Causes and treatment of PCB proofing tin beads

Pc proofing factor 1: the choice of solder paste 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 all affect the generation of the tin beads.

 

a. The metal content of the solder paste

The metal content of the solder paste has a mass ratio of about 88% to 92% and a volume ratio of about 50%. As the metal content increases, the viscosity of the solder paste increases, effectively resisting the forces generated by vaporization during the preheating process. The increase in the metal content causes the metal powder to be closely packed so that it is easier to bond without being blown off when melted. In addition, an increase in the metal content may also reduce the "collapse" after the solder paste is printed, and therefore, it is difficult to produce solder beads.

b. The oxidation degree of the metal powder of the solder paste

The higher the degree of oxidation of the metal powder in the solder paste, the greater the metal powder bonding resistance during soldering, and the solder paste is less likely to infiltrate with the pads and the components, resulting in a decrease in solderability. The experiment proves that the incidence of tin beads is proportional to the oxidation degree of the metal powder. Generally, the solder oxidation degree in the solder paste is controlled to be less than 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, resulting in a higher oxidation degree of the fine powder, and thus the phenomenon of the solder beads is intensified. Experiments have shown that when using finer-grain solder paste, it is easier to produce tin beads.

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

 

Too much soldering will cause partial collapse of the solder paste, making the solder balls easy to produce. In addition, when the activity of the flux is too weak, the ability to remove oxidation is weak, and tin beads are more likely to be produced.

e. Other considerations

After the solder paste is taken out of the refrigerator, it is opened and used without returning to the temperature. The solder paste absorbs moisture. When the preheating, the solder paste splashes to produce tin beads. The PCB is damp, the humidity in the room is too heavy, and the wind blows against the solder paste. The addition of excess thinner to the paste, excessive machine agitation time, etc. will promote the production of tin beads.

Pc proofing factor 2, the production and opening of the steel mesh

 

a. The opening of the steel mesh

We generally open the steel mesh according to the size of the pad. When printing the solder paste, it is easy to print the solder paste onto the solder resist layer, thereby producing tin beads during reflow soldering. Therefore, we open the steel mesh in such a way that the opening of the steel mesh is 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 the steel mesh

 

Steel mesh Baidu is generally between 0.12~0.17mm. Too thick will cause the solder paste to "collapse", thus producing tin beads.

Pcc proofing factor three, placement machine placement pressure

 

If the pressure is too high during the placement, the solder paste is easily squeezed onto the solder mask under the component, and the solder paste melts around the component to form solder balls during reflow soldering. Solution: Reduce the placement pressure; use a suitable stencil opening to prevent the solder paste from being squeezed outside the pad.

 

Pcc proofing factor four, the setting of the furnace temperature curve

Tin beads are produced during reflow soldering. During the preheating phase, the temperature of solder paste, PCB and components is raised to between 120 and 150 °C. The thermal shock of the components during reflow must be reduced. At this stage, the solder in the solder paste begins to vaporize, thus 

making small particles. The metal powder runs down to the bottom of the component and travels around the component to form a tin bead during the flow. At this stage, the temperature rise should not be too fast, generally should be less than 2.5 ° C / S, too fast and easy to cause solder splash, forming tin beads. Therefore, the preheating temperature and preheating speed of reflow soldering should be adjusted to control the generation of solder balls.