How to Select the Diameter of Solder Wire in Laser Soldering?

How to Select the Diameter of Solder Wire in Laser Soldering?

The diameter of solder wire significantly impacts the effectiveness of laser soldering, making proper diameter selection crucial. Songsheng Optoelectronics will explain the key factors to consider when choosing solder wire diameter—let’s take a closer look.

How to Select the Right Solder Wire Diameter for Laser Soldering

Choosing the correct solder wire diameter is essential for ensuring welding quality and efficiency. By considering factors such as the size of the components, process requirements, operator skill and preference, as well as equipment limitations, an optimal choice can be made. The right solder wire diameter not only improves welding precision and efficiency but also helps avoid common soldering issues.

Laser solder wire welding is widely used in precision electronics manufacturing, including PCB boards, sensors, connectors, FPC flexible circuits, and more.

With its high precision, non-contact heating, and high degree of automation, it is well-suited for the high-density circuit board industry, enabling automated production while improving efficiency and consistency.

In laser solder wire welding, matching the pad size with the solder wire diameter is crucial to ensuring welding quality.

I. Principles and Core Technologies of Laser Solder Wire Welding

Laser solder wire welding is a non-contact welding technology that utilizes high-energy laser as a heat source. Its fundamental principle involves precisely focusing the laser beam on the solder joint to rapidly heat and melt the solder wire, allowing it to wet the pad and ultimately form a metallurgical bond. Unlike the "thermal convection" heating of traditional soldering iron processes, laser welding employs a "surface heating" mode with extremely high energy density (reaching over 10⁶ W/cm²) and ultrafast heating rates. Moreover, the heat-affected zone is limited to a micron-scale area around the solder joint, preventing thermal damage to the component itself.

A typical laser welding system consists of a semiconductor laser, optical focusing assembly, wire-feeding mechanism, and control system. 

The welding process comprises three stages:

1. Solder Joint Preheating: The laser preheats the pad to remove surface oxidation.

2. Solder Wire Melting: The laser simultaneously heats the solder wire, melting it and allowing it to wet the pad.

3. Cooling and Solidification: After the laser stops, the molten solder rapidly cools and solidifies, forming a high-strength solder joint.

 II. Relationship Between Pad Size and Solder Wire Diameter

Empirical Formula:

It is generally recommended that the solder wire diameter should not exceed 1/2 to 2/3 of the pad width. For example, when the pad width is 1.5mm, a 0.8~1.0mm solder wire is recommended.

Solder Volume Calculation:
The required solder volume should be proportional to the pad area and welding height. Too thin a wire may result in insufficient solder melting, while too thick a wire can cause overflow or solder ball formation.

III. Key Considerations

Thermal Capacity Matching:

The thermal capacity of the pad must be balanced with the laser energy and solder wire melting rate. Large pads require thicker wires to provide sufficient molten solder, while small pads need to avoid spattering caused by excessive heat concentration.

Welding Strength:
Adequate molten solder ensures mechanical strength, but excessive solder should be avoided to prevent short circuits (e.g., bridging between adjacent pins).

Process Window:

• Thick solder wires (e.g., 1.6mm) require higher laser power and longer heating      times, which may damage heat-sensitive components.

• Thin solder wires (e.g., 0.6mm) demand higher precision in wire feeding stability and laser energy control.

IV. Application Scenarios for Different Solder Wire Diameters

Solder wire diameters range from ultra-fine 0.3mm to thicker 3mm, with each size catering to specific welding requirements.

Fine-diameter solder wire (0.3mm - 0.7mm):
Ideal for precision electronics manufacturing, such as smartphones, tablets, and other compact electronic components. The fine diameter enables superior control and minimizes the heat-affected zone, reducing thermal impact on sensitive parts.

Medium-diameter solder wire (0.8mm - 1.5mm):
Best suited for general electronic assembly and repair work, including PCBs and plug-in components. This diameter provides adequate filler material while maintaining good maneuverability and control.

Thick-diameter solder wire (1.6mm - 3mm):
Designed for larger electronic components or power systems, such as power sockets, heavy-duty connectors, and household appliances. The abundant material supply makes it perfect for applications requiring substantial filler metal.

Conclusion

Selecting the appropriate solder wire diameter is critical to ensuring welding quality and efficiency. By considering factors such as component size, process requirements, operator skill and preference, as well as equipment limitations, an optimal choice can be made.

The right solder wire diameter not only enhances welding precision and productivity but also helps prevent common soldering defects such as overheating, weak joints, and poor connections. Therefore, carefully evaluating and selecting the solder wire diameter is an essential step before initiating any laser soldering project.