Step 19: Build the cube: soldering advice
You are going to be soldering VERY close to the LED body, and you are probably going to be using really cheap LEDs from eBay. LEDs don't like heat, cheap LEDs probably more so than others. This means that you have to take some precautions in order to avoid broken LEDs.
Soldering iron hygiene
First of all, you need to keep your soldering iron nice and clean. That means wiping it on the sponge every time you use it. The tip of your soldering iron should be clean and shiny. Whenever the you see the tip becoming dirty with flux or oxidizing, that means loosing it's shinyness, you should clean it. Even if you are in the middle of soldering. Having a clean soldering tip makes it A LOT easier to transfer heat to the soldering target.
When soldering so close to the LED body, you need to get in and out quickly. Wipe your iron clean. Apply a tiny amount of solder to the tip. Touch the part you want to solder with the side of your iron where you just put a little solder. Let the target heat up for 0.5-1 seconds, then touch the other side of the target you are soldering with the solder. You only need to apply a little bit. Only the solder that is touching the metal of both wires will make a difference. A big blob of solder will not make the solder joint any stronger. Remove the soldering iron immediately after applying the solder.
Mistakes and cool down
If you make a mistake, for example if the wires move before the solder hardens or you don't apply enough solder. Do not try again right away. At this point the LED is already very hot, and applying more heat with the soldering iron will only make it hotter. Continue with the next LED and let it cool down for a minute, or blow on it to remove some heat.
We recommend using a thin solder for soldering the LEDs. This gives you a lot more control, and enable you to make nice looking solder joints without large blobs of solder. We used a 0.5 mm gauge solder. Don't use solder without flux. If your solder is very old and the flux isn't cleaning the target properly, get newer solder. We haven't experienced this, but we have heard that it can happen.
Are we paranoid?
When building the 8x8x8 LED Cube, we tested each and every LED before using it in the cube. We also tested every LED after we finished soldering a layer. Some of the LEDs didn't work after being soldered in place. We considered these things before making a single solder joint. Even with careful soldering, some LEDs were damaged.
The last thing you want is a broken LED near the center of the cube when it is finished. The first and second layer from the outside can be fixed afterwards, but any further in than that, and you'll need endoscopic surgical tools ;)