6
LET’S SOLDER!

In Chapter 5, you used a breadboard to build new circuits. A breadboard lets you quickly test new ideas and experiment with different components to see the result, but it isn’t a permanent solution. Components can easily fall out, and there can be loose wires all over the breadboard. When you want to use a circuit for a long time without changing it, it’s better to solder the circuit to a circuit board. Soldering is kind of like gluing: you melt a material called solder onto the component legs so the components stick to the circuit board. A circuit board has holes like a breadboard, and when you solder components to it, they become connected by plates of copper.

This chapter will teach you how to solder, starting with a simple LED circuit. This will give you the foundation you need to solder your own circuits later. After you’ve practiced the basics in this chapter, you’ll continue soldering in Chapter 7 to build a touch-enabled switch and a circuit that wakes you up when the sun rises in the morning.

HOW TO SOLDER

To solder a circuit, you need a few tools:

Solder

A soldering iron

A stand to hold the soldering iron

A damp sponge

Safety goggles

Solder is a mix of metals that can easily melt at high temperatures. The most commonly used solder melts at around 360 to 370 degrees Fahrenheit.

A soldering iron is a pen-shaped tool that heats up to a temperature high enough to melt solder. If you put solder on the tip of the soldering iron, the solder melts.

Soldering is quite easy once you learn the basics, and one important basic step is to take a few safety precautions.

Soldering Safety Tips

Soldering irons get extremely hot, and you can burn yourself if you touch the metal part of a soldering iron while it’s plugged in. Always return the soldering iron to its stand when you’re not using it; never lay it directly on a surface. Avoid touching joints (soldered connections) and components for a few seconds after you solder them, too. They can get hot enough to burn.

Here are some other important safety tips to keep in mind while soldering:

Keep the hot parts of the soldering iron away from the power cord.

If you’re soldering on a table, protect the surface with a piece of wood or some thick cardboard.

Always wash your hands after handling solder.

If you do get burned, don’t panic. For a minor burn, immediately cool the burn with running cold water, keeping the burned area under water for at least five minutes. Putting ice on a burn is good, too, but make sure to do the initial cooling with water right away.

Ask an adult to supervise your first few attempts at soldering, and remember: soldering is fun, but soldering irons must still be used with great care. With those tips in mind, read the rest of this section for a step-by-step guide to basic soldering.

Heat the Soldering Iron

The first step of soldering is to plug in your soldering iron and place it on your stand. Don’t forget to put on your safety goggles!

After a minute or two, check whether the iron is hot enough by touching some solder to the tip of the iron. If the solder melts, your iron is ready to go.

Clean the Soldering Iron Tip

Touch the tip of your soldering iron to a damp sponge to clean it. A clean tip transfers heat much better than a dirty one, so clean the tip often.

Tin the Soldering Iron Tip

Here’s a little trick: to heat the joint faster, add a bit of solder to the tip of the soldering iron just before you start soldering. This is called tinning. This must be done just a second or two before soldering to be effective.

Heat Both the Pin and the Pad

Place the tip of the soldering iron onto both the component leg and the copper strip. Heat the strip and leg for a couple of seconds before you move on to the next step.

Add Solder

While keeping the soldering iron tip on the joint, touch some solder to the leg and the copper strip. As the solder melts, slowly add more until there’s just enough to coat both the leg you’re soldering and the copper strip you want to connect it to. When you have enough melted solder, remove the solder wire from the joint while still keeping the soldering iron tip on the joint.

Remove the Soldering Iron

Finally, remove the soldering iron tip from the joint and place the iron in its stand. Always do this last. If you remove the iron while the solder wire is touching the solder joint, the solder wire may get stuck to the circuit board when the solder joint hardens.

Your solder joint should have a cone shape.

If you’re done soldering, then unplug the iron so it can cool down.

Watch Out for Bad Solder Joints!

