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Technical Documents - Documentos Técnicos: Electronics: Soldering and Desoldering Tools. The soldering iron.

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, and 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 and many PC technicians never touch a soldering iron, you might find one useful in some situations. The most common case is when physical damage to a system has occurred, such as when someone rips the keyboard connector off a motherboard by pulling on the cable improperly. Simple soldering skills can 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 being damaged by an external source. If a short circuit or static charge from an external device blows these fuses, the motherboard 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 and with the use of heatsinking devices clipped to the leads of the device being soldered. A heatsink is a small metal clip-on device designed to absorb excessive heat before it reaches the component the heatsink is protecting. In some cases, you can use a pair of hemostats as an effective heatsink when you solder a component.

To remove components soldered into place on a printed circuit board, you can use a soldering iron with a solder sucker. This device normally takes the form of 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. When the plunger retracts, it creates a momentary suction that draws the liquid solder away from the connection and leaves the component lead dry in the hole.

Always perform 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-mounted 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 abilities, 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 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 the board manufacturer produced by machine. Soldered joints that do not look clean can keep the component from making a good connection with the rest of the circuit. This “cold-solder joint” is normally created because you have not used enough heat. Remember that you should not practice your new soldering skills on the motherboard of a system 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 might 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 you apply 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 instantly melts 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.

 

 

 

 

 

 
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