface of the metal to be soldered, rather than simply attaching the solder to its surface. Welding the metal surface, so a good solder joint, its conductivity is generally better.

 

2. The reflow solder joints also need to have a certain strength. The main components of general tin-lead solder are tin and lead, which are relatively low in strength. So in order to increase its strength as much as possible, we are soldering Usually need to increase the welding area according to the actual situation, or use it for the lead of the component to be welded. The wire is wound, snapped, and dropped on its contact before the welding process, so we can usually see soldering The solder joint is generally a reflow solder joint surrounded by tin-lead solder.

 

2. Reflow solder joint formation process

1. First of all, the solvent used to achieve the required viscosity and screen printing performance begins to evaporate, and the temperature must rise slowly (about 3°C ​​per second) to limit boiling and splashing and prevent the formation of small tin particles. In addition, some components affect the internal Stress is more sensitive. If the external temperature of the component rises too fast, it will cause fracture.

 

2. The flux is active and the chemical cleaning action starts. The water-soluble flux and no-clean flux will have the same cleaning action, but the temperature is slightly different. Remove metal oxides and certain contamination from the metal and solder particles to be bonded.

 

3. When the temperature continues to rise, the solder particles first melt separately and begin the process of liquefaction, and begin to form solder joints.

 

4. This stage is the most important. When the individual solder particles are all melted, they combine to form liquid tin. At this time, the surface tension starts to form the surface of the solder foot, and the molecules move violently, so that the liquid tin and copper form a good alloy, thereby Realize the welding process.

 

5. In the cooling stage, if the cooling is fast (4-10°C/S), the tin point strength will be slightly larger, but it should not be too fast to cause temperature stress inside the component.

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