1, tin dipping effect

When hot liquid solder dissolves and penetrates the metal surface of the печатная плата being soldered, it is called metal bonding or metal bonding. The molecules of the mixture of solder and copper form a new alloy that is part copper and part solder. This solvent action is called tin-bonding. It forms an intermolecular bond between the various parts of the PCB, creating a metal alloy compound. The formation of good intermolecular bonds is the core of PCB welding process, which determines the strength and quality of PCB welding points. Tin can be stained only if the copper surface is free of contamination and oxide film formed due to PCB exposure to air, and the solder and the working surface need to reach the appropriate temperature.

2. Поверхностное натяжение

Everyone is familiar with the surface tension of water, the force that keeps cold water droplets spherical on a greased PCB metal plate because, in this case, the adhesion that tends to diffuse the liquid on a solid surface is less than its cohesion. Wash with warm water and detergent to reduce surface tension. The water will saturate the greased PCB metal plate and flow outward to form a thin layer, which occurs if the adhesion is greater than the cohesion.

Tin-lead solder is even more cohesive than water, making the solder spherical to minimize its surface area (for the same volume, the sphere has the smallest surface area compared to other geometries to meet the requirements of the lowest energy state). The effect of flux is similar to that of detergent on the PCB metal plate coated with grease. In addition, the surface tension is also highly dependent on the cleanliness and temperature of the PCB surface. Only when the adhesion energy is much greater than the surface energy (cohesion), the PCB can have the ideal tin adhesion.

3, with tin Angle

A meniscus is formed when a drop of solder is placed on the surface of a hot, flux-coated PCB approximately 35 ° C above the eutectic point of solder. To some extent, the ability of the metal surface of a PCB to stick tin can be evaluated by the shape of the meniscus. The metal is not solderable if the meniscus has a clear bottom cut, looks like droplets of water on a greased PCB metal plate, or even tends to be spherical. Only the meniscus stretched to a size less than 30. The small Angle has good weldability.

4. The generation of metal alloy compounds

The intermetallic bonds of copper and tin form grains whose shape and size depend on the duration and strength of the temperature at which they are welded. Less heat during welding can form a fine crystal structure, which makes the PCB form an excellent welding spot with the best strength. Too long reaction time, whether due to PCB welding time too long, too high temperature or both, will result in a rough crystalline structure that is gravelly and brittle with low shear strength.Copper is used as the metal base material of PCB, and tin-lead is used as the solder alloy. Lead and copper will not form any metal alloy compounds, but tin can penetrate into copper. The intermolecular bond between tin and copper forms metal alloy compounds Cu3Sn and Cu6Sn5 at the solder and metal junction.

The metal alloy layer (n +ε phase) must be very thin. In PCB laser welding, the thickness of metal alloy layer is 0.1mm in number class. In wave soldering and manual soldering, the thickness of intermetal bond of good welding points of PCB is more than 0.5μm. Because the shear strength of PCB welds decreases as the metal alloy layer thickness increases, it is often attempted to keep the metal alloy layer thickness below 1μm by keeping the welding time as short as possible.

The thickness of the metal alloy layer depends on the temperature and time of forming the welding spot. Ideally, the welding should be completed in about 220 ‘t 2s. Under these conditions, the chemical diffusion reaction of copper and tin will produce appropriate metal alloy binding materials Cu3Sn and Cu6Sn5 with thickness of about 0.5μm. Inadequate intermetal bonding is common in cold solder joints or solder joints that are not raised to the appropriate temperature during welding and can lead to a cut off of the PCB weld surface. In contrast, too thick metal alloy layers, common in overheated or welded joints for too long, will result in very weak tensile strength of PCB joints.