Best soldering methods for printed circuit boards
1 Tinning effect
When hot liquid solder dissolves and penetrates the metal surface to be soldered, it is called tinning of the metal or tinning of the metal. 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 tinning, 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 the solder joint. Only when the copper surface is free of pollution and there is no oxide film formed due to exposure to the air can it be tinned, and the solder and the work surface need to reach the appropriate temperature.
2 Surface tension
Everyone is familiar with the surface tension of water. This force keeps the cold water droplets on the greased metal plate in a spherical shape. This is because in this case, the adhesion force that makes the liquid on the solid surface tend to spread is less than its cohesion. Wash with warm water and detergent to reduce its surface tension, and the water will soak the greased metal plate and flow outward to form a thin layer. This will happen if the adhesion force is greater than the cohesion force.
The cohesion of tin-lead solder is even greater than that of water, making the solder spherical to minimize its surface area (a sphere has the smallest surface area compared to other geometric shapes of the same volume to meet the requirements of the lowest energy state). The effect of flux is similar to that of a detergent on a greased metal plate. In addition, the surface tension is highly dependent on the cleanliness and temperature of the surface. Ideal tinning can only occur when the adhesion energy is much greater than the surface energy (cohesion).
3 Production of metal alloy co-compounds
The intermetallic bonds between copper and tin form grains, and the shape and size of the grains depend on the duration and intensity of the temperature during welding. Less heat during welding can form a fine crystalline structure, forming a good solder joint with optimal strength. Long reaction time, whether due to long welding time or high temperature or both, will result in a coarse crystalline structure that is gritty and brittle with 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, but tin can penetrate into copper. The intermolecular bonds between tin and copper form metal alloy co-compounds Cu3Sn and Cu6Sn5 at the connection surface between solder and metal, as shown in the figure.
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 metal bond of excellent solder joints is mostly more than 0.5μm. Since the shear strength of the solder 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 below 1μm, which can be achieved by making the soldering 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, 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 alloy bonding material Cu3Sn and Cu6Sn5 with a thickness of about 0.5μm. Insufficient intermetallic bonds are common in cold solder joints or solder joints that are not raised to the appropriate temperature during soldering, which may lead to the cutting of the solder surface. On the contrary, too thick a metal alloy layer is common in solder joints that are overheated or soldered for too long, which will result in very weak tensile strength of the solder joint, as shown in the figure.
4 Wetting Angle
When a drop of solder is placed on a hot flux-coated surface at a temperature about 35℃ higher than the eutectic point of solder, a meniscus is formed. To some extent, the ability of the metal surface to be tinned can be evaluated by the shape of the meniscus. If the solder meniscus has a clear bottom cut edge, is shaped like a drop of water on a greased metal plate, or even tends to be spherical, the metal is not solderable. Good weldability is achieved only when the meniscus is stretched to a small angle of less than 30°.
