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In electronics, desoldering is the removal of solder and components from a circuit board for troubleshooting, repair, replacement, and salvage.
Desoldering tools and materials include the following:
Terminology is not totally standardised. Anything with a base unit with provision to maintain a stable temperature, pump air in either direction, etc., is often called a "station" (preceded by rework, soldering, desoldering, hot air); one, or sometimes more, tools may be connected to a station, e.g., a rework station may accommodate a soldering iron and hot air head. A soldering iron with a hollow tip and a spring-, bulb-, or electrically-operated suction pump may be called a desoldering iron. [1] Terms such as "suction pen" [2] may be used; the meaning is usually clear from the context.
Electrically operated pumps are used for several purposes in conjunction with a hand-held head connected by a tube.
Suction pumps are used to suck away molten solder, leaving previously joined terminals disconnected. They are primarily used to release through-hole connections from a PCB. The desoldering head must be designed so that the extracted solder does not solidify so as to obstruct it, or enter the pump, and can be removed and discarded easily. It is not possible to remove a multi-pin part by melting solder on the pins sequentially, as one joint will solidify as the next is melted; pumps and solder wick are among methods to remove solder from all joints, leaving the part free to be removed.
Suction pumps are also used with a suction head appropriate for each part to pick up and remove tiny surface mount devices once solder has melted, and to place parts.
Hot air pumps blow air hot enough to melt all the solder around a small surface mounted part, and can be used for soldering parts in place, and for desoldering followed by removal before the solder solidifies by a vacuum pump or with tweezers. Hot air has a tendency to oxidise metals; a non-oxidising gas, usually nitrogen, can be used instead of air, at increased cost of equipment and consumables.
A desoldering pump, colloquially known as a solder sucker, is a manually-operated device which is used to remove solder from a printed circuit board. There are two types: the plunger style and bulb style. [1] (An electrically-operated pump for this purpose would usually be called a vacuum pump.)
The plunger type has a cylinder with a spring-loaded piston which is pushed down and locks into place. When triggered by pressing a button, the piston springs up, creating suction that sucks the solder off the soldered connection. The bulb type creates suction by squeezing and releasing a rubber bulb.
The pump is applied to a heated solder connection, then operated to suck the solder away.
Desoldering braid, also known as desoldering wick or solder wick, is finely braided 18 to 42 AWG copper wire coated with rosin flux, usually supplied on a roll.
The end of a length of braid is placed over the soldered connections of a component being removed. The connections are heated with a soldering iron until the solder melts and is wicked into the braid by capillary action. The braid is removed while the solder is still molten, its used section cut off and discarded when cool. Short lengths of cut braid will prevent heat being carried away by the braid instead of heating the joint.
Desoldering requires application of heat to the solder joint and removing the molten solder so that the joint may be separated. Desoldering may be required to replace a defective component, to alter an existing circuit, or to salvage components for re-use. Use of too high a temperature or heating for too long may damage components or destroy the bond between a printed circuit trace and the board substrate. Techniques are different for through-hole and surface-mounted components.
A component with one or two connections to the PCB can usually be removed by heating one joint, pulling out an end of the component while the solder is molten (bending the other lead to do so), and repeating for the second joint. Solder filling the hole can be removed with a pump or with a pointed object made of a material which solder does not wet, such as stainless steel or wood.
If a multi-pin component need not be salvaged, it is often possible to cut the pins, then remove the residual ends one by one.
Components with more connections cannot be removed intact in the way described above unless the wire leads are long and flexible enough to be pulled out one by one. For a component such as a Dual-Inline Package (DIP), the pins are too short to pull out, and solder melted on one joint will solidify before another can be melted. A technique sometimes used is the use of a large soldering-iron tip designed to melt the solder on all pins at once; different tips are required for different packages. The component is removed while the solder is molten, most easily by a spring-loaded puller attached to it before heating.
Otherwise all joints must be freed from solder before the component can be removed. Each joint must be heated and the solder removed from it while molten using a vacuum pump, manual desoldering pump, or desoldering braid.
For through-hole technology on double-sided or multi-layer boards, special care must be taken not to remove the via connecting the layers, as this will ruin the entire board. Hard pulling on a lead which is not entirely free of solder (or with solder not thoroughly molten in the case of a soldering iron tip heating all pins) may pull out a via.
To remove and recover all components, both through-hole and surface-mount, from a board which itself is usually no longer needed, a flame or hot air gun can be used to rapidly heat all parts so they can be pulled off. Parts may be damaged, and toxic fumes emitted, if excessive temperature or prolonged heating is used.
If they do not need to be re-used, some surface-mount components can be removed by cutting their leads and desoldering the remnants with a soldering iron.
Otherwise, surface-mount components must be removed by heating the entire component to a temperature sufficient to melt the solder used, but not high or prolonged enough to damage the component. For most purposes, a temperature not exceeding 260 °C (500 °F) for a time not exceeding 10 seconds is acceptable. [3]
The entire board may be preheated to a temperature that all components can withstand indefinitely. Then localised heat is applied to the component to remove, with less heating required than from cold. Most frequently, a hot air (or hot gas) gun, with a nozzle of appropriate size and shape, is used to heat the component, with nearby components shielded from the heat if necessary, followed by removal with tweezers or a vacuum tool. Removal of multi-pin components with a soldering iron and solder removal tools is impractical, as the solder between the component and the pads remains in place, unlike solder which can be removed from a hole.
