Desoldering is as much of a skill as soldering. The aim is to remove all the solder from a component and detach the component from the PCB without damaging the component or the PCB, either from excessive heat or from rough handling.
Also called solder wick, solder braid is a ribbon of extremely fine copper strands woven together. It is usually supplied on small reels of about 1.5m of braid and comes in a variety of widths. The braid is pressed aginst a molten soldered joint, and capillary action draws the solder out of the joint and into the braid. The portion of braid which contains solder is then cut off the reel and discarded. It is usually sold treated with a flux so that the solder is drawn up more effectively.
Using Solder Braid
- First, a short length (about 50mm or 2 inches) is drawn off the reel. Do not cut it off as this will reduce the usable portion of the braid, and will make it difficult to hold (the braid is made of an excellent thermal conductor, so will get very hot).
- Next the end of this section is placed on top of the solder joint requiring desoldering.
- The soldering iron is placed on top of the braid and very gentle pressure is applied.
- As the joint melts and the braid is pressed into the molten solder, the solder will flow up the braid, turning it a silvery colour.
- If the solder braid stops pulling up solder but the joint has not been cleared, move on to the next unused bit of braid - it has become full of solder and can no longer draw it up.
- Once the solder has all been removed from the joint, withdraw the iron and the braid together. If the iron is withdrawn and the braid is left, the braid will become stuck to the pad that has just been desoldered.
- The pad should now be exposed and the component leg should be free to be removed. The pad will have a thin layer of solder left on it, turning it a bright silver colour. This will actually help with resoldering later on.
- Cut off the used portion of braid and discard - it cannot be reused.
Do not melt the joint and stick the end of the braid in. This will draw up very little solder, if any, and will serve only to overheat the joint, underlying pad and the component.
For best results, try to use a width of braid that matches the width of the joint. Failing that, 1.5mm is a good general-purpose width of braid to use.
Solder suckers are devices that use an inrush of air to suck up molder solder from a joint. They can look either like a chunky pen or like a rubber bulb. To use them, prime the solder sucker by pushing down the plunger or squeezing the bulb, place the nozzle of the device next to the joint, then heat the solder. Once it is molten, release the plunger or bulb and the solder will be sucked into the device.
This technique is good for joints with a large amount of solder on them which would use an unnecessarily large length of solder braid. It generally does not remove enough solder to allow the component to be removed, but application of braid will clear out the remaining solder.
This simply involves heating up the joint and then banging the PCB, hard, on the bench. The impact will send the solder in the joint flying off in a blob, leaving a reasonably clean pad. Beware that the blob may become stuck to the PCB further down, but due to the rapid cooling, it can easily be removed with a fingernail. The impact method only works with fairly large joints as a large thermal mass is required to keep the joint liquid until it is struck on the bench.
This method is generally not advised as it can cause unnoticed solder bridges on other parts of the PCB and could damage delicate components or the PCB. In the absence of other tools, however, it can be used to good effect.
Resistors and Capacitors
These are by far the easiest SMT components to desolder. First use solder braid to remove most of the solder from each end of the component. Then, heat each end alternately for about a second at a time until the component moves off its pads. It is wise to have a small screwdriver or pick ready to remove the component from the soldering iron in case the remaining solder causes it to stick there due to surface tension. This is quite common, as SMT components weigh very little.
ICs with two rows of pins
This method, called the "blob method," will only work with IC with a row of pins on two sides, for example SOICs, etc.
These are the next-easiest components to remove. Add solder to each row of pins until you have two long blobs of solder, one on each side of the IC. Now, melt each blob alternately, letting the soldering iron stay in the blob for just longer than it takes to melt. After a couple of alternations, the IC should slide off its pads. Once this happens, remove the iron and, if needed, clean up the leads of the IC and the PCB pads from which you just removed it with solder braid.
Large components like Quad Flat Packs (QFPs) need more drastic measures to desolder them. Since they can have over 300 pins distributed around four sides, they cannot be desoldered by the "blob method" above, as each side will cool before the rest has been melted.
The only viable way of desoldering QFPs and the like is a heat gun, which blows air hot enough to melt the solder over the device, melting all the connections at once.
Hot air from a heat gun may burn the PCB and component of interest instantly and can also melt connectors and other components if you do not use caution. Shield adjacent components by draping or wrapping aluminium foil over them.
If a large circuit board needs to be completely desoldered for any reason, consider using a small toaster oven (not a microwave oven). Set the temperature to about 180°C to 200°C, place the PCB assembly in the oven and wait for the solder to melt. Then, quickly remove as many components as you can, or, if you are in a hurry, turn the board over and tap it sharply on the backside. Repeat as many times as needed. There is no guarantee that all the components or the circuit board will emerge undamaged by repeated heatings, but it is a viable method, nonetheless.
- Do not use the same oven you use for food preparation. Most solders contain lead, a toxic heavy metal.
An electric hot plate is sometimes used in combination with the above techniques.
The hot plate set to the "pre-heat" temperature — a little below the melting point of the solder, around 100°C to 150°C (212°F to 302°F) — and allowed to stabilize before proceeding. The PCB assembly is then placed on the hot plate and warmed for a brief period. It then takes much less heat and time from the soldering iron to nudge the temperature above the melting point, using any of the above techniques
In the alternative, one can set the hot plate above the melting point of the solder, wait for all the solder to melt, then pull off each of the desired components using tweezers.
If you don't have a hot plate, you can try using a skillet on a gas or electric range, although temperature control may be more difficult. Do not use the same utensils you use for food preparation to avoid lead poisoning.