Solder On, Solder Off: Soldering and Desoldering Techniques
In the modern era, most electronics are dependent on printed circuit boards, or PCBs, to contain the various electronic components that allow them to function. These components are held onto the board by solder flux that creates a bond between the pins of a component and their corresponding pads on the PCB, with the main purpose of the solder being to provide electrical connectivity. Soldering and desoldering is performed to install or remove components on a PCB.
Soldering is most commonly achieved with a soldering iron. The iron is heated to roughly 420 degrees Celsius, which is hot enough to quickly melt the solder flux. First, the component is positioned on the PCB so that its pins are aligned with the corresponding pads on the board. A solder wire is brought in contact with the interface between the first pin and its pad. Briefly touching this wire at the interface with the heated soldering iron tip melts the solder, which flows onto the pad and covers the component pin. Once it has solidified, it creates a strong bond between the pin and the pad. This process happens within two or three seconds, which allows the user to quickly move on to the next pin and pad.
Reflow soldering is used in PCB production environments in which large numbers of surface mount devices (SMD) and their components must be soldered at the same time. SMDs are electronic components that are much smaller than their through-hole counterparts, and are soldered onto the component side of the board and do not need to be drilled. The heat-oven method of soldering is frequently used in these cases, and makes use of a specially-designed oven. First, the SMD components are placed on the board with a solder flux paste spread over all their terminals. The paste is sticky enough to keep the components in place while the board is put in the oven, and over several stages, the flux paste is heated to melt and bond the components to the board, and then cooled to solidify it in place.
Desoldering is accomplished several different ways. The first is with copper braid, where the solder flux is melted by a soldering tip, and a copper braid is used to absorb and remove the flux. This method is efficient, but slow, since it requires each individual soldered joint to be worked on one by one. Desoldering can also be done with a solder sucker, which sucks molten flux up off of the pads. Like with copper braid, a heated soldering tip is placed on the solid solder until it melts, and the solder sucker sucks the flux up and away, like a vacuum. Lastly, desoldering with a heat gun is most frequently used to desolder SMD components, but can also be used for through-hole components. In this method, the board is placed on a flat area, and a heat gun is pointed directly at the components that need to be desoldered for a few seconds. This melts the solder, loosening the components, which are then lifted away by tweezers. The problem with this method is that it is difficult to use for small individual components, since the heat can melt the solder on nearby pads. Molten flux can also flow onto nearby traces and pads, causing electrical shorts.
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