It’s important to heat both the component leg and the copper strip with the iron before touching solder to either. When you heat only the component leg and not the copper strip, the solder sticks to the leg, but there’s no connection between the solder and the copper strip. If only the strip is heated, the solder will stick to the strip, but not the leg. It could look like a good soldering joint from a distance, but the solder probably won’t be connected to the leg.

You must also make sure the solder doesn’t float over to another copper strip next to it. This will create an unintended connection between the two strips.

If your solder joint doesn’t look right, don’t worry. Just reheat the joint, the component leg, and the copper strip and add more solder to achieve the cone shape of a good solder joint. Then, you should be good to go.

PROJECT #12: SOLDER YOUR FIRST LED CIRCUIT

Now, let’s get soldering! In this project, you’ll solder the resistor and LED circuit from “Project #8: Powering an LED” on page 78 to a circuit board. The battery should make current flow in the circuit, the resistor should make sure there isn’t too much current, and the LED should light up.

Shopping List

A standard 9 V battery to power the circuit.

A 9 V battery clip (Jameco #11280, Bitsbox #BAT033) to connect the battery to the circuit.

A circuit board (Jameco #2191488, Bitsbox #HW005) with copper strips.

A standard LED (Jameco #34761, Bitsbox #OP003)

A 330 Ω resistor (Jameco #661386, Bitsbox #CR25330R) for limiting the current to the LED.

Tools

A soldering iron (for example, Jameco #116572, Bitsbox #TL031)

A stand (for example, Jameco #36329, Bitsbox #TL032) to hold the soldering iron.

A roll of standard solder wire (for example, Jameco #94570, Bitsbox #HW022)

A wet sponge to clean the tip of the soldering iron.

A wire cutter (Jameco #35482, Bitsbox #TL008) to cut off excess legs from components.

Step 1: Place the Components

Look at your circuit board and get familiar with the connection pattern on it. The circuit board I recommend is called a prototyping board, and it’s covered with little straight strips of copper, each with lots of holes. Any component leads that you solder into holes that share a strip of copper will become electrically connected.

Place your resistor and LED legs through the circuit board. The resistor should share a copper strip with the LED’s positive lead. Note that the strips shown in the figure are on the bottom side of the board, which is why they’re not copper colored in the picture. Anytime a project involves soldering to a circuit board, I’ll show shadows of the copper strips so you can see where the connections are underneath.

Step 2: Bend the Component Legs

Carefully flip your circuit board over so you can see the copper; hold the LED and resistor in place with a finger if needed so they don’t fall out. Then, slightly bend each component’s legs outward so that the LED and resistor stay in place even with the copper side up. Keep the board copper-side-up for now.

Step 3: Heat and Clean the Soldering Iron

Your components should be in place and ready to solder, so plug in and heat up the iron now. This may take a few minutes. As always, check to see whether the iron is hot enough by touching some solder to the tip to see whether it melts.

Before you start soldering, clean the tip of the soldering iron, too. Wet your sponge just a little bit and wring out any excess water. Then, wipe the tip of the iron on the sponge to remove any old solder.

Step 4: Solder the Resistor and LED

With the copper side of the circuit board facing up, solder each component leg to the board, as described in “How to Solder” on page 112. Your board should look like this:

Step 5: Trim the Legs

Right now, your LED and resistor have really long legs. You don’t want to leave them sticking out because stray metal can create unintentional paths between components. At best, that could prevent your circuit from working until you move the leg away from the component it shouldn’t be touching; at worst, an unintended connection could break a component. To prevent accidental connections, cut off each leg just above the solder joint. The legs could go flying when you cut them, so turn the board away from your eyes or wear safety goggles for protection.

Step 6: Solder the Battery Clip

This circuit also needs a way to connect the battery to the components, so solder the battery clip to the circuit board now.

The red wire from the battery clip is the positive wire; connect it to the copper strip that connects only to the resistor, not the LED. The black wire is the negative lead, so connect it to the negative side of the LED. Your circuit board should now look like this:

Step 7: Let There Be Light!

Now, let’s test your circuit. Plug in the battery, and your LED should light up!

