1. **Tin Dipping Effect of PCB**
The phenomenon where hot liquid solder dissolves and infiltrates the surface of the metal being welded is known as tin-stained metal. The molecules resulting from the mixture of solder and copper create a new alloy, incorporating elements of both. This action is referred to as tin dipping. It establishes intermolecular bonds between the materials, forming a metal alloy compound. The strength and quality of the weld joint depend significantly on the formation of robust intermolecular bonds, which is central to the welding process. For effective tin staining, the copper surface must be free from contaminants and the oxide layer created by exposure to air, while also achieving the appropriate temperature for soldering.
2. **PCB Surface Tension**
Most people are aware of water’s surface tension, which keeps cold water droplets on a greased metal plate in a spherical shape. In this scenario, the adhesion that causes the liquid to spread on the solid surface is weaker than its cohesion. Cleaning with warm water and detergent can reduce surface tension, allowing water to penetrate the greased metal plate and create a thin layer, occurring when adhesion surpasses cohesion. The cohesion of tin-lead solder exceeds that of water, causing the solder to take a spherical shape to minimize surface area, as a sphere presents the least surface area for a given volume, achieving a lower energy state. The role of flux is akin to that of a cleaner on the greased metal surface. Furthermore, surface tension is significantly influenced by the cleanliness and temperature of the surface, with ideal tin staining occurring only when adhesion energy greatly exceeds surface energy (cohesion).
3. Generation of Metal Alloy Co-Compounds in PCBs
The intermetallic bond formed between copper and tin creates grains. The shape and size of these grains are influenced by the duration and intensity of heat applied during welding. When welding, applying less heat can produce a fine crystal structure, resulting in high-quality welding points with optimal strength. Conversely, an extended reaction time—whether due to prolonged welding, excessive temperature, or both—can lead to a coarse crystal structure that is sandy, brittle, and possesses low shear strength.
Copper serves as the metal substrate, while a tin-lead alloy is utilized as the solder. Lead and copper do not form any metal alloy co-compounds. However, tin can diffuse into copper, forming intermetallic compounds, specifically Cu3Sn and Cu6Sn5, at the interface between the solder and the metal, as illustrated in the accompanying figure.
The metal alloy layer (N phase + ε phase) must be kept very thin. In laser welding, the thickness of the metal alloy layer is typically 0.1 mm. In wave soldering and manual soldering, the thickness of the intermetallic bond at optimal welding points generally exceeds 0.5 μm. Since the shear strength of the welded joint diminishes as the thickness of the metal alloy layer increases, efforts are often made to maintain this thickness at 1 μm or less, which can be achieved by minimizing the welding time.
The thickness of the metal alloy eutectic layer is contingent on the temperature and time involved in forming the welding point. Ideally, welding should be completed within approximately 2 seconds at 220°C. Under these conditions, the chemical diffusion reaction between copper and tin generates a suitable amount of metal alloy bonding materials, Cu3Sn and Cu6Sn5, with a thickness around 0.5 μm. Insufficient intermetallic bonds are frequently encountered in cold welds or those that do not reach the required temperature during welding, potentially leading to delamination of the welding surface. Conversely, a thick metal alloy layer is often observed in welded joints subjected to excessive heat or prolonged welding, resulting in significantly weakened tensile strength, as shown in the figure.
4. Tin Dipping Angle of PCB
When the eutectic point temperature of the solder is approximately 35 degrees Celsius higher than that of the solder itself, a drop of solder placed on a heated surface coated with flux forms a meniscus. The shape of this meniscus can partially indicate the wettability of tin on the metal surface. If the solder meniscus exhibits a pronounced undercut—similar to water droplets on a greased metal plate—or appears nearly spherical, the metal is not suitable for welding. Optimal weldability is only achieved when the meniscus stretches to a value less than 30 degrees; thus, a small angle is essential.
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The phenomenon where hot liquid solder dissolves and infiltrates the surface of the metal being welded is known as tin-stained metal. The molecules resulting from the mixture of solder and copper create a new alloy, incorporating elements of both. This action is referred to as tin dipping. It establishes intermolecular bonds between the materials, forming a metal alloy compound. The strength and quality of the weld joint depend significantly on the formation of robust intermolecular bonds, which is central to the welding process. For effective tin staining, the copper surface must be free from contaminants and the oxide layer created by exposure to air, while also achieving the appropriate temperature for soldering.
2. **PCB Surface Tension**
Most people are aware of water’s surface tension, which keeps cold water droplets on a greased metal plate in a spherical shape. In this scenario, the adhesion that causes the liquid to spread on the solid surface is weaker than its cohesion. Cleaning with warm water and detergent can reduce surface tension, allowing water to penetrate the greased metal plate and create a thin layer, occurring when adhesion surpasses cohesion. The cohesion of tin-lead solder exceeds that of water, causing the solder to take a spherical shape to minimize surface area, as a sphere presents the least surface area for a given volume, achieving a lower energy state. The role of flux is akin to that of a cleaner on the greased metal surface. Furthermore, surface tension is significantly influenced by the cleanliness and temperature of the surface, with ideal tin staining occurring only when adhesion energy greatly exceeds surface energy (cohesion).
3. Generation of Metal Alloy Co-Compounds in PCBs
The intermetallic bond formed between copper and tin creates grains. The shape and size of these grains are influenced by the duration and intensity of heat applied during welding. When welding, applying less heat can produce a fine crystal structure, resulting in high-quality welding points with optimal strength. Conversely, an extended reaction time—whether due to prolonged welding, excessive temperature, or both—can lead to a coarse crystal structure that is sandy, brittle, and possesses low shear strength.
Copper serves as the metal substrate, while a tin-lead alloy is utilized as the solder. Lead and copper do not form any metal alloy co-compounds. However, tin can diffuse into copper, forming intermetallic compounds, specifically Cu3Sn and Cu6Sn5, at the interface between the solder and the metal, as illustrated in the accompanying figure.
The metal alloy layer (N phase + ε phase) must be kept very thin. In laser welding, the thickness of the metal alloy layer is typically 0.1 mm. In wave soldering and manual soldering, the thickness of the intermetallic bond at optimal welding points generally exceeds 0.5 μm. Since the shear strength of the welded joint diminishes as the thickness of the metal alloy layer increases, efforts are often made to maintain this thickness at 1 μm or less, which can be achieved by minimizing the welding time.
The thickness of the metal alloy eutectic layer is contingent on the temperature and time involved in forming the welding point. Ideally, welding should be completed within approximately 2 seconds at 220°C. Under these conditions, the chemical diffusion reaction between copper and tin generates a suitable amount of metal alloy bonding materials, Cu3Sn and Cu6Sn5, with a thickness around 0.5 μm. Insufficient intermetallic bonds are frequently encountered in cold welds or those that do not reach the required temperature during welding, potentially leading to delamination of the welding surface. Conversely, a thick metal alloy layer is often observed in welded joints subjected to excessive heat or prolonged welding, resulting in significantly weakened tensile strength, as shown in the figure.
4. Tin Dipping Angle of PCB
When the eutectic point temperature of the solder is approximately 35 degrees Celsius higher than that of the solder itself, a drop of solder placed on a heated surface coated with flux forms a meniscus. The shape of this meniscus can partially indicate the wettability of tin on the metal surface. If the solder meniscus exhibits a pronounced undercut—similar to water droplets on a greased metal plate—or appears nearly spherical, the metal is not suitable for welding. Optimal weldability is only achieved when the meniscus stretches to a value less than 30 degrees; thus, a small angle is essential.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