Hot air (or gas) may be applied with tools ranging from some portable gas soldering irons such as the Weller Portasol Professional which can be fitted with a narrow hot-air nozzle, set to a temperature not controlled but approximately correct, to an industrial rework station with many facilities including hot-gas blowing, vacuum part holding, soldering iron head, and nozzles and fitting specific to particular component packages.
Quad Flat Package (QFP) chips have thin leads closely packed together protruding from the four sides of the integrated circuit (IC); usually a square IC. Removal of these chips can be problematic as it is impossible to heat all of the leads at once with a standard soldering iron. It is possible to remove them with the use of a razor blade or a high-rpm craft tool, simply by cutting off the leads. The stubs are then easy to melt off and clean with a soldering iron. Obviously this technique entails the destruction of the IC. Another method is to use a heat gun or pencil butane torch and heat up a corner, and gently pry it off, working the torch down the leads. This method often leads to traces getting lifted off the PCB where a lead did not get heated enough to cause the solder to flow.
Several vendors offer systems that use heat shields to concentrate hot air where it needs to be, protecting nearby components and avoiding damage to the board or the QFP. The extractor uses a spring system that gently pulls the IC upward when the liquid stage of solder has been reached. The IC is held by a vacuum nozzle similar to the ones used in pick & place machines. This system prevents damage to the pads on the PCB, the IC, avoids overheating surrounding components and blowing them off and also reduces the risk of operator error when using tweezers or other tools that can damage the PCB or IC.
Another way to remove these devices is to use Field's metal, an alloy which melts at around 140 °F (62 °C), lower than the boiling point of water. The metal is melted into the solder joints of the device, where it remains liquid even once cooled down to room temperature, and the chip can simply be lifted off the board. This has the advantage of not damaging the PCB or the IC, although the solder joints must be carefully cleaned of any remaining Field's metal to maintain solder joint strength after resoldering.
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.
Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.
Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
Flip chip, also known as controlled collapse chip connection or its abbreviation, C4, is a method for interconnecting dies such as semiconductor devices, IC chips, integrated passive devices and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. The technique was developed by General Electric's Light Military Electronics Department, Utica, New York. The solder bumps are deposited on the chip pads on the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry, it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is reflowed to complete the interconnect. This is in contrast to wire bonding, in which the chip is mounted upright and fine wires are welded onto the chip pads and lead frame contacts to interconnect the chip pads to external circuitry.
In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.
Tweezers are small hand tools used for grasping objects too small to be easily handled with the human fingers. Tweezers are thumb-driven forceps most likely derived from tongs used to grab or hold hot objects since the dawn of recorded history. In a scientific or medical context, they are normally referred to as just "forceps", a name that is used together with other grasping surgical instruments that resemble pliers, pincers and scissors-like clamps.
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.
Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.
Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.
Reflow soldering is a process in which a solder paste is used to temporarily attach anywhere from one to thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or by soldering individual joints with a hot air pencil.
In electronics, rework is the repair or refinish of a printed circuit board (PCB) assembly, usually involving desoldering and re-soldering of surface-mounted electronic components (SMD). Mass processing techniques are not applicable to single device repair or replacement, and specialized manual techniques by expert personnel using appropriate equipment are required to replace defective components; area array packages such as ball grid array (BGA) devices particularly require expertise and appropriate tools. A hot air gun or hot air station is used to heat devices and melt solder, and specialised tools are used to pick up and position often tiny components. A rework station is a place to do this work—the tools and supplies for this work, typically on a workbench. Other kinds of rework require other tools.
ColdHeat was an American company founded to develop and market products using the proprietary graphite-like compound Athalite. The composite material is claimed by the manufacturer to have the unusual ability to conduct large amounts of heat and return to room temperature in a short amount of time.
A heat gun is a device used to emit a stream of hot air, usually at temperatures between 100 and 550 °C, with some hotter models running around 760 °C, which can be held by hand. Heat guns usually have the form of an elongated body pointing at what is to be heated, with a handle fixed to it at right angles and a pistol grip trigger in the same pistol form factor as many other power tools.
A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.
Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.
A soldering station is a multipurpose power soldering device designed for electronic components soldering. This type of equipment is mostly used in electronics and electrical engineering. Soldering station consists of one or more soldering tools connected to the main unit, which includes the controls, means of indication, and may be equipped with an electric transformer. Soldering stations may include some accessories – holders and stands, soldering tip cleaners, etc.
Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.
Soldering is a process of joining two metal surfaces together using a filler metal called solder. The soldering process involves heating the surfaces to be joined and melting the solder, which is then allowed to cool and solidify, creating a strong and durable joint.
Decapping (decapsulation) or delidding of an integrated circuit is the process of removing the protective cover or integrated heat spreader (IHS) of an integrated circuit so that the contained die is revealed for visual inspection of the micro circuitry imprinted on the die. This process is typically done in order to debug a manufacturing problem with the chip, or possibly to copy information from the device, to check for counterfeit chips or to reverse engineer it. Companies such as TechInsights and ChipRebel decap, take die shots of, and reverse engineer chips for customers. Modern integrated circuits can be encapsulated in plastic, ceramic, or epoxy packages.
