The phenomenon of solder beading is one of the main defects in surface mount production. Its diameter is about 0.2-0.4mm. It mainly appears on one side of the chip resistive component, which not only affects the appearance of board-level products, but also more serious Because of the dense components on the printed board, it will cause a short circuit during use, which will affect the quality of electronic products. Therefore, it is very important to understand the reasons for its occurrence and strive to control it most effectively.
The causes of solder beads are caused by many factors. The temperature and time in reflow soldering, the printing thickness of solder paste, the composition of solder paste, the production of templates, the mounting pressure, and the external environment will all be affected in all aspects of the production process. The formation of solder beads has an impact.
Solder beads are produced when the board is passed through the reflow soldering furnace. The reflow curve can be divided into four stages: preheating, heat preservation, reflow and cooling. The main purpose of the preheating stage is to heat the printed board and the surface mount components above to between 120-150 degrees, so that the volatile solvent in the solder paste can be removed and the thermal vibration of the components can be reduced. Therefore, vaporization will occur inside the solder paste during this process. At this time, if the bonding force between the metal powders in the solder paste is less than the force generated by vaporization, a small amount of solder paste will flow away from the pad. When the temperature is close to the peak of the curve during the reflow soldering stage, this part of the solder paste will also melt, and then squeeze out from under the chip resistive components to form solder beads. It can be seen from the process that the higher the preheating temperature, the faster the preheating speed, which will cause splashing in the atmospheric phenomenon, and the easier it is to form tin beads. Therefore, we can adopt a moderate preheating temperature and preheating speed to control the formation of solder beads.
The selection of solder paste also affects the quality of soldering. The metal content in the solder paste, the oxide content of the solder paste, the particle size of the metal powder in the solder paste, and the printing thickness of the solder paste on the printed circuit board all affect the solder bead to varying degrees. The formation.
1: The metal content in the solder paste:
The mass ratio of the metal content in the solder paste is about 90-91%, and the volume ratio is about 50%. When the metal content increases, the viscosity of the solder paste increases, which can more effectively resist the force generated by vaporization during the preheating process. In addition, the increase in metal content makes the metal powders tightly arranged, so that they have more opportunities to combine and are not easy to be blown away during gasification. The increase of the metal content can also reduce the tendency of the solder paste to collapse after printing, so it is not easy to form solder beads.
2: The oxide content in the solder paste:
The oxide content in the solder paste also affects the welding effect. The higher the oxide content, the greater the resistance during the bonding process after the metal powder is melted. During the reflow soldering stage, the surface of the metal powder The content of oxides will increase, which is disadvantageous? "Wetting" leads to the formation of tin beads.
3: The particle size of the metal powder in the solder paste:
The metal powder in the solder paste is very small and spherical, with a diameter of about 20-75um. When mounting fine-pitch and ultra-fine-pitch components, it is advisable to use metal powder with a smaller particle size The solder paste is about 20-45um. The overall surface area of the solder particles is greatly increased due to the shrinkage of the metal powder. The finer powder has a higher oxide content, which will alleviate the tin bead phenomenon.
4: The printing thickness of the solder paste on the printed circuit board:
The printing thickness of the solder paste is a main parameter in the production, and the printing thickness of the solder paste is usually between 0.15-0.20mm. If it is too thick, it will cause "collapse" to promote tin beading. Formed. When making the template, the size of the pad determines the size of the printed hole on the template. Usually, in order to avoid excessive solder paste printing, the size of the printed hole is made to be less than 10% of the contact area of the corresponding pad. We have done such a practice, and the results show that this will reduce the tin bead phenomenon to a considerable extent.
If the mounting pressure is too high during the placement process, when the component is pressed on the solder paste, a part of the solder paste may be squeezed under the component. During the reflow soldering stage, this part of the solder paste melts to form tin beads. Appropriate placement pressure should be selected during placement.
Solder paste usually needs to be refrigerated, but before use, it must be returned to room temperature before it can be opened for use. Sometimes the solder paste is opened when the temperature is too low, which will cause moisture on the surface, and the moisture in the solder paste will also cause Sikkim beads are formed.
In addition, the external environment also affects the formation of tin beads. We have encountered such a situation. When the printed boards are stored in a damp warehouse for a long time, small water droplets are found in the vacuum bag containing the printed boards. Moisture will affect the welding effect. Therefore, if conditions permit, high-temperature drying of printed boards and components before mounting will effectively inhibit the formation of tin beads.
The shorter the time the solder paste is in contact with air, the better. This is the basic principle of using solder paste.
After taking out a part of the solder paste, close the lid immediately, especially the inner lid must be pressed down tightly to squeeze all the air between the lid and the solder paste, otherwise it may be scrapped within a few days.
Summer is the season most likely to produce solder balls. In summer, the air temperature is high. When taking out the solder paste from the refrigerated place, be sure to leave it at room temperature for 4-5 hours before opening the lid. If the solder paste can be used up in a short period of one month, refrigeration is not recommended.
It can be seen that there are many factors that affect the formation of tin beads, and it is far from enough to adjust a certain parameter. We need to study how to control various factors in the production process, so as to achieve the best welding results.
The above points are the characteristics of PCBA failure. Next time, FASTPCBA will analyze the PCB failure analysis steps for everyone.
date：2021-08-20 09:56:17
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