Soldering is not too difficult when you understand what is going on, and how to do it correctly.

Before we pick up the soldering iron, lets discuss what we are doing, and why we do it, then we can watch a movie.

Step 1: Metallurgy

The first thing to understand is metallurgy of solder.

Solder is a mixture of 2 different metals, and the amounts of each metal in this alloy, and adding gold or silver, changes the properties of solder. Soldering is joining two pieces of metal together by adding a third (the third is actually an alloy in this case.) It is different than welding, welding you are making one piece of metal by joining two pieces of the same type of metal using a third peice of the same kind of metal.
The alloy we use in electronics soldering is called eutectic solder, and its an alloy of 63/37 percent lead/tin. Some eutectic solder may include 1% silver or gold. The reason we use this specific ratio of lead/tin is because it has very little eutectic state. Eutectic state is in between liquid, and solid, its a sort of "plasticy" state where the solder isnt quite liquid, and it isnt quite solid. The quicker the solder goes from liquid to solid, the less chance of something disturbing the solder joint. When the solder is cooling it must remain still while the metal transforms from liquid to solid, if there is any motion during this time while its not totally liquid, and not totally solid (aka eutectic), it will cause what is commonly known as a cold solder joint. The point of understanding eutectic solder, is that its still important to not disturb solder joints while they are cooling, even though its designed to make it harder to make bad solder joints.

Step 2: Oxides and Flux

Flux is another important big part of good soldering. but to understand why you need flux, you have to understand oxides. Oxides are formed when things are in direct contact with oxygen, heat accelerates oxides, but they can also form when parts are stored over a long period of time. In electronic manufacturing, old parts can be a real issue, but with hobby stuff, you can usually work around extra oxides, by using a little more flux, or by pre-tinning. When you heat metals, oxides form really fast, and you need to remove oxides or they will contaminate your solder, and prevent you from making a good electrical connection. The key is to remove oxides with flux while soldering. You need enough flux to cover the entire solder joint during the entire time that the solder is liquid. The flux helps remove oxides, but its real function is to keep oxides from forming during the soldering process. Sometimes you will come across parts that have a lot of oxides, you may use mechanical means to remove oxides before trying to solder. Pencil eraser, sandpaper, steel wool, or other abrasives may be required to get difficult oxides off, it can also save using a lot of flux, and ultimately make better solder joints.

Generally, you should use lot of flux, there are exception where you need to control the flow, but generally you want plenty of flux, for good wetting/flowing, and to prevent cold solder joints. You may need to add more flux if you dont get it right the first time. reheating it without fluxing is often a waste of time. There is such a thing as too much flux, but too much flux doesnt effect the quality, just how much you are wasting, and how much you have to clean when you are done soldering.

Oxides form on your soldering iron tip, and cleaning your tip will help you transfer heat better, because the oxides make an insulative barrier that prevents the the heat transfer, effectively making your soldering iron cooler, and harder to use. Those oxides can also contaminate your solder, making a bad solder joint. Its a good idea to get in the practice of cleaning your tip every time you pick up your iron. You can solder a whole row at a time, but each time you pick it up, clean, you clean the tip. Its also a good idea to put a little solder on the tip when you arent using it for a while. this helps reduce tip corrosion and makes the tip last longer.

Flow and wetting You want good flow from one piece of metal to the other, flux helps remove oxides, and when that happens the metal flows nicely from one to the other, and you should see good wetting on both parts. Wetting is the appearance of smooth shiny surface of the liquid solder, and it should completely cover the pads/lands and parts. Sometimes you have to move the iron around a little bit, to get good wetting. If you overheat the solder, it may become chunky, lumpy, dull and wont flow very well. Solder can be re-flowed many times, but each time you heat it, it starts to separate, and after several re-heatings it may get lumpy and chunky, and have dull finish. You can remove it, and add fresh solder if it gets too lumpy.

Step 3: Soldering

Soldering
If you have a tiny bit of solder on your tip, it helps conduct the heat from the iron to the part. Sometimes you want to use enough solder to solder the joint, but usually you use just enough for the iron to make good contact with the solder joint. Give the iron a second to heat up the part, then add solder until you have a nice fillet (inside curve). If you have too much solder, the solder joint will be convex (outside curve), sometimes excessive solder is tolerated, but rarely is insufficient solder, so its usually best to err toward excessive, if you must err. I personally try for minimal sufficient solder. I typically use the hottest iron I have, because I find that I get the best results by moving fast with a lot of heat. I get more consistent looking solder joints, and because im moving fast, but consistant. I have less thermal load duration, but more thermal shock, which means less overall heat, but it heats and cools very rapidly. You try to lay the tip in a way that gets the best contact, but you often have to move or wiggle it, to help transfer heat, and get the solder to flow nicely.

