Soldering and Desoldering Tools
In certain situations–such as repairing a broken wire, making cables, reattaching a component to a circuit board, removing and installing chips that are not in a socket, or adding jumper wires or pins to a board–you must use a soldering iron to make the repair.
Although virtually all repairs these days are done by simply replacing the entire failed board, you may need a soldering iron in some situations. The most common case would be where there was physical damage to a system, such as where somebody had ripped the keyboard connector off of a motherboard by pulling on the cable improperly. Simple soldering skills could save the motherboard in this case.
Most motherboards these days include I/O components such as serial and parallel ports. Many of these ports are fuse-protected on the board; however, the fuse is usually a small soldered-in component. These fuses are designed to protect the motherboard circuits from damage from an external source. If a short circuit or static charge from an external device blows these fuses, the board can be saved if you can replace them.
To perform minor repairs such as these, you need a low-wattage soldering iron–usually about 25 watts. More than 30 watts generates too much heat and can damage the components on the board. Even with a low-wattage unit, you must limit the amount of heat to which you subject the board and its components. You can do this with quick and efficient use of the soldering iron, as well as with the use of heat-sinking devices clipped to the leads of the device being soldered. A heat sink is a small metal clip-on device designed to absorb excessive heat before it reaches the component that the heat sink is protecting. In some cases, you can use a pair of hemostats as an effective heat sink when you solder a component.
To remove components that originally were soldered into place from a printed circuit board, you can use a soldering iron with a solder sucker. This device normally is constructed as a small tube with an air chamber and a plunger-and-spring arrangement. (I do not recommend the squeeze-bulb type of solder sucker.) The unit is “cocked” when you press the spring-loaded plunger into the air chamber. When you want to remove a device from a board, you use the soldering iron from the underside of the board, and heat the point at which one of the component leads joins the circuit board until the solder melts. As soon as melting occurs, move the solder-sucker nozzle into position, and press the actuator. This procedure allows the plunger to retract and creates a momentary suction that inhales the liquid solder from the connection and leaves the component lead dry in the hole.
Always do the heating and suctioning from the underside of a board, not from the component side. Repeat this action for every component lead joined to the circuit board. When you master this technique, you can remove a small component in a minute or two with only a small likelihood of damage to the board or other components. Larger chips that have many pins can be more difficult to remove and resolder without damaging other components or the circuit board.
TIP: These procedures are intended for Through-Hole devices only. These are components whose pins extend all the way through holes in the board to the underside. Surface mount devices are removed with a completely different procedure, using much more expensive tools. Working on surface-mounted components is beyond the capabilities of all but the most well-equipped shops.
If you intend to add soldering and desoldering skills to your arsenal of capabilities, you should practice. Take a useless circuit board and practice removing various components from the board, then reinstall the components. Try to remove the components from the board by using the least amount of heat possible. Also, perform the solder-melting operations as quickly as possible, limiting the time that the iron is applied to the joint. Before you install any components, clean out the holes through which the leads must project and mount the component in place. Then apply the solder from the underside of the board, using as little heat and solder as possible.
Attempt to produce joints as clean as the joints that the board manufacturer performed by machine. Soldered joints that do not look clean may keep the component from making a good connection with the rest of the circuit. This “cold-solder joint” normally is created because you have not used enough heat. Remember that you should not practice your new soldering skills on the motherboard of a system that you are attempting to repair! Don’t attempt to work on real boards until you are sure of your skills. I always keep a few junk boards around for soldering practice and experimentation.
TIP: When first learning to solder, you may be tempted to set the iron on the solder and leave it there until the solder melts. If the solder doesn’t melt immediately when applying the iron to it, you’re not transferring the heat from the iron to the solder efficiently. This means that either the iron is dirty, or there is debris between it and the solder. To clean the iron, take a wet sponge and drag it across the tip of the iron.
If after cleaning the iron there’s still some resistance, try to scratch the solder with the iron when it’s hot. Generally, this removes any barriers to heat flow and will instantly melt the solder.
No matter how good you get at soldering and desoldering, some jobs are best left to professionals. Components that are surface-mounted to a circuit board, for example, require special tools for soldering and desoldering, as do other components that have high pin densities.
I upgraded an IBM P75 portable system by replacing the 486DX-33 processor with a 486DX2-66 processor. This procedure normally would be simple (especially if the system uses a ZIF socket), but in this particular system, the 168-pin 486DX chip was soldered into a special processor card. To add to the difficulty, there were surface-mounted components on both sides of the card–even the solder side.
Needless to say, this was a very difficult job that required a special piece of equipment called a hot air rework station. The hot air rework station uses blasts of hot air to solder or desolder all of the pins on a chip simultaneously. To perform this replacement job, the components on the solder side of the board were protected with special heat-resistant masking tape, while the hot air was directed at the 168 pins of the 486 chip, allowing it to be removed. Then the replacement chip was inserted into the holes in the board, a special solder paste was applied to the pins, and the hot air was used again to solder all 168 pins simultaneously.
The use of professional equipment such as this resulted in a perfect job that cannot be told from the factory original. Attempting a job like this with a conventional soldering iron probably would have damaged the expensive processor chips, as well as the even more expensive multilayer processor card.