PCB soldering is a crucial process in electronic product assembly. The quality of soldering directly impacts circuit and device performance.
Excellent soldering ensures circuit stability and reliability. Conversely, poor soldering methods can damage components, complicate testing, and pose hidden risks to electronic equipment reliability.
1. Classification and characteristics of PCB soldering
Soldering is classified into three categories: fusion welding, contact welding, and brazing.
– Fusion welding involves heating the joint to a molten state without applying pressure, e.g., arc welding, gas welding.
– Contact welding requires applying pressure (heated or not) during welding, e.g., ultrasonic welding, pulse welding, friction welding.
– Soldering, specifically in electronic assembly, uses low melting point alloys like tin and lead, commonly known as “solder.”
2. Principles of PCB soldering
While solder
The so-called soldering is a process where solder and the metal to be joined are heated to the optimal temperature simultaneously, allowing molten solder to fill the gap between the metals, forming a metallurgical alloy combination. Microscopically, soldering comprises two essential processes: wetting and diffusion.
1. Wetting (lateral flow)
Wetting, also known as lateral flow, refers to the formation of a uniform, smooth, continuous, and firmly adhered solder layer on the metal surface. The degree of wetting primarily depends on the cleanliness of the surface and the surface tension of the solder. Despite the apparent smoothness, metal surfaces exhibit numerous irregularities, grain boundaries, and scars visible under a microscope. Through capillary action, molten solder wets and diffuses along these surface imperfections, ensuring proper solder flow.
2. Diffusion (vertical flow)
In addition to wetting, molten solder also diffuses into the solid metal surface during soldering. For instance, when soldering copper components with tin-lead solder, various types of diffusion occur, including surface, grain boundary, and intragranular diffusion. In tin-lead solder, lead participates primarily in surface diffusion, while tin and copper atoms mutually diffuse due to their differing metal properties. This selective diffusion results in the formation of new alloys at the interface, enhancing the bond between solder and the metal workpiece.
3. Metallurgical Bonding
As a consequence of diffusion, an alloy layer forms where tin atoms interact with copper metal, creating a robust solder joint. For example, soldering copper parts with tin-lead solder at temperatures between 250°C to 300°C results in the formation of Cu3Sn and Cu6Sn5 alloys at the interface. Higher temperatures can lead to additional intermetallic compounds like Cu31Sn8. The thickness of the solder joint interface typically ranges from 3 to 10 micrometers, influenced by temperature and soldering duration.
III. PCB Soldering Elements
1. Base Metal Weldability
Solderability refers to the ability of liquid solder and base metal to dissolve into each other, depending on atomic radius, periodic table position, and crystal structure compatibility. Tin-lead solder exhibits good solubility with most metals, except those rich in chromium or aluminum alloys, which require surface treatments like tin or silver plating to enhance solderability.
2. Cleanliness of Soldering Parts
Both solder and base metal surfaces must be free from oxide layers and contaminants for effective soldering. Oxides hinder metal atom diffusion, preventing proper wetting and joint formation. Oxidized component pins or PCB pads often lead to poor solder joints, known as “cold soldering.”
3. Flux
Flux removes oxide films, purifies soldering surfaces, and ensures smooth, bright solder joints. Rosin-based flux is commonly used in electronic assembly.
4. Soldering Temperature and Time
Ideal soldering temperatures range from 240°C to 250°C, with a maximum limit of 260°C to avoid joint quality degradation. Soldering times typically range from 2 to 3 seconds, crucial for achieving both wetting and diffusion processes effectively.
5. Soldering Method
Choosing the correct soldering method and adhering to proper procedures are critical for achieving reliable solder joints.
IV. Quality Standards for PCB Solder Joints
1. Electrical Performance
High-quality solder joints form a solid alloy layer between solder and metal, ensuring excellent electrical conductivity. Superficial soldering that merely coats metal surfaces without alloy formation is unacceptable.
2. Mechanical Strength
Solder joints must withstand mechanical stresses, including vibrations encountered in electronic equipment operation. Tin-lead solder, known for relatively low strength, may require additional measures like increasing solder surface area or mechanically reinforcing component leads.
3. Solder Quantity
Appropriate solder volume ensures mechanical strength without excessive cost or risk of solder bridging (short circuits). Ideal solder joints for PCBs fill pads evenly, forming a skirt-shaped appearance.
4. Surface Appearance
Quality solder joints exhibit a bright, uniform surface finish due to the residual resin component of the flux, which prevents oxidation.
5. Defects
Avoid solder joint defects such as burrs and voids, which compromise electronic performance and safety, especially in high-voltage circuits prone to electrical discharge.
6. Cleanliness
Maintaining clean solder joint surfaces is crucial to prevent corrosion, leakage, or short circuits caused by contaminants or moisture absorption.
