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Workingbear initially thought that wave soldering machines should have been relegated to museums by now! However, despite the decades-long development of Surface Mount Technology (SMT), many PCBs still undergo wave soldering processes. But nowadays, most wave soldering processes employ Selective Mask Wave Soldering, rather than the old method of immersing entire boards into solder baths.
In Selective Mask Wave Soldering, the original wave soldering machine is still used, but the PCB is placed in a wave soldering carrier. Only the pins or solder joints of components requiring wave soldering (typically through-hole components) are exposed to the solder, while other components are shielded with a mask of carrier. It’s a bit like wearing a lifebuoy in a swimming pool – the parts covered by the lifebuoy won’t get wet. Similarly, in the wave soldering process, the areas covered by the carrier won’t get soldered, eliminating the risk of re-melting solder or components falling off.
Another method, known as Selective Soldering machines, utilizes small solder pots with nozzle and automated positioning PCB. The PCB is first secured, and then the program controls the movement of the PCB to align it with the solder nozzle beneath. This allows the solder to make contact with the through-hole devices’ pins, achieving automated soldering. However, this article does not delve into the specifics of selective soldering machines and processes.
However, not all PCBs are suitable for Selective Mask Wave Soldering processes. There are design limitations to consider. The primary condition is that the components selected for wave soldering must maintain a certain distance from other non-soldered components, creating a keep-out area for the creation of the solder mask carrier.
Design Guidance for Selective Mask Wave Soldering Carrier and PCB Layout:
Components with through-hole pins too close to the edge of the carrier’s opening may suffer from solder insufficiency due to shadow effects. It’s recommended to maintain a distance of at least 1.27mm from the edge of the carrier’s opening to the solder joints of components needing wave soldering. Additionally, a 45° angled slope should be added to the edge of the carrier’s opening to facilitate solder flow.
The mask carrier must cover components not requiring wave soldering to prevent the risk of re-melting and solder dropping.
The thickness of the mask carrier’s edge walls should be at least 0.05 inches (1.27mm) to effectively isolate solder penetration into components not requiring wave soldering.
Components on the wave soldering surface should be less than 0.15 inches (3.8mm) in height, as taller components may not be covered by the wave soldering mask carrier.
The material of the wave soldering carrier should not react with solder and should withstand repeated high-temperature cycles without deformation. It should have low heat absorption, minimal heat shrinkage, and be lightweight. Aluminum alloy is commonly used, but synthetic materials are also utilized.
In the beginning, PCBs were predominantly designed for traditional through-hole devices (THDs), necessitating wave soldering for all boards, which were typically single-sided only.
The advent of Surface Mount Technology (SMT) led to the hybrid use of SMT and wave soldering. Many electronic components couldn’t transition to SMT processes in the beginning, particularly traditional through-hole devices. Thus, PCBs were designed with all through-hole components on one side for wave soldering, while Surface Munt Devices on the other side required adhesive to prevent them from falling into the wave soldering bath. While most boards now follow a two-sided SMT process today, there are still a few components unsuitable for complete SMT conversion still, leading to the emergence of selective mask wave soldering processes.
Workingbear initially thought that wave soldering machines should have been relegated to museums by now! However, despite the decades-long development of Surface Mount Technology (SMT), many PCBs still undergo wave soldering processes. But nowadays, most wave soldering processes employ Selective Mask Wave Soldering, rather than the old method of immersing entire boards into solder baths.
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