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, which cannot be eliminated, but can be changed.
The main measures to reduce the surface tension and viscosity in PCBA welding are as follows.
① Increase the temperature. Increasing the temperature can increase the molecular distance in molten solder and reduce the attraction of molecules in liquid solder to surface molecules. Therefore, increasing temperature can reduce viscosity and surface tension.
② Adjust the proportion of metal alloy. The surface tension of Sn is very high, and adding Pb can reduce the surface tension. When the content of Pb in Sn-Pb solder is increased, the surface tension decreases obviously when the content of Pb reaches 37%.
③ Increase the active agent. This can effectively reduce the surface tension of the solder and remove the oxide layer on the surface of the solder.
④ Improve the welding environment. Pcba welding under nitrogen protection or vacuum welding can reduce high-temperature oxidation and improve wettability.
What is the role of surface tension in PCBA welding, and how to reduce the surface tension and viscosity?
The role of surface tension in welding;
The surface tension is opposite to the wetting force, so the surface tension is one of the factors unfavorable to wetting.
No matter reflow soldering, wave soldering or manual soldering, surface tension is an unfavorable factor for forming good solder joints. However, the surface tension can be used in SMT chip processing reflow soldering. When the solder paste reaches the melting temperature, under the action of balanced surface tension, the self-alignment effect will occur, that is, when the placement position of components deviates a little, the components can be automatically pulled back to the approximate target position under the action of surface tension. Therefore, the surface tension makes the requirements of reflow process for mounting accuracy more relaxed, and it is easier to achieve high automation and high speed.
At the same time, because of the characteristics of “reflow” and “self-positioning effect”, SMT reflow soldering process has stricter requirements on pad design and component standardization. If the surface tension is unbalanced, even if the mounting position is very accurate, there will be welding defects such as component position deviation, erection and bridging after welding.
During wave soldering, due to the size and height of SMC/SMD components, or because tall components block short components and block the oncoming tin wave flow, and because of the influence of the surface tension of tin wave flow, the shadowing effect is caused, and a flow blocking area which cannot be infiltrated by liquid solder is formed on the back of the components, resulting in solder leakage.
