1 PCB surface tension

The cohesion of tin lead solder is even greater than that of water, making the solder 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 function of the flux is SIMilar to that of the cleaner on the metal plate coated with grease. In addition, the surface tension of the PCB is also highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is far greater than the surface energy (cohesion) can ideal tin adhesion occur.

2 Production of metal alloy compounds

The intermetallic bond between copper and tin forms grains. The shape and size of grains depend on the duration and strength of the temperature during welding. Less heat during welding can form fine crystalline structure, forming excellent welding points with the best strength. The processing reaction time of PCB chip is too long. Whether it is due to too long PCB welding time, too high temperature or both, it will lead to rough crystalline structure, which is sandy and brittle, with low shear strength.

3 Tin DIPping angle

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

4 Tinning

When the hot liquid solder dissolves and penetrates into the metal surface to be welded, it is calLED metal tin or metal tin. The molecules of the mixture of soldering tin and copper form a new kind of alloy that is partly copper and partly solder. This solvent action is called dipping tin, which forms intermolecular bonds between various parts to form a metal alloy compound. The formation of good intermolecular bond is the core of PCB welding process, which determines the strength and quality of welding points. Only the copper surface is free from pollution, and no oxide film formed due to exposure to air can be stained with tin, and the soldering tin and working surface need to reach appropriate temperature.

5 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 complexes. However, tin can penetrate into copper. The intermolecular bond between tin and copper forms metal alloy complexes Cu3Sn and Cu6Sn5 at the junction surface of solder and metal, as shown in the figure.

Metal alloy layer (n-phase+ ε Phase) must be very thin. In laser welding, the order of magnitude of the thickness of the metal alloy layer is 0.1mm. In wave soldering and manual soldering of PCB boards, the thickness of the metal bonds at the welding points of excellent PCBs mostly exceeds 0.5 μ m 。 As the shear strength of PCB solder joints decreases with the increase of metal alloy layer thickness, it is often tried to keep the thickness of metal alloy layer at 1 μ M below, which can be achieved by making the PCB welding time as short as possible.

The thickness of the metal alloy eutectic layer depends on the temperature and time of forming the solder joint. Ideally, the circuit board welding should be completed in about 220't 2s. 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 。 Inadequate intermetallic bonds are commonly found in cold welding points or welding points that have not been raised to an appropriate temperature during welding, which may result in the cutting off of the welding surface. On the contrary, too thick metal alloy layer is commonly used in welding points that are overheated or welded for too long, which will lead to very weak tensile strength of welding points

This paper mainly introduces the method of false soldering and false soldering caused by improved PCB soldering in PCB chip processing