Make: Electronics

by Charles Platt

Millions of people have learned how to do it, and statistically, you are unlikely to be less coordinated than all of them. I have a lifelong problem with a tremor in my hands that makes it difficult for me to hold small things steadily. I also get impatient with repetitive detail work. If I can solder components, almost anyone should be able to.

  Myth #2: Soldering involves poisonous chemicals.

  Modern solder contains no lead. You should avoid inhaling the fumes for prolonged periods, but that also applies to everyday products such as bleach and paint. If soldering was a significant health hazard, we should have seen a high death rate among electronics hobbyists decades ago.

  Myth #3: Soldering is hazardous.

  A soldering iron is less hazardous than the kind of iron that you might use to iron a shirt, because it delivers less heat. In fact, in my experience, soldering is safer than most activities in a typical home or basement workshop. That doesn’t mean you can be careless!

  Soldering alternatives

  As recently as the 1950s, connections inside electronic appliances such as radio sets were still being hand-soldered by workers on production lines. But the growth of telephone exchanges created a need for a faster way to make large numbers of rapid, reliable point-to-point wiring connections, and “wire wrap” became a viable alternative.

  In a wire-wrapped electronics project, components are mounted on a circuit board that has long, gold-plated, sharp-cornered square pins sticking out of the rear. Special silver-plated wire is used, with an inch of insulation stripped from its ends. A manual or power-driven wire-wrap tool twirls the end of a wire around one of the pins, applying sufficient tension to “cold-weld” the soft silver plating of the wire to the pin. The wrapping process exerts sufficient pressure to make a very reliable joint, especially as 7 to 9 turns of wire are applied, each turn touching all four corners of the pin.

  During the 1970s and 1980s, this system was adopted by hobbyists who built their own home computers. A wire-wrapped circuit board from a hand-built computer is shown in Figure 3-37. The technique was used by NASA to wire the computer in the Apollo spacecraft that went to the moon, but today, wire-wrapping has few commercial applications.

  The widespread industrial use of “through-hole” components, such as the chips on early desktop computers, encouraged development of wave soldering, in which a wave or waterfall of molten solder is applied to the underside of a preheated circuit board where chips have been inserted. A masking technique prevents the solder from sticking where it isn’t wanted.

  Today, surface-mount components (which are significantly smaller than their through-hole counterparts) are glued to a circuit board with a solder paste, and the entire assembly is then heated, melting the paste to create a permanent connection.

  Figure 3-37. This picture shows some of the wire-wrapping in Steve Chamberlin’s custom-built, retro 8-bit CPU and computer. “Back in the day,” connecting such a network of wires with solder joints would have been unduly time-consuming and prone to faults. Photo credit: Steve Chamberlin.

  Tools

  Eight most common soldering errors

  1. Not enough heat.

  The joint looks OK, but because you didn’t apply quite enough heat, the solder didn’t melt sufficiently to realign its internal molecular structure. It remained granular instead of becoming a solid, uniform blob, and you end up with a “dry joint,” also known as a “cold joint,” which will come apart when you pull the wires away from each other. Reheat the joint thoroughly and apply new solder.

  A leading cause of underheated solder is the temptation to use the soldering iron to carry solder to the joint. This results in the cold wires reducing the temperature of the solder. What you should do is touch the soldering iron to heat the wires first, and then apply the solder. This way, the wires are hot and help to melt the solder, which wants to stick to them.

  Because this is such a universal problem, I’ll repeat myself: Never melt solder on the tip of the iron and then use it to carry the solder to the joint.

  You don’t want to put hot solder on cold wires. You want to put cold solder on hot wires.

  2. Too much heat.

  This may not hurt the joint, but can damage everything around it. Vinyl insulation will melt, exposing the wire and raising the risk of short circuits. You can easily damage semiconductors, and may even melt the internal plastic components of switches and connectors.

  Damaged components must be desoldered and replaced, which will take time and tends to be a big hassle (see “Tools: Desoldering” on page 109 for advice).

  3. Not enough solder.

  A thin connection between two conductors may not be strong enough. When joining two wires, always check the underside of the joint to see whether the solder penetrated completely.

  4. Moving the joint before the solder solidifies.

  You may create a fracture that you won’t necessarily see. It may not stop your circuit from working, but at some point in the future, as a result of vibration or thermal stresses, the fracture can separate just enough to break electrical contact. Tracking it down will then be a chore. If you clamp components before you join them, or use perforated board to hold the components steady, you can avoid this problem.

  5. Dirt or grease.

  Electrical solder contains rosin that cleans the metal that you’re working with, but contaminants can still prevent solder from sticking. If any component looks dirty, clean it with fine sandpaper before joining it.

  6. Carbon on the tip of your soldering iron.

  The iron gradually accumulates flecks of black carbon during use, and they can act as a barrier to heat transfer. Wipe the tip of the iron on the little sponge mounted in the base of your soldering iron stand or your helping hand.

