PCB Tech - The most suitable welding method for PCB

1. Tin dipping effect of PCB

When the hot liquid solder dissolves and penetrates into the surface of the metal to be welded, it is called metal stained with tin or metal stained with tin. The molecules of the mixture of solder and copper form a new alloy with part of copper and part of solder. This solvent action is called tin dipping. It forms an intermolecular bond between each part to form a metal alloy co compound. The formation of good intermolecular bond is the core of welding process, which determines the strength and quality of welding joint. Only the surface of copper is free of pollution and oxide film formed due to exposure to air can be stained with tin, and the solder and working surface need to reach an appropriate temperature.

2. PCB surface tension

Everyone is familiar with the surface tension of water, which keeps the cold water droplets on the greased metal plate spherical, because in this example, the adhesion that makes the liquid tend to diffuse on the solid surface is less than its cohesion. Clean with warm water and detergent to reduce its surface tension. The water will infiltrate the metal plate coated with grease and flow out to form a thin layer. This will happen if the adhesion is greater than the cohesion.

The cohesion of tin lead solder is even greater than that of water, so that the solder is a sphere to minimize its surface area (under the same volume, the sphere has the smallest surface area compared with other geometric shapes to meet the requirements of the lowest energy state). The effect of flux is similar to that of cleaner on metal plate coated with grease. In addition, the surface tension is highly dependent on the cleanliness and temperature of the surface. Ideal tin staining can occur only when the adhesion energy is much greater than the surface energy (cohesion).

3. Generation of metal alloy co compounds of PCB

The intermetallic bond between copper and tin forms grains. The shape and size of grains depend on the duration and strength of temperature during welding. When welding, less heat can form fine crystal structure and form excellent welding points with the best strength. Too long reaction time, whether due to too long welding time, too high temperature or both, will lead to rough crystal structure, which is sandy, brittle and low shear strength.

Copper is used as the metal substrate and tin lead is used as the solder alloy. Lead and copper will not form any metal alloy co compounds. However, tin can penetrate into copper. The intermolecular bond between tin and copper forms metal alloy co compounds cu3sn and Cu6Sn5 on the connecting surface between solder and metal, as shown in the figure.

Metal alloy layer (N phase)+ ε Phase) must be very thin. In laser welding, the thickness of metal alloy layer is 0.1mm. In wave soldering and manual soldering iron welding, the thickness of intermetallic bond at excellent welding points mostly exceeds 0.5 μ m . Since the shear strength of the welded joint decreases with the increase of the thickness of the metal alloy layer, it is often tried to keep the thickness of the metal alloy layer at 1 μ M or less, this can be achieved by making the welding time as short as possible.

The thickness of the metal alloy eutectic layer depends on the temperature and time of forming the welding point. Ideally, the welding should be completed within about 2S of 220't. under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount of metal alloy bonding materials cu3sn and Cu6Sn5, with a thickness of about 0.5 μ m . Insufficient intermetallic bonds are common in cold welds or welds that do not rise to the appropriate temperature during welding, which may lead to the cutting of the welding surface. On the contrary, too thick metal alloy layer is common in welded joints with excessive heating or welding for too long, which will lead to very weak tensile strength of welded joints, as shown in the figure.

4. Tin dipping angle of PCB

When the eutectic point temperature of solder is about 35 degree Celsius higher than that of solder, when a drop of solder is placed on the hot surface coated with flux, a meniscus is formed. To some extent, the ability of tin on the metal surface can be evaluated by the shape of the meniscus. If the solder meniscus has an obvious undercut, such as water droplets on a greased metal plate, or even tends to be spherical, the metal is not weldable. Only the meniscus is stretched to a value less than 30. Good weldability can only be achieved at a small angle.