Editing Desoldering (section)

==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 technology]] on double-sided or multi-layer boards, special care must be taken not to remove the [[Via (electronics)|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 technology|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 {{convert|260|C|F}} for a time not exceeding 10 seconds is acceptable.<ref>{{Cite web |url=http://www.welwyn-tt.com/products/resistors/pbfree-faqs.asp |title=Typical guidelines on SMT soldering, Welwyn: "Components with Pb-free finish may be reflowed with peak temperatures of 260°C (10 seconds)." |access-date=2012-05-03 |archive-url=https://archive.today/20130209164647/http://www.welwyn-tt.com/products/resistors/pbfree-faqs.asp |archive-date=2013-02-09 |url-status=dead }}</ref>

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 [[Cooper Tools|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===
[[Image:Desoldering with hot air.jpg|thumb|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.

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 tweezers|vacuum nozzle]] similar to the ones used in [[SMT placement equipment|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&nbsp;°F (62&nbsp;°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.