Desoldering

From Wikipedia, the free encyclopedia

In electronics, desoldering is the removal of solder and components from a circuit for troubleshooting, repair purposes and to salvage components. Electronic components are often mounted on a circuit board and it is usually desirable to avoid damaging the circuit board, surrounding components, and the component being removed.

Specialized tools, materials, and techniques have been devised to aid in the desoldering process.

Desoldering tools and materials include:

• Desoldering braids
• Heat guns
• Vacuum plungers (solder suckers)
• Removal alloys
• Removal fluxes
• Vacuum and pressure pumps with specialized heater tips and nozzles
• Rework stations, used to repair printed circuit boards that fail factory test.

[edit] 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 high value components for possible re-use. Excess application of heat may damage electronic components, or destroy the bond between a printed circuit trace and the underlaying substrate.

A single joint can usually be quickly undone with a soldering iron and use of either vacuum bulb, plunger, or desoldering braid. Once the solder has been melted, the vacuum device is applied to withdraw solder from the joint by a rapid movement of air. Desoldering braid draws up the molten solder by capillary action; the braid usually contains the same flux as solder.

Multiple-lead devices require special care in removal. For through-hole mounted devices, special soldering bits that facilitate even distribution of heat over all the leads at one time can be used. For factory re-work, a temperature controlled rework station may contain such bits, hot-air systems, and a vacuum system to facilitate removal of the part. Large surface-mounted components require special tooling for production desoldering, although various and improvised methods can be successfully applied for field service. In some instances it is better to destroy the part by cutting off its leads and removing the leads individually, instead of risking damage to the printed circuit board.

Hobbyists may even use a torch or hot air gun to rapidly heat all parts on a board and remove them; such methods are extremely difficult to apply without damaging parts or boards, and may result in toxic fumes.

[edit] Quad Flat Packs (QFP)

QFP chips have thin leads closely packed together protruding from the four corners of the integrated circuit (IC); usually a square IC. Removal of these chips can be problematic. 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. This technique however, can lead to 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 an internal spring system that gently pulls the IC when the liquid stage of solder has been reached. The IC is hold 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 heating surrounding components and blowing the 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.

[edit] References

This article does not cite any references or sources. (May 2008) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.