1. Immersion Tin Effect
When hot liquid solder dissolves and penetrates the metal surface of the PCB, it forms a tin-copper alloy. This alloy is a combination of solder and copper molecules, creating a new metallic compound. Effective bonding between various parts of the PCB is crucial for a strong and high-quality solder joint. To achieve this, the copper surface must be clean, free of oxide films, and exposed to appropriate temperatures for proper soldering.
2. Surface Tension
Surface tension, as seen in water, keeps droplets spherical on a grease-coated PCB because the liquid’s cohesive forces are stronger than its adhesive forces on the solid surface. Washing with warm water and detergent reduces surface tension, allowing water to spread and form a thin film. Tin-lead solder has higher cohesion than water, making it form spheres to minimize its surface area, thus achieving a low-energy state.
1. The effect of flux is comparable to that of detergent on greased PCB metal plates. Additionally, the surface tension is significantly influenced by the cleanliness and temperature of the PCB surface. Only when the adhesion energy substantially exceeds the surface energy (cohesion), can PCB be considered an ideal material for immersion tin.
2. Zhan Tin Corner
When the eutectic temperature of the solder is approximately 35°C higher than the eutectic temperature of the solder itself, a droplet of solder placed on the surface of the heated PCB forms a meniscus. To some extent, the adhesion of the PCB metal surface to tin can be assessed by the characteristics of this meniscus. If the solder meniscus exhibits a pronounced undercut and resembles a droplet on a grease-coated PCB metal plate, or appears nearly spherical, it indicates that the metal cannot be satisfactorily soldered. Optimal weldability is observed when the length of the meniscus is less than 30 and the angle is small.
3. The production of metal alloys
The intermetallic bonds formed between copper and tin create crystal grains, with the shape and size of these grains influenced by the duration and intensity of the soldering temperature. Limited heat during the soldering process fosters a fine crystal structure, enabling the PCB to achieve strong solder joints. Conversely, prolonged reaction times, whether due to excessive PCB soldering duration, elevated temperatures, or both, can lead to a coarse crystal structure characterized by brittleness and low shear strength. Copper acts as the metal substrate for the PCB, while a tin-lead alloy is utilized for soldering. Although lead and copper do not form metallic alloy compounds, tin can diffuse into copper. The intermolecular bond of tin and copper generates metal alloy compounds, specifically Cu3Sn and Cu6Sn5, at the interface between solder and metal.
4. The metal alloy layer (N phase + ε phase) must remain remarkably thin. In PCB laser welding, the thickness of this metal alloy layer measures approximately 0.1mm. In wave soldering and manual soldering processes, the thickness of the intermetallic bonding layer in PCB circuit board solder joints exceeds 0.5μm. Since the shear strength of PCB solder joints diminishes as the metal alloy layer thickness increases, efforts are typically made to minimize soldering time, keeping the thickness of the metal alloy layer below 1 μm.
5. The thickness of the co-composite layer of the metal alloy is contingent upon the temperature and duration during which the solder joints are formed. Ideally, welding should be completed in approximately 2 seconds at 220°C. Under these conditions, the chemical diffusion reaction between copper and tin generates an adequate quantity of the metal alloy connecting materials Cu3Sn and Cu6Sn5, with a thickness of about 0.5μm. Insufficient metal-to-metal connectivity generally arises in cold solder joints or when the temperature fails to reach the optimal level during the soldering process, potentially severing the PCB soldering surface. Conversely, in instances of overheating or protracted soldering time, an excessively thick metal alloy layer may render the tensile strength of the PCB solder joint remarkably weak.
6. Reasons for using high TG materials
In addition to the standard FR-4 materials commonly employed in PCB production, some customers specify the use of high TG materials. This leads to the question: why choose high TG materials in PCB manufacturing?
The term TG in PCB production refers to glass transition temperature, which denotes the temperature at which the material transitions from a glassy state to a softened state. The circuit board must possess flame-resistant properties, preventing combustion at specific temperatures while instead softening. The temperature at which this occurs is identified as the glass transition temperature (Tg), a value that correlates with the dimensional stability of the PCB. A higher Tg value indicates superior temperature resistance for the PCB. As the temperature rises beyond a certain threshold in high Tg PCBs, the substrate transitions from “glassy” to “rubbery.” This threshold is referred to as the glass transition temperature (Tg). Essentially, Tg is the temperature at which the substrate can endure high temperatures (°C). In contrast, common PCB substrate materials may experience phenomena such as expansion, deformation, or melting, along with a significant decline in mechanical and electrical properties at elevated temperatures.
7. Enhanced Tg of the substrate strengthens and improves heat resistance, moisture resistance, chemical resistance, and stability for Shenzhen circuit board prototypes. The higher the Tg value, the better the board’s temperature resistance and overall performance, especially in lead-free manufacturing processes, where high Tg materials find increased application.
8. High Tg signifies enhanced heat resistance. With the rapid advancement of the electronics industry, notably in products like computers that demand high functionality and multilayer designs, superior heat resistance in PCB substrate materials has become essential. The rise of high-density mounting technologies, such as SMT and CMT, further necessitates robust heat resistance in substrates to accommodate small apertures, fine wiring, and reduced thickness. This demand significantly contributes to the adoption of high TG materials in PCB production.
9. Consequently, in PCB fabrication, discrepancies between standard FR-4 and high Tg FR-4 materials lie in mechanical strength, dimensional stability, adhesion, and water absorption, particularly under hot conditions and post moisture absorption. Performance differences manifest in thermal decomposition, thermal expansion, and other attributes, with high Tg products demonstrating clear advantages over conventional PCB substrate materials.
