When the hot liquid solder dissolves and penetrates into the metal surface to be welded, 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 effect is called tin dipping, which forms intermolecular bonds between the various parts to form a metal alloy co-compound.
The formation of good intermolecular bonds is the core of the soldering process, which determines the strength and quality of printed circuit board solder joints.
Only the surface of the copper is free from 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.
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 printed circuit board 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 minimize its surface area (with the same volume, the sphere has the smallest surface area compared to other geometries to meet the needs of the lowest energy state).
The effect of flux is similar to the effect of detergent on printed circuit boards and metal plates coated with grease. In addition, 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), In order to produce the ideal dip tin.
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 optimum strength.
Excessive reaction time, whether due to excessive soldering time or high temperature or both, will result in a rough crystalline structure of the printed circuit board, which is gritty and brittle with low 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, but 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 the laser welding of printed circuit boards, the thickness of the metal alloy layer is on the order of 0.1mm. more than 0.5μm.
Since the shear strength of the weld decreases with the thickness of the metal alloy layer, an attempt is made to keep the thickness of the metal alloy layer below 1 μm, which can be achieved by keeping the welding time as short as possible.
The thickness of the metal alloy co-compound layer depends on the temperature and time of forming the solder joint. Ideally, the printed circuit board soldering should be completed within about 2s at 220't. Under this condition, the chemical diffusion reaction of copper and tin will be Appropriate amounts of metal alloy binding materials Cu3Sn and Cu6Sn5 were produced with a thickness of about 0.5 μm.
Inadequate printed circuit board metal-to-metal bonds are commonly seen in cold solder joints or solder joints that are not raised to the proper temperature during soldering, and can result in cut-off of the solder surface.
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.
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, in a way, the ability of the metal surface to wet with tin It can be assessed by the shape of the meniscus.
A metal is not solderable if the solder meniscus has a distinct undercut edge in the shape of a greased printed circuit board, a drop of water on a metal plate, or even tends to be spherical. Only when the meniscus is stretched to a small angle less than 30° has good weldability.