Factor 1: The selection of solder paste directly affects the quality of soldering
The metal content in the solder paste, the degree of oxidation of the metal powder, and the size of the metal powder can all affect the generation of tin beads.
a. Metal content of solder paste
The metal content in the solder paste is about 88% to 92% by mass and 50% by volume. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during preheating. The increase in the metal content makes the metal powder tightly packed, making it easier to combine when melted without being blown apart. In addition, the increase in metal content may also reduce the “slump” of the solder paste after printing, so it is less likely to produce solder beads.
b. Oxidation degree of metal powder of solder paste
The higher the oxidation degree of the metal powder in the solder paste, the greater the bonding resistance of the metal powder during soldering, and the infiltration between the solder paste and the pads and components is not easy, resulting in reduced solderability. Experiments show that the occurrence rate of tin beads is proportional to the degree of oxidation of the metal powder. Generally, the solder oxidation degree in the solder paste is controlled below 0.05%, and the maximum limit is 0.15%
c. Size of metal powder in 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 a higher degree of oxidation of the finer powder, which intensifies the phenomenon of solder beads. Experiments have shown that solder balls are more likely to be produced when a finer particle of solder paste is used.
d. The amount of flux in the solder paste and the activity of the flux
Too much flux will cause local collapse of the solder paste, which will make solder balls easy to produce. In addition, when the activity of the flux is too weak, the ability to remove oxidation is weak, and it is more likely to produce tin beads.
e. Other precautions
After the solder paste is taken out of the refrigerator, it is opened and used without returning to the temperature, which causes the solder paste to absorb moisture, and the solder paste splashes during preheating to produce solder beads. PCB is damp, the indoor humidity is too heavy, the wind blows against the solder paste, excessive diluent added to the solder paste, excessive machine stirring time, etc. will promote the generation of tin beads.
Factor 2. The production and opening of steel mesh
a. Opening of steel mesh
We generally open the stencil according to the size of the pad. When printing solder paste, it is easy to print the solder paste on the solder mask layer, thereby producing solder beads during reflow soldering. Therefore, we open the steel mesh in this way, making the opening of the steel mesh 10% smaller than the actual size of the pad, and the shape of the opening can be changed to achieve the desired effect.
b. Thickness of steel mesh
The stencil Baidu is generally between 0.12 and 0.17mm. Too thick will cause the “collapse” of the solder paste, resulting in tin beads.
Factor 3. The placement pressure of the placement machine
If the mounting pressure is too high, the solder paste is easily squeezed onto the solder mask under the component, and the solder paste melts and runs around the component to form solder beads during reflow soldering.
Solution: reduce the mounting pressure; use a suitable stencil opening form to prevent the solder paste from being squeezed out of the pad.
Factor 4. The setting of furnace temperature curve
Solder beads are produced during reflow soldering. In the preheating stage, the temperature of the solder paste, PCB and components rises to between 120 and 150 °C, and the thermal shock of the components during reflow must be reduced. At this stage, the flux in the solder paste begins to vaporize, thereby making small particles The metal powder runs to the bottom of the component separately, and runs around the component to form tin beads when the flow is applied. At this stage, the temperature rise should not be too fast, generally less than 2.5℃/S, too fast will 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 tin beads.
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