This section is going to look at Desoldering. Specifically, removing parts from an existing PCB Board. In the world of repair, or when scrounging parts, this is a topic that is just as important as soldering. Unfortunately, it is not covered many places, and you can find a lot of mis-information about desoldering. Over the years I have seen many, many, PCBs destroyed by improper desoldering techniques. Although there are a few different methods, what I will cover here is the techniques that I have found work best for me.
For our purposes, desoldering can be described as the process of removing existing solder from a printed circuit board (PCB) without damaging the part, or the board. This would naturally also incorporate removing the part physically from the board. In order to do the job correctly, you will need just a couple of tools. Namely, a properly sized soldering iron, and a desoldering pump, or Solder-Sucker. Additionally, we will cover the occasional use of desoldering braid.
The first thing you will need is a properly sized soldering iron. The goal, much the same as soldering, is to heat the joint up enough to melt the solder before the PCB gets hot enough to delaminate the circuit trace. For desoldering, you want an iron with enough tip mass to source a large amount of heat quickly. Use an iron with 25-35 watts of power with a tip about twice the mass of the one you would select for soldering the same connection. I find a 33 watt iron with a 1/4" to 5/16" chisel tip works well for most conventional through hole applications. Many people cause damage by trying to desolder with too SMALL an iron. I cannot stress this enough. You need a lot of heat, fast. Do not try to use your little delicate Antex iron to desolder!
The most important tool you need is a desoldering pump. This can be as fancy and expensive as a desoldering station with a built in vacuum pump. But, for most uses, a simple Solder Sucker pump is all that is needed. With some practice, you can do almost as quick a job with it (and just as good), as with the expensive station. My favorites by far are the Edsyn SoldaPullt FULL SIZE solder suckers. (At left, top down. A soldapullt Silverstat Model AS196 Anti-Static. A soldapullt model DS-017, and a Soldavac Model DS-101). Incidentally, do not even bother with the cute little miniature ones, they do not have nearly enough suction.
The first key to doing a good job is to keep your solder sucker effective by maintaining it! Clean it out frequently. These instructions are for a Soldapullt, but most are similar.
Begin by 'cocking' the plunger until it is ready for use. Then twist the top part till it unlatches, and remove the plunger assembly from the barrel. Clean all the old solder and grease from the piston area, and the 'O' ring seal. Shake the excess solder from the barrel, and clean out the tip. And don't forget to remove the ring of solder that will form at the bottom of the piston. After cleaning it, re-grease the plunger and 'O' ring with a silicon based grease. Make sure you use grease, not silicon rubber or heat sink compound!!! While there are many types of grease you can use, my favorite is silicon dielectric grease. It is the right consistency, and normally on hand. You can find the Silicone dielectric grease (at left is Permatex item# 22058) at an auto parts store.
Thoroughly wipe a small amount of grease on the 'O' ring, and reinstall the plunger. You can test the seal by holding your finger firmly over the tip and releasing the plunger. The plunger should stay down or move up VERY slowly. After a while you will get the 'feel' for a properly functioning solder sucker. Clean the solder sucker frequently, and re-grease as necessary, and it will do a good job for you.
A note about tips. The tip is designed to touch hot parts, as you will see later. Although it is likely made of Teflon, the tip will gradually melt and distort. While the tip does not have to be perfect, at some point you will need to replace it. Make sure you keep replacements on hand, and use your own judgement when the tip is no longer effective. On many solder suckers, the tip can be pushed out the end of the barrel by pressing down very firmly on the plunger. It will go past the point you normally push it to cock the device. This will push the tip assembly out. Then, simply push a new tip into the barrel. If any of this does not appear to work, read the instructions first!
While it may come in handy for some special situations, I do not routinely recommend the use of desoldering braid. Using braid often ends up damaging the PCB. Use the Solder Sucker instead. With that said, it still can be handy to have some braid on hand for occasional special uses. Be sure to get desoldering braid made for the purpose. At right is a 1.7meter dispenser spool from Multicore. Size AB. Solder wick is designed to wick solder very effectively. Despite some well-meaning advice to the contrary, do not use the braid from Coax, you will end up damaging the board. If, you are extremely desperate, and decide to use braid, make sure it is pure Copper, not steel, or an alloy, and get it from the smallest coax you can find (tighter weave).
We will start by desoldering a simple two lead component. 'Cock' your solder sucker by pushing the plunger down till it latches. Hold it in one hand ready to use, and your iron in the other. Place your iron so it makes good contact with the majority of the joint. If necessary melt a small amount of fresh solder on the tip to make a good thermal connection to the old joint. Heat the joint until the solder just begins to melt. Place the solder sucker tip on the joint, as close to the center as you can, and slightly overlapping your iron tip. Do not be afraid to touch the joint with the solder sucker. You need it as close as possible. Let the solder heat a short while longer (1 sec or so) and then push the release. The solder should be drawn up into the solder sucker. Remove your iron. The entire process should last about 2-3 seconds. No more.
See if all, or most, of the solder has been removed. If some solder is left, DO NOT try to remove the rest by re-heating the joint. Instead, let the joint cool, then properly RESOLDER the connection, and try again. Never 'work' the joint. The circuit trace will lift. Always allow time for the joint to cool between applications of heat. The idea is to heat only the metal and the solder, not the PCB or the glue that holds the trace.
If you have removed most of the solder, then use a small pick or a screwdriver to gently pry/push the lead away from the edge of the hole. It will often be up against one side of the PCB hole. Push the lead away from this point towards the center of the hole. You will most often feel a small click as it frees itself. At this point you should be able to remove the lead. If you need to, re-solder the joint, and try again. DO NOT force or pry the lead out. Do not continue heating the joint trying to get a small bit of solder out. Re-do it!
A note here about old solder... As solder ages over the years it crystallizes. This makes it much harder to melt. If you have an old joint, or board, it is often necessary to first solder the connections, adding a small amount of solder to the existing joint. This 'freshens' the solder and makes it much easier to remove. On an old board, I often start by re-soldering the component before trying to remove it. This does wonders.
When removing an IC, follow the same procedures. Stagger the leads you are desoldering to keep from concentrating the heat in one place. Suck the solder from all the leads, then use your pick to push the leads away from the sides. Most times an IC's leads will be against the side of the hole away from the body. Push the leads in towards the center of the chip. You will hear and feel the tiny click as the lead breaks free from the remaining solder in the hole. You may sometimes find one or two leads that need to be resoldered and the done again. Take your time, a little patience, and it will all end up with no damage.
Exceptions. Occasionally you will find a lead that defies all efforts to remove the solder. In this case after using the solder sucker, you can try your desoldering braid to remove the rest of the solder. Lay the braid carefully over the hole. Often sticking the component lead through the center of the braid works well. Then, put your iron on top of the braid. Heat the braid first. It should wick the remaining solder into itself. Never re-use a piece of braid that has absorbed any solder. Move to a fresh section. And be very careful to not overheat the joint. Occasionally it makes sense to heat the connection and pull the component away from the board while heating. Be very careful to not overheat, or over-pry the component. You will also still be faced with removing the solder from the hole to insert a new component. Finally, although I hate to do it, very occasionally an IC may be removed by clipping the leads first close to the body, and then removing each separate lead individually. This should rarely, if ever, be necessary, but be aware of the method.
Also be aware that there are special Iron tips available shaped to heat all the leads of an IC or other component at the same time, while pulling the component out. I have never found a use for these, and do not like the method, but they are available.