Generally speaking, the causes of tin beads are multifaceted and comprehensive. The printing thickness of the solder paste, the composition and oxidation degree of the solder paste, the production and opening of the template, whether the solder paste absorbs moisture, the component mounting pressure, the solderability of the components and the pad, the setting of the reflow soldering temperature, the outside Environmental influences may be the cause of solder beads.

 

 

      The diameter of the tin beads is roughly between 0.2mm and 0.4mm, and there are some that exceed this range, and they are mainly concentrated around the chip resistance-capacitance components. The presence of solder beads not only affects the appearance of electronic products, but also lays down hidden dangers to the quality of the products. The reason is that modern printed boards have high component density and small spacing, and solder beads may fall off during use, which will cause component short-circuits and affect the quality of electronic products.

 

 

1. The metal content of the solder paste.

      The mass ratio of the metal content in the solder paste is about 88% to 92%, and the volume ratio is about 50%. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during the preheating process.

 

      In addition, the increase of the metal content makes the metal powders tightly arranged, so that they can be more easily combined without being blown away when they are melted. In addition, the increase in metal content may also reduce the "sagging" of the solder paste after printing, so it is not easy to produce solder beads.

 

2. Metal oxidation degree of solder paste.

      In the solder paste, the higher the degree of metal oxidation, the greater the resistance to the bonding of the metal powder during soldering, and the less wetting between the solder paste and the pads and components, resulting in reduced solderability. Experiments show that the incidence of solder beads is directly proportional to the degree of oxidation of the metal powder. Generally, the oxidation degree of the solder in the solder paste should be controlled below 0.05%, and the maximum limit is 0.15%.

 

3. The particle size of the metal powder in the solder paste.

      The smaller the particle size of the powder in the solder paste, the larger the overall surface area of ​​the solder paste, which results in a higher degree of oxidation of the finer powder, and thus the phenomenon of solder beading is aggravated. Our experiments show that when choosing a finer-grained solder paste, solder powder is more likely to be produced.

 

4. The printing thickness of the solder paste on the printed circuit board.

      The thickness of the solder paste after printing is an important parameter of printing, usually between 120~200um. Too thick solder paste will cause the solder paste to "collapse" and promote the generation of solder beads.

 

5. The amount of flux in the solder paste and the activity of the flux.

      Too much amount of solder will cause partial collapse of the solder paste, which will make solder beads easy to produce. In addition, when the activity of the flux is small, the deoxidation ability of the flux is weak, so that tin bead is likely to be generated. The activity of no-clean solder paste is lower than that of rosin and water-soluble solder pastes, so it is more likely to produce solder beads.

 

      In addition, the solder paste must be thawed for more than 3 hours before use. Otherwise, the solder paste will easily absorb moisture, and the solder will spatter during reflow soldering and cause tin beads.

 

6. Steel mesh opening

      Proper template opening shape and size will also reduce the generation of solder balls.

 

7. Cleaning of bad printed circuit boards

      When cleaning the poorly printed circuit board, if it is not cleaned, there will be some solder paste on the surface of the printed board and in the via holes, and tin beads will be formed during soldering. Therefore, it is necessary to strengthen the responsibility of the operator in the production process, and the cleaning method of the circuit board, strictly follow the process requirements for production, and strengthen the quality control of the process.

 

8. Component mounting pressure and solderability of components.

      If the component is placed under too much pressure, the solder paste will easily be squeezed onto the solder mask under the component, and the solder will melt and run around the component to form solder beads during reflow soldering.

Solution:

      Reduce the pressure during placement, and use the recommended template opening form above to prevent the solder paste from being squeezed out of the pad. In addition, the solderability of components and pads also has a direct impact. If the oxidation degree of components and pads is severe, solder beads will also be produced.

 

      After the hot-air leveled pad is printed with the solder paste, the ratio of solder to flux is changed, so that the ratio of flux is reduced. The smaller the pad, the more serious the imbalance. This is also a cause of solder beads.

 

      In summary, the production of solder beads is a very complicated process. We should consider comprehensively when adjusting the parameters and explore experience in production to achieve the best control of solder beads.