Desoldering

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Solders can be removed using a vacuum plunger (on the right) and a soldering iron. Vacuum plunge.jpg
Solders can be removed using a vacuum plunger (on the right) and a soldering iron.

In electronics, desoldering is the removal of solder and components from a circuit board for troubleshooting, repair, replacement, and salvage.

Electronics physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter.

Solder metal alloy used to join together metal pieces with higher melting points

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid". In fact, solder must first be melted in order to adhere to and connect the pieces together after cooling, which requires that an alloy suitable for use as solder have a lower melting point than the pieces being joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Contents

Tools

Desoldering with a desoldering gun. Desoldering with desoldering gun 2.jpg
Desoldering with a desoldering gun.

Desoldering tools and materials include the following:

Heat gun type of home appliance

A heat gun is a device used to emit a stream of hot air, usually at temperatures between 100 °C and 550 °C (200-1000 °F), with some hotter models running around 760 °C (1400 °F), 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 trigger, in the same general layout as a handgun, hence the name. A lighter duty heat gun is similar to a portable Hair dryer.

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.

Pumps

Electrically operated pumps are used for several purposes in conjunction with a hand-held head connected by a tube.

Pump Device that moves fluids (liquids or gases) by mechanical action

A pump is a device that moves fluids, or sometimes slurries, by mechanical action. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumps.

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.

Nitrogen Chemical element with atomic number 7

Nitrogen is the chemical element with the symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is generally accorded the credit because his work was published first. The name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas; this name is instead used in many languages, such as French, Russian, Romanian and Turkish, and appears in the English names of some nitrogen compounds such as hydrazine, azides and azo compounds.

Desoldering pump

A typical spring-loaded solder sucker Solder sucker.jpg
A typical spring-loaded solder sucker
A solder sucker partially dismantled showing the spring Solder sucker with spring taken out.jpg
A solder sucker partially dismantled showing the spring

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

A solder wick on a reel Solder wick rolled.jpg
A solder wick on a reel
Solder wick, before use Solder wick-close up-part PNrdeg0104.jpg
Solder wick, before use
... and soaked with solder and residue Solder wick-close up-solder impurities PNrdeg0112.jpg
... and soaked with solder and residue

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.

Technique

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.

Through-hole

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 mounted devices 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.

Surface mount

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.

If they may not be destroyed, 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. The most usual tool is a hot air (or hot gas) gun with a nozzle of appropriate size and shape 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 packages

Desoldering an IC with a JBC hot air system Desoldering with hot air.jpg
Desoldering an IC with a JBC hot air system

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.

A system under the JBC brand uses extractor shields that concentrates heat where it needs to be, protect surrounding components and avoids damage to the board or the QFP. The system takes advantage of the properties of solder by melting it with hot air. The extractor has 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 removes the risk of having operator errors by using tweezers or other tools that damage the PCB or IC.

Another way to remove one of these devices is to use Field's metal. Take some of the Field's metal wire, and solder it into all the leads of the chip. Fields metal melts at around 140 °F (62 °C) less than water's boiling point. Once it's applied to all the leads, it stays molten, and the chip can simply be lifted off the board. This has the advantage of not damaging the PCB or the IC.

Related Research Articles

Soldering iron

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.

Point-to-point construction Method for assembling electrical components

Point-to-point construction is a non-automated method of construction of electronics circuits widely used before the use of printed circuit boards (PCBs) and automated assembly gradually became widespread following their introduction in the 1950s. Circuits using thermionic valves were relatively large, relatively simple, and used large sockets, all of which made the PCB less obviously advantageous than with later complex semiconductor circuits. Point-to-point construction is still widespread in power electronics where components are bulky and serviceability is a consideration, and to construct prototype equipment with few or heavy electronic components. A common practice, especially in older point-to-point construction is to use the leads of components such as resistors and capacitors to bridge as much of the distance between connections as possible, often removing the need to add additional wire between the components.

Ball grid array

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 method for producing electronic circuits

Surface-mount technology (SMT) is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface-mount device (SMD). In industry, it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board. Both technologies can be used on the same board, with the through-hole technology used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

Brazing metal-joining technique by high-temperature molten metal filling

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.

Flux (metallurgy) type of chemicals used in metallurgy

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.

Induction heating is the process of heating an electrically conducting object by electromagnetic induction, through heat generated in the object by eddy currents. An induction heater consists of an electromagnet, and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flowing through the resistance of the material heat it by Joule heating. In ferromagnetic materials like iron, heat may also be generated by magnetic hysteresis losses. The frequency of current used depends on the object size, material type, coupling and the penetration depth.

Tweezers tools used for picking up objects too small to be easily handled with the human hands

Tweezers are small tools used for picking up objects too small to be easily handled with the human fingers. The word is most likely derived from tongs, pincers, or scissors-like pliers 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 forceps.

Wave soldering

Wave soldering is a bulk soldering process used in the manufacture 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

Reflow soldering is a process in which a solder paste is used to temporarily attach one or 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 or under an infrared lamp or by soldering individual joints [unconventionally] with a desoldering hot air pencil.

Rework (electronics) term for the refinishing operation or repair of an electronic printed circuit board (PCB) assembly

Rework is the term for the refinishing operation or repair of an electronic 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.

ColdHeat

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.

Foundry factory that produces metal castings

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 aluminium 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

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.

Oil burner

An oil burner is a heating device which burns #1, #2 and #6 heating oils, diesel fuel or other similar fuels. In the United States ultra low #2 diesel is the common fuel used. It is dyed red to show that it is road-tax exempt. In most markets of the United States heating oil is the same specification of fuel as on-road un-dyed diesel.

HASL or hot air solder leveling is a type of finish used on printed circuit boards (PCBs).

Soldering station a multipurpose power soldering device for electronic components soldering

Soldering Station – 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.

Soldering

Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained lead, but environmental and health concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

Component placement is an electronics manufacturing process that places electrical components precisely on printed circuit boards (PCBs) to create electrical interconnections between functional components and the interconnecting circuitry in the PCBs (leads-pads). The component leads must be accurately immersed in the solder paste previously deposited on the PCB pads. The next step after component placement is soldering.

References

  1. 1 2 McComb, Gordon; Shamieh, Cathleen (2009), Electronics For Dummies (2nd ed.), For Dummies, p. 251, ISBN   978-0-470-28697-5.
  2. "Terminology: commercial equipment described as "hot air system" with "suction pen" (in this case a vacuum-style IC handler)". Archived from the original on 2012-06-08. Retrieved 2012-05-01.
  3. "Typical guidelines on SMT soldering, Welwyn: "Components with Pb-free finish may be reflowed with peak temperatures of 260°C (10 seconds)."". Archived from the original on 2013-02-09. Retrieved 2012-05-03.

Further reading