When hot liquid solder dissolves and penetrates the metal surface of the PCB, it forms a tin-copper alloy. This alloy is a combination of solder and copper molecules, creating a new metallic compound. Effective bonding between various parts of the PCB is crucial for a strong and high-quality solder joint. To achieve this, the copper surface must be clean, free of oxide films, and exposed to appropriate temperatures for proper soldering.
2. Surface Tension
Surface tension, as seen in water, keeps droplets spherical on a grease-coated PCB because the liquid’s cohesive forces are stronger than its adhesive forces on the solid surface. Washing with warm water and detergent reduces surface tension, allowing water to spread and form a thin film. Tin-lead solder has higher cohesion than water, making it form spheres to minimize its surface area, thus achieving a low-energy state.
1. The effect of flux is comparable to that of detergent on greased PCB metal plates. Additionally, the surface tension is significantly influenced by the cleanliness and temperature of the PCB surface. Only when the adhesion energy substantially exceeds the surface energy (cohesion), can PCB be considered an ideal material for immersion tin.
2. Zhan Tin Corner
When the eutectic temperature of the solder is approximately 35°C higher than the eutectic temperature of the solder itself, a droplet of solder placed on the surface of the heated PCB forms a meniscus. To some extent, the adhesion of the PCB metal surface to tin can be assessed by the characteristics of this meniscus. If the solder meniscus exhibits a pronounced undercut and resembles a droplet on a grease-coated PCB metal plate, or appears nearly spherical, it indicates that the metal cannot be satisfactorily soldered. Optimal weldability is observed when the length of the meniscus is less than 30 and the angle is small.
3. The production of metal alloys
The intermetallic bonds formed between copper and tin create crystal grains, with the shape and size of these grains influenced by the duration and intensity of the soldering temperature. Limited heat during the soldering process fosters a fine crystal structure, enabling the PCB to achieve strong solder joints. Conversely, prolonged reaction times, whether due to excessive PCB soldering duration, elevated temperatures, or both, can lead to a coarse crystal structure characterized by brittleness and low shear strength. Copper acts as the metal substrate for the PCB, while a tin-lead alloy is utilized for soldering. Although lead and copper do not form metallic alloy compounds, tin can diffuse into copper. The intermolecular bond of tin and copper generates metal alloy compounds, specifically Cu3Sn and Cu6Sn5, at the interface between solder and metal.
4. The metal alloy layer (N phase + ε phase) must remain remarkably thin. In PCB laser welding, the thickness of this metal alloy layer measures approximately 0.1mm. In wave soldering and manual soldering processes, the thickness of the intermetallic bonding layer in PCB circuit board solder joints exceeds 0.5μm. Since the shear strength of PCB solder joints diminishes as the metal alloy layer thickness increases, efforts are typically made to minimize soldering time, keeping the thickness of the metal alloy layer below 1 μm.
5. The thickness of the co-composite layer of the metal alloy is contingent upon the temperature and duration during which the solder joints are formed. Ideally, welding should be completed in approximately 2 seconds at 220°C. Under these conditions, the chemical diffusion reaction between copper and tin generates an adequate quantity of the metal alloy connecting materials Cu3Sn and Cu6Sn5, with a thickness of about 0.5μm. Insufficient metal-to-metal connectivity generally arises in cold solder joints or when the temperature fails to reach the optimal level during the soldering process, potentially severing the PCB soldering surface. Conversely, in instances of overheating or protracted soldering time, an excessively thick metal alloy layer may render the tensile strength of the PCB solder joint remarkably weak.
6. Reasons for using high TG materials
In addition to the standard FR-4 materials commonly employed in PCB production, some customers specify the use of high TG materials. This leads to the question: why choose high TG materials in PCB manufacturing?
The term TG in PCB production refers to glass transition temperature, which denotes the temperature at which the material transitions from a glassy state to a softened state. The circuit board must possess flame-resistant properties, preventing combustion at specific temperatures while instead softening. The temperature at which this occurs is identified as the glass transition temperature (Tg), a value that correlates with the dimensional stability of the PCB. A higher Tg value indicates superior temperature resistance for the PCB. As the temperature rises beyond a certain threshold in high Tg PCBs, the substrate transitions from “glassy” to “rubbery.” This threshold is referred to as the glass transition temperature (Tg). Essentially, Tg is the temperature at which the substrate can endure high temperatures (°C). In contrast, common PCB substrate materials may experience phenomena such as expansion, deformation, or melting, along with a significant decline in mechanical and electrical properties at elevated temperatures.
7. Enhanced Tg of the substrate strengthens and improves heat resistance, moisture resistance, chemical resistance, and stability for Shenzhen circuit board prototypes. The higher the Tg value, the better the board’s temperature resistance and overall performance, especially in lead-free manufacturing processes, where high Tg materials find increased application.
8. High Tg signifies enhanced heat resistance. With the rapid advancement of the electronics industry, notably in products like computers that demand high functionality and multilayer designs, superior heat resistance in PCB substrate materials has become essential. The rise of high-density mounting technologies, such as SMT and CMT, further necessitates robust heat resistance in substrates to accommodate small apertures, fine wiring, and reduced thickness. This demand significantly contributes to the adoption of high TG materials in PCB production.
9. Consequently, in PCB fabrication, discrepancies between standard FR-4 and high Tg FR-4 materials lie in mechanical strength, dimensional stability, adhesion, and water absorption, particularly under hot conditions and post moisture absorption. Performance differences manifest in thermal decomposition, thermal expansion, and other attributes, with high Tg products demonstrating clear advantages over conventional PCB substrate materials.
