1. **Tin-Dipping Effect**
When hot liquid solder dissolves and penetrates the surface of the metal to be welded, it is referred to as metal dip tinning. The molecules of the solder and the metal form a new alloy, consisting partly of copper and partly of solder. This solvent action, known as tinning, creates an intermolecular bond that results in a metal alloy eutectic. The formation of strong intermolecular bonds is crucial to the welding process, influencing the strength and quality of PCB solder joints. For effective tinning, the copper surface of the printed circuit board must be clean and free of oxide films caused by exposure to air, and the solder and surface need to reach the appropriate temperature.
2. **Surface Tension**
Surface tension, a familiar concept with water, causes cold water droplets on a grease-coated metal printed circuit board to remain spherical. This occurs because the adhesive forces between the liquid and the solid surface are weaker than the cohesive forces within the liquid. Washing with warm water and detergent can reduce surface tension, allowing water to penetrate the grease-coated metal and spread out in a thin layer, provided that the adhesive forces are stronger than the cohesive forces.
Tin-lead solder exhibits even higher cohesion than water, causing solder spheres to minimize their surface area (as a sphere has the smallest surface area for a given volume, achieving the lowest energy state). The effect of flux on soldering is similar to that of a cleaner on a grease-coated metal plate. Additionally, surface tension is highly dependent on the cleanliness and temperature of the surface. Ideal adhesion occurs only when the adhesive energy significantly exceeds the cohesive energy (surface tension) of the tin.
3. The production of metal alloys
The intermetallic bond between copper and tin forms crystal grains. The shape and size of these grains depend on the duration and strength of the temperature during soldering. Less heat during welding can produce a fine crystalline structure, resulting in a strong and reliable welding point. Conversely, a prolonged reaction time—whether due to extended welding duration or excessively high temperature—or both, will lead to a rough crystalline structure that is granular and brittle, with reduced shear strength.
The soldering method for printed circuit boards uses copper as the metal base material and tin-lead as the solder alloy. Lead and copper do not form any metal alloys. However, tin can diffuse into the copper, creating intermolecular bonds within the solder. This interface with the metal forms metal alloy eutectic compounds Cu3Sn and Cu6Sn5.
The metal alloy layer (n phase + epsilon phase) must be very thin. In laser welding, this layer typically measures around 0.1 mm in thickness. In wave soldering and manual soldering, the thickness of the intermetallic bond in good solder joints is usually more than 0.5 μm. As the shear strength of the solder joint decreases with an increase in the thickness of the metal alloy layer, it is often preferable to maintain this layer below 1 μm. This can be achieved by minimizing the welding time.
4. Zhan Tin Kok
When the eutectic point temperature of solder is about 35°C higher, a drop of solder placed on a hot flux-coated surface forms a meniscus. The ability of the metal surface to accept tin can be evaluated by the shape of this meniscus. If the solder meniscus exhibits a pronounced undercut edge, resembling a drop of water on a greased metal plate, or even becomes spherical, the metal is not weldable. Good weldability is indicated when the meniscus stretches to a size less than 30 and maintains a small angle.
The above outlines the best soldering practices for PCB printed boards. I hope this information is helpful.
When hot liquid solder dissolves and penetrates the surface of the metal to be welded, it is referred to as metal dip tinning. The molecules of the solder and the metal form a new alloy, consisting partly of copper and partly of solder. This solvent action, known as tinning, creates an intermolecular bond that results in a metal alloy eutectic. The formation of strong intermolecular bonds is crucial to the welding process, influencing the strength and quality of PCB solder joints. For effective tinning, the copper surface of the printed circuit board must be clean and free of oxide films caused by exposure to air, and the solder and surface need to reach the appropriate temperature.
2. **Surface Tension**
Surface tension, a familiar concept with water, causes cold water droplets on a grease-coated metal printed circuit board to remain spherical. This occurs because the adhesive forces between the liquid and the solid surface are weaker than the cohesive forces within the liquid. Washing with warm water and detergent can reduce surface tension, allowing water to penetrate the grease-coated metal and spread out in a thin layer, provided that the adhesive forces are stronger than the cohesive forces.
Tin-lead solder exhibits even higher cohesion than water, causing solder spheres to minimize their surface area (as a sphere has the smallest surface area for a given volume, achieving the lowest energy state). The effect of flux on soldering is similar to that of a cleaner on a grease-coated metal plate. Additionally, surface tension is highly dependent on the cleanliness and temperature of the surface. Ideal adhesion occurs only when the adhesive energy significantly exceeds the cohesive energy (surface tension) of the tin.
3. The production of metal alloys
The intermetallic bond between copper and tin forms crystal grains. The shape and size of these grains depend on the duration and strength of the temperature during soldering. Less heat during welding can produce a fine crystalline structure, resulting in a strong and reliable welding point. Conversely, a prolonged reaction time—whether due to extended welding duration or excessively high temperature—or both, will lead to a rough crystalline structure that is granular and brittle, with reduced shear strength.
The soldering method for printed circuit boards uses copper as the metal base material and tin-lead as the solder alloy. Lead and copper do not form any metal alloys. However, tin can diffuse into the copper, creating intermolecular bonds within the solder. This interface with the metal forms metal alloy eutectic compounds Cu3Sn and Cu6Sn5.
The metal alloy layer (n phase + epsilon phase) must be very thin. In laser welding, this layer typically measures around 0.1 mm in thickness. In wave soldering and manual soldering, the thickness of the intermetallic bond in good solder joints is usually more than 0.5 μm. As the shear strength of the solder joint decreases with an increase in the thickness of the metal alloy layer, it is often preferable to maintain this layer below 1 μm. This can be achieved by minimizing the welding time.
4. Zhan Tin Kok
When the eutectic point temperature of solder is about 35°C higher, a drop of solder placed on a hot flux-coated surface forms a meniscus. The ability of the metal surface to accept tin can be evaluated by the shape of this meniscus. If the solder meniscus exhibits a pronounced undercut edge, resembling a drop of water on a greased metal plate, or even becomes spherical, the metal is not weldable. Good weldability is indicated when the meniscus stretches to a size less than 30 and maintains a small angle.
The above outlines the best soldering practices for PCB printed boards. I hope this information is helpful.