  7. Inappropriate materials.

  Electronic solder is designed for electronic components. It will not work with aluminum, stainless steel, or various other metals. You may be able to make it stick to chrome-plated items, but only with difficulty.

  8. Failure to test the joint.

  Don’t just assume that it’s OK. Always test it, by applying manual force if you can (see Figures 3-38 and 3-39 for the ideal protocol) or, if you can’t get a grip on the joint, slip a screwdriver blade under it and flex it just a little, or use small pliers to try to pull it apart. Don’t be concerned about ruining your work. If your joint doesn’t survive rough treatment, it wasn’t a good joint.

  Of the eight errors, dry/cold joints are by far the worst, because they are easy to make and can look OK.

  Figure 3-38. Test result of a bad solder joint.

  Figure 3-39. Test result of a good solder joint.

  Your Second Solder Joint

  Time now to try your pencil-style soldering iron. Once again, you must leave it plugged in for a good five minutes to make sure it’s hot enough. In the meantime, don’t forget to unplug your other soldering iron, and put it somewhere safe while it cools.

  This time I’d like you to align the wires parallel with each other. Joining them this way is a little more difficult than joining them when they cross each other, but it’s a necessary skill. Otherwise, you won’t be able to slide heat-shrink tubing over the finished joint to insulate it.

  Figures 3-40 through 3-44 show a successful joint of this type. The two wires do not have to make perfect contact with each other; the solder will fill any small gaps. But the wires must be hot enough for the solder to flow, and this can take an extra few seconds when you use the low-wattage pencil-style iron.

  Be sure to feed the solder in as shown in the pictures. Remember: don’t try to carry the solder to the joint on the tip of the iron. Heat the wires first, and then touch the solder to the wires and the tip of the iron, while keeping it in contact with the wires. Wait
until the solder liquifies, and you will see it running eagerly into the joint. If this doesn’t happen, be more patient and apply the heat for a little longer.

  Figure 3-40.

  Figure 3-41.

  Figure 3-42.

  Figure 3-43. This and the preceding three figures show how joining two wires that are parallel is more difficult, and the low-wattage, pencil-type soldering iron will require longer to heat them sufficiently for a good joint. Thinner solder can be used.

  Figure 3-44. The finished joint has enough solder for strength, but not so much solder that it will prevent heat-shrink tubing from sliding over it.

  Tools

  Desoldering

  Desoldering is much, much harder than soldering. Two simple tools are available:

  Suction pump. First, you apply the soldering iron to make the solder liquid. Then you use this simple gadget to try to suck up as much of the liquid as possible. Usually it won’t remove enough metal to allow you to pull the joint apart, and you will have to try the next tool. Refer back to Figure 3-10.

  Desoldering wick or braid. Desoldering wick, also known as braid, is designed to soak up the solder from a joint, but again, it won’t clean the joint entirely, and you will be in the awkward position of trying to use both hands to pull components apart while simultaneously applying heat to stop the solder from solidifying. Refer back to Figure 3-11.

  I don’t have much advice about desoldering. It’s a frustrating experience (at least, I think so) and can damage components irrevocably.

  Theory

  Soldering theory

  The better you understand the process of soldering, the easier it should be for you to make good solder joints.

  The tip of the soldering iron is hot, and you want to transfer that heat into the joint that you are trying to make. In this situation, you can think of the heat as being like a fluid. The larger the connection is between the soldering iron and the joint, the greater the quantity of heat, per second, that can flow through it.

  For this reason, you should adjust the angle of the soldering iron so that it makes the widest possible contact. If it touches the wires only at a tiny point, you’ll limit the amount of heat flow. Figures 3-45 and 3-46 illustrate this concept. Once the solder starts to melt, it broadens the area of contact, which helps to transfer more heat, so the process accelerates naturally. Initiating it is the tricky part.

  The other aspect of heat flow that you should consider is that it can suck heat away from the places where you want it, and deliver it to places where you don’t want it. If you’re trying to solder a very heavy piece of copper wire, the joint may never get hot enough to melt the solder, because the heavy wire conducts heat away from the joint. You may find that even a 40-watt iron isn’t powerful enough to overcome this problem, and if you are doing heavy work, you may need a more powerful iron.

  As a general rule, if you can’t complete a solder joint in 10 seconds, you aren’t applying enough heat.

  Figure 3-45. With only a small surface area of contact between the iron and the working surface, an insufficient amount of heat is transferred.

  Figure 3-46. A larger area of contact between the soldering iron and its target will greatly increase the heat transfer.

  Adding Insulation

  After you’ve succeeded in making a good inline solder connection between two wires, it’s time for the easy part. Choose some heat-shrink tubing that is just big enough to slide over the joint with a little bit of room to spare.

  Figure 3-47. Other members of your family should understand that although a heat gun looks like a hair dryer, appearances may be deceptive.

