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Why are solder beads produced in PCBA processing and what are the reasons?
2024-01-03
PCBA processing may have the production of tin beads, next, STG Electronic will analyze the factors that affect the production of tin beads.
Factor 1: The selection of solder paste in PCBA processing technology directly affects the quality of soldering
The metal content in the solder paste, the oxidation degree of the metal powder, and the size of the metal powder can all affect the generation of solder balls.
a. The metal content of the solder paste, the mass ratio of the metal content in the solder paste is about 88%-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 forces generated by vaporization during the preheating process. The increase in metal content makes the metal powders tightly packed, making it easier to bond without being blown away when melted. In addition, the increase in metal content may also reduce the "collapse" of the solder paste after printing, so it is not easy to produce solder beads.
b. The oxidation degree of the metal powder of the solder paste, the higher the oxidation degree of the metal powder in the solder paste, the greater the binding resistance of the metal powder during soldering, and the less easy it is to infiltrate between the solder paste and the pads and components, resulting in the reduction of solderability. Experiments show that the incidence of tin beads is directly proportional to the oxidation degree of metal powder. In general, the oxidation degree of solder in solder paste is controlled below 0.05%, and the maximum limit is 0.15%
c. The size of the metal powder in the solder paste, the smaller the particle size of the metal powder in the solder paste, the larger the overall surface area of the solder paste, which leads to the higher oxidation degree of the finer powder, so the phenomenon of solder beads is aggravated. Experiments have shown that when using finer particles of solder paste, it is easier to produce solder beads.
d. The amount of flux in the solder paste and the activity of the flux, too much solder dose will cause the local collapse of the solder paste, so that the solder beads are easy to generate. In addition, if the activity of the flux is too weak, the ability to remove oxidation is weak, and it is easier to produce solder beads.
e. Other precautions, the solder paste is taken out of the refrigerator and opened without reheating, causing the solder paste to absorb moisture, and the solder paste splashes during preheating and produces solder beads; PCB moisture, indoor humidity is too heavy, wind blows on the solder paste, excessive thinner is added to the solder paste, and the machine stirring time is too long, etc., will promote the production of solder balls.
Factor 2: PCBA processing technology steel mesh production and opening
a. The opening of the steel mesh, we generally open the steel mesh according to the size of the pad, and when printing solder paste, it is easy to print the solder paste on the solder mask, so as to produce solder beads during reflow soldering. Therefore, we open the stencil in such a way that the stencil opening is 10% smaller than the actual size of the pad, and we can change the shape of the stencil to achieve the desired effect.
b. The thickness of the steel mesh, the steel mesh Baidu is generally between 0.12-0.17mm, too thick will cause the "collapse" of the solder paste, resulting in tin beads.
Factor 3: If the placement pressure of the PCBA processing technology placement machine is too high, the solder paste will be easily squeezed onto the
solder mask layer under the component, and the solder paste will melt and run to the surrounding area of the component to form tin beads during reflow soldering. Solution: Reduce the placement pressure, adopt a suitable stencil opening form to avoid the solder paste being squeezed to the outside of the pad.
Factor 4: The setting of the furnace temperature curve of the PCBA processing process, the tin beads are generated during overflow soldering. In the preheating stage, the temperature of the solder paste, PCB and components rises to between 120~150 °C, and the thermal shock of the components during reflow must be reduced. At this stage, the temperature rise should not be too fast, generally less than 2.5 °C/S, too fast can easily cause solder splash and form tin beads. Therefore, the preheating temperature and preheating speed of reflow soldering should be adjusted to control the generation of solder beads.
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