Step 8: What If the Soldered LED Circuit Doesn’t Work?

If your LED doesn’t light, check that you don’t have any unintentional connections. Is solder connecting two joints that shouldn’t connect? Are extra long component legs making contact?

Next, inspect your solder joints closely. Do any of them look like the examples of bad connections described in “Watch Out for Bad Solder Joints!” on page 117? If so, you might need to flow a little more solder: warm up your iron again and try to make sure your solder joints look like little pyramids.

Check the placement of the LED, too. Did you connect it correctly? Because you’ve cut off the excess legs, you can’t see which leg is longer anymore, but if you look closely at the LED, you should see that one side of the base of the plastic housing is flattened. This is the negative side, which should be connected to the battery clip’s black wire. If you’ve placed it the wrong way, here’s one quick fix: use your wire cutters to cut the battery clip leads, removing the clip from the circuit board. Now, solder the positive battery lead where the negative lead used to be, and solder the negative battery lead where the positive lead used to be. This works because it doesn’t matter which side of the LED the resistor is on.

OOPS! HOW DO I REMOVE A SOLDERED COMPONENT?

Sometimes, you’ll make mistakes when soldering. For example, what if you soldered the battery clip the wrong way or used a 33,000 Ω resistor instead of a 330 Ω one? Well, don’t worry: even the most experienced engineers mess up at soldering sometimes, and there’s a way to fix your circuit when it happens. In these cases, you need to desolder, which just means to remove the solder from a solder joint.

Solder wick is a very useful tool for desoldering, and I suggest keeping some at your side any time you solder. The solder wick is made of braids of copper thread, so it’s also called desoldering braid.

When you place a piece of solder wick on top of a solder joint and heat both together, the solder should melt as usual, and the solder wick should absorb the liquid solder just as a dry cloth absorbs water. When you remove the solder wick, the solder should be on the wick instead of on the circuit board!

PROJECT #13: DESOLDER THE BATTERY CLIP

Knowing how to desolder is very useful, and not just for fixing soldering mistakes. For example, desoldering will let you exchange a broken component in a circuit for a fresh one—or you can desolder a component to reuse it in another circuit. In this project, I’ll show you how to desolder your battery clip from the previous project.

Shopping List

The circuit from “Project #12: Solder Your First LED Circuit” on page 118

Tools

A soldering iron (for example, Jameco #116572, Bitsbox #TL031)

A stand (for example, Jameco #36329, Bitsbox #TL032) to hold the soldering iron.

A wire cutter (Jameco #35482, Bitsbox #TL008) to cut the solder wick.

Solder wick (Jameco #153462, Bitsbox #HW082) to remove the solder.

Step 1: Heat the Soldering Iron

Plug in your soldering iron and wait for it to heat. To test the temperature, touch some solder wire to the tip; the solder will melt when the iron is warm enough.

Step 2: Place the Solder Wick on the Solder Joint

Lay the end of your solder wick on top of the solder joint for one battery clip wire.

Step 3: Heat the Solder Joint and the Solder Wick

Place the tip of the heated soldering iron on top of the solder wick, directly over the solder joint you want to desolder.

After a few seconds, the solder should melt and flow onto the braid. Lift the braid from the board, together with the soldering iron.

Step 4: Trim the Used Solder Wick

Remove the braid from the joint and look at it. The braid should have solder in it.

Much like a dirty cloth, this bit of wick is too dirty to use anymore. Use your wire cutter to cut off the piece that has solder inside.

Now, inspect your solder joint. If there’s no solder left connecting the component leg to the circuit board, then you should be able to remove the wire from the hole. If not, repeat Steps 2 and 3 with a fresh bit of solder wick, and then repeat Step 3 until you can remove the wire.

Step 5: Remove the Other Battery Clip Wire

Repeat Steps 2 to 4 for the other battery clip wire, and you should be able to completely remove the battery clip from the circuit board. At this point, the copper side of your circuit board should look something like this: