Selective soldering pertains to the method of selectively soldering components to molded modules and PCBs that can be damaged by heat during the reflow process in traditional SMT assembly processing. It is typically conducted after the SMT oven reflow process to avoid damaging certain components. Some of the processes that are used in selective soldering are:
• Selective aperture tooling over a wave solder – The tools protect the areas that were previously soldered during the SMT reflow soldering process. That way, only the parts to be selectively soldered are exposed through the window or aperture of the tools. Once the set-up is complete, the PCB and tool assembly are passed over equipment for wave soldering.
• Mass selective dip solder fountain – This is one type of selective aperture soldering where special tooling comes with apertures, which lets the solder to pump right through it. After that, the PCB is passed over a selective-solder fountain.
• Laser selective soldering system – The latest system in selective soldering can import CAD-based board layouts to accurately position the laser directly to solder a specific point on the PCB. IT can eliminate thermal stress and produces flexibility along with high-quality solder joints.
• Miniature wave solder fountain – This is a tool-less selective soldering method that utilizes a small round-pumped solder wave that looks like the end of a crayon or pencil to solder PCB. This type of soldering process may be slower, but it ensures accuracy. In this process, the PCBs may either be fixed, and a wave solder pot is moved under the PCB or the PCB can be moved over the solder bath or fixed wave to make it go through the selective soldering process.
Selective soldering typically requires longer contact times with solders and high-temperature solders due to the smaller mass of molten solder touching the board and the heat dissipating quickly from the area of contact to the rest of the PCB. Longer contact times and high solder temperatures can be more demanding on the flux being used. High-quality fluxes for selective soldering can produce IPC Class III solder joints on 1.6mm-thick PCBs. Alcohol based fluxes with less than 6% rosin and activator are ideal for 2.4mm thick PCBs.
• Selective aperture tooling over a wave solder – The tools protect the areas that were previously soldered during the SMT reflow soldering process. That way, only the parts to be selectively soldered are exposed through the window or aperture of the tools. Once the set-up is complete, the PCB and tool assembly are passed over equipment for wave soldering.
• Mass selective dip solder fountain – This is one type of selective aperture soldering where special tooling comes with apertures, which lets the solder to pump right through it. After that, the PCB is passed over a selective-solder fountain.
• Laser selective soldering system – The latest system in selective soldering can import CAD-based board layouts to accurately position the laser directly to solder a specific point on the PCB. IT can eliminate thermal stress and produces flexibility along with high-quality solder joints.
• Miniature wave solder fountain – This is a tool-less selective soldering method that utilizes a small round-pumped solder wave that looks like the end of a crayon or pencil to solder PCB. This type of soldering process may be slower, but it ensures accuracy. In this process, the PCBs may either be fixed, and a wave solder pot is moved under the PCB or the PCB can be moved over the solder bath or fixed wave to make it go through the selective soldering process.
Selective soldering typically requires longer contact times with solders and high-temperature solders due to the smaller mass of molten solder touching the board and the heat dissipating quickly from the area of contact to the rest of the PCB. Longer contact times and high solder temperatures can be more demanding on the flux being used. High-quality fluxes for selective soldering can produce IPC Class III solder joints on 1.6mm-thick PCBs. Alcohol based fluxes with less than 6% rosin and activator are ideal for 2.4mm thick PCBs.
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