1 tinning effect When hot liquid solder dissolves and penetrates the surface of the metal being welded, it is called metal tin or metal is tinned. The molecule of the mixture of solder and copper forms a new part of copper, which is a solder alloy. This solvent is called tin, which forms an intermolecular bond between the various parts to form a metal alloy complex. The formation of good intermolecular bonds is the core of the welding process, which determines the strength and quality of the solder joints. Only the surface of the copper is not contaminated, there is no oxide film formed by exposure to the air, and the solder and the working surface need to reach an appropriate temperature.
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2 surface tension
Everyone is familiar with the surface tension of water, which keeps the cold water droplets on the greased metal plate spherical, since in this case the adhesion of the liquid on the solid surface tends to spread less than its cohesion. Wash with warm water and detergent to reduce the surface tension, water will infiltrate the greased metal plate and flow outward to form a thin layer, this will happen if the adhesion is greater than the cohesion.
The cohesion of tin-lead solder is even larger than that of water, making the solder a sphere to minimize its surface area (the same volume, the sphere has the smallest surface area compared to other geometries, to meet the minimum energy requirements) . The role of the flux is similar to the effect of the cleaning agent on the greased metal sheet. In addition, the surface tension is highly dependent on the degree of cleanliness and temperature of the surface. Only when the adhesion energy is much greater than the surface energy (cohesion) can the ideal stain tin.
3 Metal alloy synthesis
The intermetallic bonds of copper and tin form grains, and the shape and size of the grains depend on the duration and strength of the temperature at the time of soldering. Less heat during welding forms a fine crystalline structure, resulting in an excellent solder joint with optimum strength. The reaction time is too long, whether it is due to too long welding time or too high temperature or both, it will lead to a rough crystalline structure, which is gravel and brittle, and has a small shear strength.
Copper is used as the metal substrate, tin-lead is used as the solder alloy, and lead and copper do not form any metal alloy complex. However, tin can penetrate into the copper, and the intermolecular bonds of tin and copper form a metal on the joint surface of the solder and the metal. The alloying compounds Cu3Sn and Cu6Sn5 are shown in the figure. 
The metal alloy layer (n-phase + ε phase) must be very thin. In laser welding, the thickness of the metal alloy layer is 0.1 mm. In wave soldering and manual soldering, the thickness of the intermetallic bond of the excellent solder joint mostly exceeds 0.5 μm. Since the shear strength of the solder joint decreases as the thickness of the metal alloy layer increases, it is often tried to keep the thickness of the metal alloy layer below 1 μm, which can be achieved by making the soldering time as short as possible.
The thickness of the metal alloy composite layer depends on the temperature and time at which the solder joint is formed. Ideally, the soldering should be completed within 220 't for about 2 seconds. Under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount of metal. The alloy bonding materials Cu3Sn and Cu6Sn5 have a thickness of about 0.5 μm. Insufficient intermetallic bonds are common at cold solder joints or solder joints that are not raised to the proper temperature during soldering, which may result in the cutting of the soldered surface. Conversely, a too thick layer of metal alloy, common in solder joints that are overheated or soldered for too long, will result in very weak tensile strength at the solder joint, as shown. 
4 Dip tin angle
When the temperature of the eutectic point of the solder is about 35 ° C higher, when a drop of solder is placed on the hot flux-coated surface, a meniscus is formed, and to some extent, the metal surface is tinned. It can be evaluated by the shape of the meniscus. If the solder meniscus has a distinct undercut, shaped like a water droplet on a greased metal plate, or even tends to be spherical, the metal is not solderable. Only the meniscus is stretched to a less than 30. The small angle has good weldability.
Four efficient methods for improving PCB soldering
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