Viscosity and surface tension are important properties of PCBA solders, and excellent PCBA solders should have low viscosity and surface tension when they are melted. Whether it is reflow soldering, wave soldering or manual soldering, surface tension is an unfavorable factor for the formation of 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 and cannot be eliminated, but it can be changed.
The main measures to reduce surface tension and viscosity in PCBA welding processing are as follows:
① 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.
②Adjust the proportion of metal alloy. The surface tension of Sn is very large, and increasing Pb can reduce the surface tension. It can be seen from the figure that when the content of lead is increased in the Sn-PbPCBA solder, when the content of Pb reaches 37%, the surface tension is significantly reduced.
③ Increase the active agent. This can effectively reduce the surface tension of PCBA solder and also remove the surface oxide layer of PCBA solder.
④Improve PCBA welding processing environment. The use of nitrogen-protected PCBA welding processing or vacuum PCBA welding processing can reduce high temperature oxidation and improve wettability.
2. The role of surface tension in PCBA welding processing
The direction of surface tension and wetting force are opposite, so surface tension is one of the factors that are not conducive to wetting.
Whether it is reflow soldering, wave soldering or manual soldering, surface tension is an unfavorable factor for the formation of 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 the balanced surface tension, it will produce the self-alignment effect (Self Alignment), that is, when the component placement position has a small deviation, under the action of the surface tension, the component can automatically Pulled back to the approximate target position.
Therefore, the surface tension makes the reflow process relatively loose requirements for placement accuracy, and it is easier to achieve a high degree of automation and high speed.
At the same time, because of the characteristics of "reflow" and "self-positioning effect", SMT reflow soldering process has more stringent 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 processing defects such as component position offset, tombstones, and bridges will occur after the PCBA welding process.
During wave soldering, due to the size and height of the SMC/SMD component body, or because the tall component blocks the short component and blocks the oncoming tin wave flow, the shadow effect is caused by the surface tension of the tin wave flow. A baffle area that cannot be wetted by the liquid PCBA solder is formed on the back, causing solder leakage.