What is Selective Wave Soldering

The electronics industry is experiencing intensified competition, with a strong trend towards miniaturization in almost all electronic products. In the PCBA processing industry, wave soldering and reflow soldering are widely used soldering processes for different components. Reflow soldering is typically used for surface-mount components, which align with the miniaturization trend and has become the mainstream method for mass PCBA manufacturing. Through-hole components are increasingly being replaced by SMD components. However, there are still instances where THT technology is necessary, requiring the use of wave soldering to assemble components onto the PCB surface. While traditional PCBA boards can use conventional wave soldering, more complex double-sided PCBA boards may require a more advanced selective wave soldering process. This eGuide will introduce the selective wave soldering technique to address the challenges of soldering complex circuit DIP components.

What is Selective Wave Soldering?

Selective wave soldering, also known as robot soldering, is an advanced through-hole equipment developed specifically for DIP component soldering. It is a specialized form of wave soldering. Utilizing a selective wave soldering machine in your through-hole PCB assembly process can provide a unique advantage for your electronics projects.

Since 2000, there has been a significant increase in the market share and utilization rate of SMD components. In electronic projects that require mixed PCB assembly, the utilization rate of SMD components can account for up to 70%. Following the concept of double-sided PCB assembly, manufacturers typically perform reflow soldering on the A side of the circuit board and then wave soldering on the B side. However, this two-step high-temperature soldering process can potentially damage components that are not resistant to high temperatures, particularly integrated circuits. Manual soldering for PCB that require both SMT and THT assembly can be inconsistent and time-consuming. To overcome these challenges, selective wave soldering machines have gained popularity.

Based on the design principle, selective wave soldering machines can be classified into two types:

  • Two-module design: Integrating the flux spray area, preheating, and soldering area.
  • Three-module design: Separating the flux spraying area, preheating area, and soldering area.

Selective Wave Soldering Process Principle

The operator inputs instructions into the equipment, which then performs program control based on the requirements of the circuit board design file. The selective wave soldering tank and nozzle move to the designated positions for soldering repairs, and the flux and solder are sprayed precisely from the nozzle. Equipment consists of three main stages: fluxing, preheating, and soldering. The equipment operator manages the programming, and the three stages work sequentially to complete the soldering process.

Selective soldering targets the pin surface of the circuit board, ensuring that electronic components are not affected during the wave soldering process. All machine operations are automatic, with the circuit board securely fixed in the frame during selective soldering. The subsequent operations are carried out automatically based on the pre-programmed instructions in the process control system for that specific circuit board.

For each solder joint, the operator controls the flux application, preheating time, and solder position to minimize any adverse effects on other solder joints on the board. Unlike wave soldering, which affects the entire circuit board uniformly, selective soldering performs independent soldering steps, focusing on one spot at a time until the entire circuit board is soldered. The following is the specific operation process for selective wave soldering:

  1. Perform an equipment check before starting up.
  2. Power on the machine.
  3. Set the boiler temperature and preheating temperature.
  4. Add a specific concentration of flux into the storage box.
  5. Adjust the air pressure and flow rate of the spray tank to optimize the spray effect.
  6. Once the temperature reaches the set value, start the tin pump.
  7. Spray flux onto the designated soldering area.
  8. Preheat the area to activate the flux.
  9. Solder each spot according to the welding plan, one spot at a time.
  10. After soldering, power off the equipment and clean the work area.

Selective wave soldering can be divided into two processes based on the distribution of solder joints:

  • Drag Soldering: This process is suitable for soldering circuit board components such as pin headers and connectors that have closely spaced solder joints. It enables faster and more efficient soldering work.
  • Spot Soldering: This method is suitable for widely distributed solder spot layouts. It is less efficient and not as fast as the drag soldering process.

Based on the working principle, selective wave soldering can be categorized into two types:

  • Inline selective wave soldering: These are fully automatic, integrated wave soldering machines with larger footprints. They offer real-time monitoring data but are more expensive and better suited for large-scale PCBA factories.
  • Offline selective wave soldering: These are semi-automatic machines with a lower price and purchase threshold. The offline type operates as an independent unit within the production line. The flux spraying machine and selective soldering machine are combined into a split-type 1+1 configuration. The preheating module follows the soldering part, with manual transmission and a human-machine collaboration approach.

Features of Selective Soldering

When using traditional wave soldering, the operator applies flux to the entire circuit board and immerses the soldering surface in molten tin for soldering. Selective soldering, on the other hand, is different. The operator precisely controls the soldering process by setting the position of the flux, soldering time, and wave crest height to achieve accurate soldering. This characteristic offers several advantages:

  • No need for custom fixtures or trays to support PCBA during soldering.
  • Better welding quality compared to traditional wave soldering.
  • Precise soldering reduces the consumption of solder bars, resulting in cost savings.
  • Prevents high temperatures from damaging integrated circuits and causing circuit board deformations.
  • Intelligent soldering selection, as the shape of the solder nozzle can be adjusted, allowing for different parameters to be set during soldering.
  • Output data for improved consistency.
  • Reduces the content of flux residues, minimizing the risk of short circuits caused by chemical corrosion.

However, like any technology, selective wave soldering also has its disadvantages:

  • Different projects require different parameters, necessitating significant time investment by operators to program the machine before soldering.
  • As the number of PCB layers increases, the complexity of circuit board assembly and selective soldering also increases.
  • Selective wave soldering is a more expensive assembly and soldering method.
  • Overall, the advantages of selective wave soldering outweigh the disadvantages, particularly for complex PCB assemblies that require precise and high-quality soldering.

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