  Heat Guns Get Hot, Too!

  Notice the chromed steel tube at the business end of your heat gun. Steel costs more than plastic, so the manufacturer must have put it there for a good reason—and the reason is that the air flowing through it becomes so hot that it would melt a plastic tube.

  The metal tube stays hot enough to burn you for several minutes after you’ve used it. And, as in the case of soldering irons, other people (and pets) are vulnerable, because they won’t necessarily know that the heat gun is hot. Most of all, make sure that no one in your home ever makes the mistake of using a heat gun as a hair dryer (Figure 3-47).

  This tool is just a little more hazardous than it appears.

  Slide the tubing along until the joint is centered under it, hold it in front of your heat gun, and switch on the gun (keeping your fingers away from the blast of superheated air). Turn the wire so that you heat both sides. The tubing should shrink tight around the joint within half a minute. If you overheat the tubing, it may shrink so much that it splits, at which point you must remove it and start over. As soon as the tubing is tight around the wire, your job is done, and there’s no point in making it any hotter. Figures 3-48 through 3-50 show the desired result. I used white tubing because it shows up well in photographs. Different colors of heat-shrink tubing all perform the same way.

  Figure 3-48. Slip the tubing over your wire joint.

  Figure 3-49. Apply heat to the tubing.

  Figure 3-50. Leave the heat on the tubing until it shrinks to firmly cover the joint.

  I suggest you next practice your soldering skills on a couple of practical projects. In the first one, you can add color-coded, solid-core wires to your AC adapter, and in the second one, you can shorten the power cord for a laptop power supply. You can use your larger soldering iron for both of these tasks, because neither of them involves any heat-sensitive components.

  Modifying an AC Adapter

  In the previous chapter, I mentioned the irritation of being unable to push the wires from your AC adapter into the holes of your breadboard. So, let’s fix this right now:

  1. Cut two pieces of solid-conductor 22-gauge wire—one of them red, the other black or blue. Each should be about 2 inches long. Strip a quarter-inch of insulation from both ends of each piece of wire.

  2. Trim the wire from your AC adapter. You need to expose some fresh, clean copper to maximize your chance of getting the solder to stick.

  I suggest that you make one conductor longer than the other to minimize the chance of the bare ends touching and creating a short circuit. Use your meter, set to DC volts, if you have any doubt about which conductor is positive.

  Solder the wires and add heat-shrink tubing as you did in the practice session. The result should look like Figure 3-51.

  Figure 3-51. Solid-core color-coded wires, soldered onto the wires from an AC adapter, provide a convenient way to feed power to a breadboard. Note that the wires are of differing lengths to reduce the risk of them touching each other.

  Shortening a Power Cord

  When I travel, I like to minimize everything. It always annoys me that the power cord for the power supply of my laptop is 4 feet long. The thinner wire that connects the power supply to the computer is also 4 feet long, and I just don’t need that much wire.

  After searching exhaustively I couldn’t find any laptop power cables shorter than 3 feet, so I decided to shorten one myself. If you feel no need to do this, you should try the following procedure on an old extension cord, just as an exercise. You do need to go through these steps to acquire some practice in soldering heavier, stranded wire and using heat-shrink tubing:

  1. Use your wire cutters to chop the wire, and then a utility knife to split the two conductors, with one shorter than the other. When splicing a power cord or similar cable containing two or more conductors, it’s good to avoid having the joints opposite each other. They fit more snugly if they are offset, and there’s less risk of a short circuit if a joint fails.

  Figure 3-52.

  Figure 3-53.

  Figure 3-54.

  Figure 3-55.

 
Figure 3-56.

  Figure 3-57.

  2. Strip off a minimal amount of insulation. One-eighth of an inch (3 mm) is sufficient. The automatic wire strippers that I mentioned in the shopping list in Chapter 1 are especially convenient, but regular wire strippers will do the job.

  3. Cut two pieces of heat-shrink tubing, each 1 inch long, big enough to slide over the separate conductors in your cable. Cut a separate 2-inch piece of larger tubing that will slide over the entire joint when it’s done. The steps described so far are illustrated in Figures 3-52 through 3-58.

  Figure 3-58. Figures 3-52 through 3-58 illustrate the sequence of steps to prepare for making a shortened power cord for a laptop computer power supply.

  4. Now for the most difficult part: activating your human memory. You have to remember to slide the tubing onto the wire before you make your solder joint, because the plugs on the ends of the wires will prevent you from adding any heat-shrink tubing later. If you’re as impatient as I am, it’s very difficult to remember to do this every time.

  5. Use your helping hand to align the first joint. Push the two pieces of wire together so that the strands intermingle, and then squeeze them tight between finger and thumb, so that there are no little bits sticking out. A stray strand of wire can puncture heat-shrink tubing when the tubing is hot and soft and is shrinking around the joint.