PCB Blog - Soldering method of PCB board

1、The role of tin dip
When the hot liquid solder dissolves and penetrates into the metal surface of the PCB board to be soldered, it is called metal tinning or metal tinning. The molecules of the mixture of solder and copper form a new alloy that is partly copper and partly solder. This solvent action is called tin dip, which forms an intermolecular bond between the various parts to form a metal alloy co-compound. The formation of good intermolecular bonds is the welding process, which determines the strength and quality of the welded joint. Only the surface of the copper is free of contamination, and there is no oxide film formed by exposure to the air to be tinned, and the solder and the working surface need to reach an appropriate temperature.

2、Surface tension
Everyone is familiar with the surface tension of water, the force that keeps cold water droplets on a greased metal plate spherical, because in this case the adhesion force that makes the liquid tend to spread on a solid surface is less than its cohesive force. Wash with warm water and detergent to reduce its surface tension. The water will wet the greased metal sheet and flow outward to form a thin layer, which can happen if adhesion is greater than cohesion.

The cohesion of tin-lead solder is even greater than that of water, making the solder spherical to give its surface area (the surface area of a sphere compared to other geometries at the same volume to meet the energy state requirements). The action of the flux is similar to that of the cleaner on the greased metal plate. In addition, the surface tension is also highly dependent on the cleanliness and temperature of the surface. Only when the adhesion energy is much greater than the surface energy (cohesion) can ideal adhesion occur. tin.

3、The formation of metal alloy co-compounds
The intermetallic bonds of copper and tin form grains, the shape and size of which depend on the duration and strength of the temperature during soldering. Less heat during welding creates a fine grained structure, resulting in an excellent weld with strength. Excessive reaction time, whether due to excessive welding time or high temperature or both, can result in a coarse crystalline structure that is gritty and brittle with less shear strength. Using copper as the metal substrate and tin-lead as the solder alloy, lead and copper will not form any metal alloy co-compounds, however, tin can penetrate into copper, and the intermolecular bonds of tin and copper form metal at the connection surface of solder and metal. Alloy co-compounds Cu3Sn and Cu6Sn5.

The metal alloy layer (n-phase + ε phase) must be very thin. In laser welding, the thickness of the metal alloy layer is on the order of 0.1mm. In wave soldering and manual soldering, the thickness of the intermetallic bonds of good solder joints is mostly more than 0.5μm. Since the shear strength of the welded joint decreases with the thickness of the metal alloy layer, it is often tried to keep the thickness of the metal alloy layer below 1 μm, which 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 at which the solder joint is formed. Ideally, the soldering should be completed within about 2s at 220't. Under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount of metal The thickness of alloy bonding materials Cu3Sn and Cu6Sn5 is about 0.5μm. Insufficient metal-to-metal bonds are commonly seen in cold solder joints or solder joints that have not been raised to the proper temperature during soldering, and can result in severed joints. Conversely, a metal alloy layer that is too thick, often seen in overheated or soldered joints for too long, will result in very weak joint tensile strengths.

4、Dip tin corner
About 35°C above the eutectic temperature of the solder, when a drop of solder is placed on a hot flux-coated surface, a meniscus is formed, to some extent, the ability of the metal surface to wet with tin It can be estimated by the shape of the meniscus. A metal is not solderable if the solder meniscus has a distinct undercut edge, like a drop of water on a greased metal plate, or even tends to be spherical. Only the meniscus is stretched to a less than 30. The small angle has good weldability on PCB board.