By focusing on these key elements and quality standards, PCB soldering can consistently produce reliable and durable electronic assemblies.
Excellent soldering ensures circuit stability and reliability. Conversely, poor soldering methods can damage components, complicate testing, and pose hidden risks to electronic equipment reliability.
1. Classification and characteristics of PCB soldering
Soldering is classified into three categories: fusion welding, contact welding, and brazing.
– Fusion welding involves heating the joint to a molten state without applying pressure, e.g., arc welding, gas welding.
– Contact welding requires applying pressure (heated or not) during welding, e.g., ultrasonic welding, pulse welding, friction welding.
– Soldering, specifically in electronic assembly, uses low melting point alloys like tin and lead, commonly known as “solder.”
2. Principles of PCB soldering
While solder
The so-called soldering is a process where solder and the metal to be joined are heated to the optimal temperature simultaneously, allowing molten solder to fill the gap between the metals, forming a metallurgical alloy combination. Microscopically, soldering comprises two essential processes: wetting and diffusion.
1. Wetting (lateral flow)
Wetting, also known as lateral flow, refers to the formation of a uniform, smooth, continuous, and firmly adhered solder layer on the metal surface. The degree of wetting primarily depends on the cleanliness of the surface and the surface tension of the solder. Despite the apparent smoothness, metal surfaces exhibit numerous irregularities, grain boundaries, and scars visible under a microscope. Through capillary action, molten solder wets and diffuses along these surface imperfections, ensuring proper solder flow.
2. Diffusion (vertical flow)
In addition to wetting, molten solder also diffuses into the solid metal surface during soldering. For instance, when soldering copper components with tin-lead solder, various types of diffusion occur, including surface, grain boundary, and intragranular diffusion. In tin-lead solder, lead participates primarily in surface diffusion, while tin and copper atoms mutually diffuse due to their differing metal properties. This selective diffusion results in the formation of new alloys at the interface, enhancing the bond between solder and the metal workpiece.
3. Metallurgical Bonding
As a consequence of diffusion, an alloy layer forms where tin atoms interact with copper metal, creating a robust solder joint. For example, soldering copper parts with tin-lead solder at temperatures between 250°C to 300°C results in the formation of Cu3Sn and Cu6Sn5 alloys at the interface. Higher temperatures can lead to additional intermetallic compounds like Cu31Sn8. The thickness of the solder joint interface typically ranges from 3 to 10 micrometers, influenced by temperature and soldering duration.
III. PCB Soldering Elements
1. Base Metal Weldability
Solderability refers to the ability of liquid solder and base metal to dissolve into each other, depending on atomic radius, periodic table position, and crystal structure compatibility. Tin-lead solder exhibits good solubility with most metals, except those rich in chromium or aluminum alloys, which require surface treatments like tin or silver plating to enhance solderability.
2. Cleanliness of Soldering Parts
Both solder and base metal surfaces must be free from oxide layers and contaminants for effective soldering. Oxides hinder metal atom diffusion, preventing proper wetting and joint formation. Oxidized component pins or PCB pads often lead to poor solder joints, known as “cold soldering.”
3. Flux
Flux removes oxide films, purifies soldering surfaces, and ensures smooth, bright solder joints. Rosin-based flux is commonly used in electronic assembly.
4. Soldering Temperature and Time
Ideal soldering temperatures range from 240°C to 250°C, with a maximum limit of 260°C to avoid joint quality degradation. Soldering times typically range from 2 to 3 seconds, crucial for achieving both wetting and diffusion processes effectively.
5. Soldering Method
Choosing the correct soldering method and adhering to proper procedures are critical for achieving reliable solder joints.
IV. Quality Standards for PCB Solder Joints
1. Electrical Performance
High-quality solder joints form a solid alloy layer between solder and metal, ensuring excellent electrical conductivity. Superficial soldering that merely coats metal surfaces without alloy formation is unacceptable.
2. Mechanical Strength
Solder joints must withstand mechanical stresses, including vibrations encountered in electronic equipment operation. Tin-lead solder, known for relatively low strength, may require additional measures like increasing solder surface area or mechanically reinforcing component leads.
3. Solder Quantity
Appropriate solder volume ensures mechanical strength without excessive cost or risk of solder bridging (short circuits). Ideal solder joints for PCBs fill pads evenly, forming a skirt-shaped appearance.
4. Surface Appearance
Quality solder joints exhibit a bright, uniform surface finish due to the residual resin component of the flux, which prevents oxidation.
5. Defects
Avoid solder joint defects such as burrs and voids, which compromise electronic performance and safety, especially in high-voltage circuits prone to electrical discharge.
6. Cleanliness
Maintaining clean solder joint surfaces is crucial to prevent corrosion, leakage, or short circuits caused by contaminants or moisture absorption.
By focusing on these key elements and quality standards, PCB soldering can consistently produce reliable and durable electronic assemblies.
