焊接原理

Principles of Soldering

 润湿

Wetting

　　 润湿是焊接行为中的主角，其接合即是利用液态焊锡润湿在基材上而达到接合的效果，这种现象正如水倒在固体表面上完全一样，不同的是当温度降低后，焊锡凝固而形成焊接点。当焊锡润湿在基材上时，两者之间以化学键结合，而形成一种连续性的结合。但在实际情况下基材常因受空气及周边环境的侵蚀，而会有一层氧化层。氧化层会阻挡焊锡而无法达到好的润湿效果，其现象正如水倒在满是油脂的盘上，水只聚集在部分地方，无法全面均匀分布在盘子上。如果我们未能将氧化层除去，即使勉强沾上焊锡，其结合力量也是非常弱的。一涂有油脂的金属薄板浸到水中，没有润湿现象，不管它上面所涂的油脂多薄，它可能完全看不到，但水会成球状的水滴，一摇即掉。因此，水并未润湿或粘在金属薄板上。如将此金属薄板放入清洗剂中加以清洗，并小心地干燥。再把它浸入水中，液体将完全地扩散到金属薄板的表面而形成一薄、均匀的膜层，再怎么摇也不会掉，即它已经润湿了此金属薄板。因此，当焊锡表面和金属表面也很干净时，焊锡一样会润湿金属表面。其清洁水准的要求比水于金属板上还要高很多，因为焊锡和金属之间必需是紧密的连接。而极薄的氧化层也将防碍金属表面上焊锡的润湿作用。

Wetting is the leading role in soldering, and its bonding is made by wetting substrates using liquid solders to achieve the bonding effect. This phenomenon is just like water being poured on the surfaces of solids. What is different is that after temperature drops, the solder will be condensed and forms welds. When solders wet the substrates, both are bonded by chemical bonds and form a kind of successive bonding. However, as the case may be, substrates are eroded by air and surrounding environments, hence a coat of oxide. The oxide coat will stop solders from achieving the wetting effect. The phenomenon is just like water being poured on a plate full of grease, and water is only concentrated on part of places and it is unable to be well distributed on the plate. If we are unable to remove the oxide coat, even it is reluctantly soldered, the bonding strength is very weak. When metal plates applied with grease are dipped in water, there is no wetting. No matter how thin the grease is applied to the surface, it will be completely invisible. But water will become sphere shaped water drops and will drop the time it is shaken. Therefore, water is not wetted or stuck to the thin metal plates. If metal plates are placed in cleaning agents to be rinsed and are made dry with care, and then dipped in water, the liquids will be completely spread on the surfaces of such metal plates forming a thin and well distributed film. It will not drop however you shake it, that is, it has already wetted such a metal plate. Therefore, when soldering surfaces and metal surfaces are also very clean, solders will also wet the surfaces of such metal. The requirement of the cleaning level is more than the water on metal, for there must be close bonding between solders and metals. The extremely thin oxide coat will also hinder the solder wetting effect on metal surfaces.

　　 助焊剂的作用就像溶剂对涂有油脂的金属薄板一样，溶剂去除油脂，让水润湿金属表面和减少表面张力。助焊剂的功能为

（1）清除金属表面氧化物，

（2）降低焊锡表面张力，增加其扩散能力。

The soldering flux is just like the solvents to the metal plates applied with grease, in which the solvents remove grease and allow water to wet the metal surface and reduce the surface tension. The function of soldering flux is 

(1) to clear oxide on metal surfaces, 

(2)to reduce tension on the solder surface so as to enhance the diffusion capacity.

 润湿的热动力平衡

Wetted thermodynamic equilibrium

         当焊锡润湿在基层金属上，静止下来时，亦即是力平衡的状态。下图是焊锡力平衡状态图。

When solder wetting is wetted on the metal at the basic level, when it becomes still, that is the thermodynamic state. The following graph is the chart of the solder thermodynamic state.

  由上图得知PSF=PLS +PLF·COSθ

PSF表示在基底金属和助焊剂流体之间的界面张力；

PLS表示熔化焊料与基底金属的界面张力；

PLF表示在熔化焊料和助焊剂流体之间的界面张力；

θ表示液态焊料和基板之间的接触角。

         当焊锡滴在固体表面呈圆球时PSF＞PLS +PLF·COSθ,此时开始扩散,θ角度逐渐变小,PLE·COSθ值变大，直到力量平衡为止。

1.＞90°，如果整个系统力量达到平衡时θ＞90°，则表示PSF的值小，亦即其液体的扩散力差．

2．90°＞θ＞M，我们称为边际润湿（marginal wetting）。通常的M＞75°，这种润湿也是不能接受的润湿。

3．θ＜M，此种称为良好润湿（good wetting）。M值要求低于75°。

            由上述说明角度越小表示润湿越好，采用化学和物理方法。两者都可获得很低的θ值。物理方法是在焊接过程中，处理所要焊接材料的表面张力。原则上采用：

（1）低表面张力的助焊剂，

（2）低表面张力的焊料。

From the above chart, we know PSF=PLS +PLF·COSθ

PSF indicates the tension of the interface between the basic level metal and soldering flux fluid;

PLS indicates the tension between the interfaces of the melted solders and bottom metal;

PLF indicates the tension of interfaces between the melted solders and soldering flux fluids;

θ indicates the contact angle between the liquid solders and substrates.

When the solders drip on the solid surface in a shape of a round ball,PSF＞PLS +PLF·COSθ,starts diffusion,and Angleθ becomes smaller,PLE·COSθbecomes larger until the strength is balanced.

1.θ＞90°，If the strength of the entire system lives up to balance,θ＞90°，then it indicates that the value of PSF is small，the diffusion capacity of liquids is poor．

2．90°＞θ＞M，We call it marginal wetting. Generally, M＞75°，and this kind of wetting is unacceptable wetting.

3．θ＜M，This is called good wetting. M value is required to be lower than 75°。

Seen from above, the smallerθangle it is, the better wetting it appears. By applying chemical and physical approaches, both can achieve a very low θvalue。The physical approach is that during the welding process, the surface tension of the required welding materials is processed. In principle,we apply:

 (1) soldering flux with low surface tension; and

 (2) solders with low surface tension.