In the ever-evolving PCB design and manufacturing industry, the pursuit of improved quality and performance remains a constant. In the past, PCB designers relied on labor-intensive methods, such as laying out through-hole circuit boards at four times their actual size on a light table using black tape and sticky dots. However, the advent of early CAD systems brought about a significant transformation, simplifying the design process. Yet, the continuous advancements in small fine-pitch surface mount technology necessitated further adaptations.
As PCB layout technology grew more sophisticated to accommodate shrinking parts and pin pitches, the manufacturing of circuit boards also had to undergo significant changes. The fabrication processes that once worked well for wide, hand-taped traces needed substantial refinement to handle the smaller features required for BGAs and other modern high-density, fine-pitch components. Similarly, the assembly processes had to keep pace with these developments to ensure proper soldering while effectively managing the thermal requirements of numerous individual components. Thankfully, the selective soldering process has emerged as a valuable solution, contributing significantly to soldering efficiency and achieving improved outcomes.
Comparison of Soldering Methods |
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| Advantage | Disadvantage | |
| Automated (wave, reflow) | The greatest soldering throughput can solder entire sides of boards in a continuous process | Dense placement of SMT and through-hole components on double-sided assembly may inhibit masking pallet and require a secondary solder process |
| Selective | A middle-ground between automated and manual soldering, selective soldering can combine the speed of automation with the flexibility of hand soldering | The upfront time investment to prepare boards for the selective soldering process may not be time-efficient if little soldering is required overall |
| Manual | The least restrictive soldering method
Excellent for large components (e.g., connectors) that are incompatible with other soldering technologies |
Costly and time-consuming
Requires a trained technician Quality has a higher variance |
What Is the Selective Soldering Process?
A selective soldering system is a machine that will solder through-hole leads on a board or a panel. It uses a solder reservoir and pump system to deliver molten solder to the leads coming through the bottom side of the board. This solder mechanism is built on an assembly that moves in multiple directions, which is pre-programmed by the operators for the board or panel being assembled. In addition to movement, the programming also grants operators precise control of the solder parameters, namely the application time and the temperature. The programming can be developed from the Gerber data generated for board fabrication.
Selective solder fills the process gap between the automated wave solder process and manual hand-soldering. Wave solder is the first choice for through-hole component soldering, as it is quick and takes very little preparation or programming, yet there are limitations for boards with dense parts placement on the bottom side. Some boards may require a custom pallet to mask off surface mount components on the bottom side, allowing the solder wave to contact only the through-hole pins. In cases where the surface mount parts are too close for a masking pallet, the only other option would be to manually solder the through-hole leads – a time-consuming process that could introduce human error. With the selective soldering process, however, the potential problems with manual soldering can be entirely sidestepped.
Summarizing the Selective Soldering Process |
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What Are the Benefits of Selective Soldering?
Some major disadvantages of manual hand-soldering are that it takes time, even with a trained technician, and it requires that the circuit board undergo considerable localized heating to achieve a good solder joint. Heat is an aging mechanism for PCBs which can eventually result in reliability problems for the board, the components, and already-completed solder joints. In contrast, the selective soldering system allows operators to manipulate soldering variables and achieve reliable solder joints quicker with minimal additional heat. During the selective soldering process, the operator:
- Programs the nozzle coordinates to precisely apply the molten solder.
- Outlines the nozzle travel time for adequate solder heating time.
- Programs how much solder to dispense and the operating temperature
This much control affords the operator greater precision in soldering without manipulating a soldering iron. Soldering work that used to require a team of technicians hand-soldering through-hole leads due to wave solder system incompatibility can now be performed quickly, autonomously, and with a high level of quality using the selective solder system.
How Selective Soldering Aids CM Assembly
Consider soldering a backplane with connectors on both sides. The tall profile of the connectors makes it extremely challenging to create a custom pallet for wave soldering. Traditionally, this soldering task would be rectified using a handheld soldering iron, requiring the expertise and time of a trained technician. However, with the automated tip of a selective solder nozzle, the job can be completed in significantly less time and consistently yield high-quality results.
Contract manufacturers often default to wave soldering for efficient bulk assembly, especially when dealing with high-volume lots. If only a small amount of additional soldering is required after wave soldering, manual hand-soldering may still be feasible in less time than running boards through the selective soldering process. However, if a board necessitates more than minor touch-up soldering, the selective soldering system is a superior choice regarding speed, accuracy, and reduced thermal load on the board.
At VSE, our team of engineers is committed to building electronics for our customers. We carefully evaluate and implement customized assembly plans, prioritizing production quality while minimizing costs.
