Viscosity and surface tension are important properties of PCBA solder, and good PCBA solder should have low viscosity and surface tension when melted. Whether it is reflow, wave or hand soldering, surface tension is a detriment to forming good solder joints.
1. Measures to change surface tension and viscosity
Viscosity and surface tension are important properties of PCBA solder. A good PCBA solder should have low viscosity and surface tension when melted. Surface tension is the nature of matter, it cannot be eliminated, but it can be changed.
The main measures to reduce surface tension and viscosity in PCBA welding process are as follows:
i. Increase the temperature. Increasing the temperature can increase the molecular distance in the molten PCBA solder and reduce the attraction of the molecules in the liquid PCBA solder to the surface molecules. Therefore, increasing the temperature can reduce the viscosity and surface tension.
ii. Adjust the metal alloy ratio. The surface tension of Sn is very large, and the addition of Pb can reduce the surface tension. When the content of lead is increased in Sn-PbPCBA solder, when the content of Pb reaches 37%, the surface tension is obviously reduced.
iii. increase the active agent. This can effectively reduce the surface tension of the PCBA solder, and can also remove the surface oxide layer of the PCBA solder.
iv. Improve the PCBA welding processing environment. Using nitrogen to protect PCBA welding process or vacuum PCBA welding process can reduce high temperature oxidation and improve wettability.
2. The role of surface tension in PCBA welding
Surface tension is in the opposite direction to the wetting force, so surface tension is one of the factors that are detrimental to wetting.
Whether it is reflow, wave or hand soldering, surface tension is a detriment to forming good solder joints. But surface tension can be used again in SMT chip processing reflow soldering.
When the solder paste reaches the melting temperature, under the action of the rebalanced surface tension, the self-alignment effect (Self Alignment) will occur, that is, when the component placement position deviates a little, under the action of surface tension, the components can automatically is pulled back to the approximate target position.
Therefore, the surface tension makes the reflow process looser on the placement accuracy requirements, and it is easier to achieve a high degree of automation and high speed.
At the same time, it is precisely because of the characteristics of “reflow” and “self-positioning effect” that the SMT reflow soldering process has stricter requirements in terms of pad design and component standardization.
If the surface tension is unbalanced, even if the placement position is very accurate, PCBA welding defects such as component position deviation, tombstones, and bridges will occur after PCBA welding processing.
During wave soldering, due to the size and height of the SMC/SMD component body itself, or because high components block short components, the oncoming tin wave flow is blocked, and the shadow effect is caused by the influence of the surface tension of the tin wave flow. The back side forms a blocking area that liquid PCBA solder cannot infiltrate, resulting in solder leakage.