Whether it is reflow soldering, wave soldering, or manual soldering, surface tension is a detrimental factor in forming good solder joints. But in SMT surface mount processing and reflow soldering, surface tension can be utilized again - when the solder paste reaches the melting temperature, on the balanced surface

1、 Measures to change surface tension and viscosity

Viscosity and surface tension are important properties of solder. Excellent solder should have low viscosity and surface tension when melted. Surface tension is the nature of matter and cannot be eliminated, but it can be changed.

The main measures to reduce surface tension and viscosity in PCBA welding are as follows:

① Raise the temperature. Raising the temperature can increase the molecular distance inside the molten solder and reduce the attraction of molecules on the surface of the liquid solder. Therefore, heating can reduce viscosity and surface tension.

② Adjust the proportion of metal alloys. The surface tension of Sn is high, and increasing Pb can reduce the surface tension. From the figure, it can be seen that increasing the lead content in Sn-Pb solder significantly reduces surface tension when the Pb content reaches 37%.

③ Add active agents. This can effectively reduce the surface tension of the solder and also remove the surface oxide layer of the solder.

④ Improve the welding environment. Using nitrogen protected PCBA welding or vacuum welding can reduce high-temperature oxidation and improve wettability.

2、 The role of surface tension in welding

The direction of surface tension and wetting force is opposite, so surface tension is one of the factors that is not conducive to wetting.

Whether it is reflow soldering, wave soldering, or manual soldering, surface tension is a detrimental factor in forming good solder joints. But surface tension can also be utilized in SMT surface mount processing and reflow soldering.

When the solder paste reaches the melting temperature, under the action of balanced surface tension, a self alignment effect will occur. That is, when the placement position of the component deviates slightly, under the action of surface tension, the component can be automatically pulled back to the approximate target position.

Therefore, surface tension makes the reflow process have relatively loose requirements for mounting accuracy, making it easier to achieve high automation and high speed.

At the same time, due to the characteristics of "reflow" and "self positioning effect", the SMT reflow soldering process has stricter requirements for pad design, component standardization, and other aspects.

If the surface tension is unbalanced, even if the mounting position is very accurate, welding defects such as component position deviation, standing monument, and bridging may occur after welding.

During peak soldering, due to the size and height of the SMC/SMD component body itself, or the blocking of the incoming tin wave flow by the high component blocking the low component, and the shadow effect caused by the surface tension of the tin wave flow, a flow blocking zone that liquid solder cannot penetrate is formed on the back of the component body, resulting in solder leakage.