Desoldering is often required when you need to replace defective components, build or alter an electrical circuit, or salvage components from a discarded piece of equipment to reuse the parts later.
The operation requires that you apply heat to a soldered joint to melt the solder, and then remove the molten solder to separate the components or remove them from a circuit while leaving all parts cleaned from any solder.
The use of excessive heat or heating for too long will often damage the components, burn the PCB board, or separate the bond between the patterned conductive layer and the board substrate. The following will give you a better understanding of the skills and help avoid some of those mishaps.
The Melting Point of Solder
Lead-alloy solder is rarely used these days, due to health hazard issues, and has been substituted by different alloys solder types that don’t contain any lead. The common alloys used for electrical are 60/40 Sn-Pb which melts at 370°F (188°C), and 63/37 Sn-Pb for electrical and electronic work with a melting point of 361°F (183°C).
With lead-free solder, the lead element is replaced with a combination of different elements, such as tin, silver, brass, copper, and a few more, to create a “soft solder” with the most commonly used alloy having a melting point range of 360°F (180°C) to 370°F (190°C) common for electronic work.
When doing soldering work, you have to keep in mind that the soldering iron has to heat up the joint enough that it is the joint itself that melts the solder and not the iron, meaning that the iron has to reach a higher than melting point to effectively and quickly bring the joint to the solder’s melting point.
The higher tip temperature allows you to apply the iron to the area, apply the solder, and remove it all within about one to two seconds, avoiding unnecessary overheating of the area and component.
You should therefore be careful when choosing your soldering iron or soldering gun that it will not generate enough heat to damage components, burn, or delaminate the patterned conductive layers from the sandwiched structure of the printed circuit board.
Choosing the Right Tool to Melt Solder
The ideal temperature for a specific soldering task will be determined by your intended objectives, and in consideration of your soldering skills and experience.
Too much heat or heat applied for too long on components’ leads or on a PCB board can damage both or either one, and the final quality of your work will also be affected by the temperature applied to it.
For example, a 60 Watts iron doesn’t have to be resting on the PCB board’s patterned conductor layer for very long for the conductor layer to start peeling off or for the board to get scorched (burn marks or melting on the circuit board).
At the other end of the spectrum, a 15 Watts iron will need a prolonged contact period for a component or the PCB board to get hot enough to melt the solder, during which time more heat is transferred throughout the components and the board, thus affecting their integrity.
For normal soldering work on motherboards or PCB boards, a 25 Watts soldering iron could be good enough, although a 30 Watts iron will provide faster heating and melting without any risks of causing damage from overheating.
For desoldering work, however, where you most likely want to remove the components, you should probably opt for a 40 Watts iron for an easier and faster melting of the solder and its removal.
With a working temperature ranging between 824° to 932°F (440-500°C), this iron is more capable of more intense application, making it better suited for working on audio equipment, RC (resistors and capacitors) equipment repairs, and other higher-level electronic applications.
Preheating Your Iron
Before applying your iron to the components or the PCB board, it should be preheated to the melting point of the solder. This is easily done by using some solder wire to test the iron for its hotness. When your soldering iron starts melting the solder, you’ll know that it’s hot enough and ready to use.
If you find the iron rather slow to heat up and melt the solder, clean the tip from all dirt, oxidation, and corrosion, and check to make sure the tip is tight enough to make good contact with the iron’s element. Adding a coating of flux will also increase the heat transfer to melt the solder.
Note # 1 - A soldering heat sink is a cheap but worthy tool that, when clamped between the soldering iron and the component itself, will preserve the component from being submitted to extreme or excessive heat.
Flux and Rosin-Core Solder
Flux is basically a substance that is applied onto metal surfaces to promote fluidity and to remove objectionable impurities such as oil, oxidation, or other dirt that can prevent the solder from forming a solid bond.
The rosin flux is used to coat the surfaces prior to soldering to chemically clean the metal and also prevent oxidation, but it also helps in lowering the melting temperature, improving the mechanical strength and electrical contact of electrical joints.
Note # 2 - When flux is not readily available, you can always substitute with Vaseline.
Rosin-core solder is a modified solder wire available in both lead-alloy and lead-free solder and basically offers a wire with a hollow center to create a core that is then filled with rosin. It, therefore, doesn’t require any more flux to be applied to the joint.
Solder Removal Tools
There are several different tools that can be used to remove solder from electrical joints. Some of these tools are better for removing larger amounts of solder, some for cleaning off PCB boards more effectively, cleaning off all residues by absorbing it, or soaking it all up.
1. Electric Desoldering Pump
Electric desoldering pumps (vacuum pump) are more cumbersome and awkward to use for the fact that the handle with its suction tip is hooked to a separate vacuum pump through a plastic hose, making it bulky with the pump’s physical size.
The molten solder sucked up by the pump can sometimes get clogged up along the tube and even inside the vacuum pump, making the cleaning labor-intensive.
2. Electric Desoldering Vacuum Pump
The electric desoldering vacuum pump is a more compact tool shaped and handled as a handgun with a suction tip that sucks up the molten solder. The tool is most often incorporated with a heat source to melt the solder prior to extracting it.
Because it does have its own heat source, this tool obviously doesn’t need an external soldering iron to assist in desoldering. Depending on options and capabilities, their price range can run up to about $300.
3. Mechanical Plunger Suction Pump
The mechanical plunger-type suction pump is a more compact portable tool that consists of a short aluminum or ABS tube—or shell—with a removable suction tip at one end, and inside, which is a piston onto which an O-ring provides the suction.
The piston can often also be fitted with a thin stem at the bottom end to unclog the orifice of the tip when the piston is armed, should the solder get stuck and plug the opening.
On its opposite end, the piston is attached to a long stem that extends out through the top cover of the tool to provide the means to push the piston in to activate or “arm” the suction tool.
An extension spring in the top chamber of the apparatus is also attached to the top of the piston, with the other end secured at the top of the cylinder to provide enough tension while the piston is pushed in to quickly retract the piston and pull in the molten solder when the release button is pushed in.
The tool is easily cleaned by unscrewing the tip and working or sliding the piston in and out.
The mechanical plunger pump will, however, require a soldering iron to melt the solder prior to removing it with the tool.
4. Desoldering Wick
The desoldering wick, also known as a solder wick or a desoldering braid, is made of a bunch of tiny copper wires—18 to 42 AWG—coated with rosin flux prior to being finely braided into a sort of ribbon spooled on a protective plastic casing ready for use by simply pulling on the end.
After pulling out a short piece of the wick from its spool, open up the strands of wire by making the ribbon wider by compressing it together. A loosened up and wider ribbon will be able to wick up more of the solder than if it is stretched thin and tight.
You can then simply lay the end of it on the soldered joint and then apply the heat over the soldered connection to melt the solder.
The molten solder is then wicked up into the braid by capillary action, and once the end of the braid is saturated with solder, you remove it while still molten and simply cut the end off from the braid and discard it. You just repeat this until all the solder is removed and the joint is perfectly cleaned off.
No matter which tool you’re going to use to do your desoldering, the desoldering wick is the one tool that complements all the other tools. it is the only tool with which you can successfully and completely clean a surface or a component to its original state, except for an added tinned coating ready for solder.