Tinning

Soldering LED cubes requires soldering relatively thick dense metal that takes a bit longer to heat than typical PCB soldering, and pre-tinning helps a lot, it means that most of the heating is done to the wire, but less heat when the more sensitive electronic components are involved. Copper, and zinc coated wires tin quite easily, but the steel music wire is more difficult to get good flow on. I start by sanding the wire to remove as many oxides as possible. using a lot of flux, and a lot of heat, and some scrubbing action with the soldering iron, you an get the solder to flow, but it takes some time and effort. If you tried soldering the LED without pre-tinning, you would probably melt the LED before you get all 4 good solder joints. You probably wont ever need to tin LED leads, they are almost always take solder well. Many parts are pre-tinned, but sometimes they sit for a long time before being used, and need to be re-tinned to keep the oxides from contaminating the solder, sometimes just adding more flux is good enough.

Tining insulated wires helps by causing the soft insulation to shrink back before you solder it, rather than after you solder it. After you strip and tin a couple wires, you will find that the insulation shrinks back exposing more wire that you started with. As you tin it, it will shrink back a bit more, but it will get to a point where it doesnt shrink much. You should be careful at this point to not push the wire through the softened insulation. After you shrink back the insulation, you may need to trim down the exposed, tinned lead. The idea amount of space between the insulation and solder joint is 1/2 the insulation width. with solder going right up to, but not under the insulation. If your insulated wire has many solder joints then you can cut many pieces of wire, or you may be able to strip a bunch off of the end of the wire, and strip bits of insulation, and slide them down the stripped end. When wiring the back side of of the charliecube64 circuit board, each wire goes from an micro-controller pin to 4 spire leads, so a total of 5 solder joints on one wire. I start by stripping about 1/2" off the end of the wire, then measure with the wire about how far i need insulation, cut the insulation at that spot, then pull the insulation down to about 1/8" from the end of the exposed wire. I measure the next segment, cut, and slide it down, and repeat this process until I have each insulation segment about right, and twist and tin the wire between the insulation. The insulation will shrink up when tinning, so try to compensate when cutting segments to get the best results.

Soldering

How much solder you should have has always been questionable. Typically you want leaner solder joints with an nice fillet, or concave curve between the lead and pad. If you have convex curve on your solder joint, that is typically considered excessive solder, but electrically its no different, however excessive solder joints are more prone to shorting with other nearby excessive solder joints. The most accepted technique for soldering is to put the iron on the joint touching both surfaces with the iron, and touching the solder to the opposite side of the solder joint (being careful not to touch the solder to the iron), and waiting till the far side of the joint melts the solder, and it flows across the whole joint, then lifting the iron away. This is a good technique because it assures that the whole joint is up to temp before the solder flows. Using this technique, it makes it harder to make a bad solder joint, but its not the only way, and its not the best for every situation, but it is the best way for most people, most of the time,
If your parts are pre-tinned, then you can put them together, add a bit of flux, clean your tip, put just as much solder as you want on your tip, and touch the tip to the joint to flow the solder from your tip to the joint. This technique removes the adding solder after your parts are up to temp, so you really have to watch closely to see that the solder continues to flow and heat the area. If you remove the iron before the solder joint is up to temp, it will make a cold solder joint, so its a bit trickier than the standard method. What I find works best for me, is to keep the heat long enough to get good all around wetting, then remove the iron, and blow on the solder joint to help it cool. There is another technique, its kind of frowned upon, but when done properly, you can get good results. what you do is after heating the solder joint, you feed solder in while pulling the iron away. It will make the solder cool faster, but you risk making a cold solder joint. When you solder all day long, every day, you get to know how much solder to feed, and when to pull the iron/ You really gotta pay attention to what your are doing, or you may make bad solder joints. When you are working with printed circuit boards, you can preheat your next solder joint with the back part of the tip, and solder faster, and more consistently. This is called drag soldering, and its a very common technique for doing surface mount chips and through hole IC, connectors, and other parts with long row of leads to solder. When doing this type of soldering, lots of flux, and lots of heat yield the nicest, and fastest results. Sometimes its faster and easier to reflow a whole part, than to try to pull out a solder bridge with drag soldering.

Step 4: Surface Mount Vs Through Hole

I hear a lot of people complaining that surface mount soldering is more difficult than through hole. I have to disagree.

Surface mount soldering is so very well engineered that the solder provides surface tension, pulling the solder to the entire surface, and aligning the part at the same time. A good pair of tweezers helps a lot with SMT.

Hot air is also a great way to do surface mount work, especially repair work, where you have to remove a part with a lot of leads. Hot air is also a great way to completely de-laminate and burn up a printed circuit board, be careful!

Sometimes people use a hotplate for SMT reflow, which seems to work very good for many folks.

If you are using a technique that heats up a large area, like with hot air, or hotplate, a simple and very effective way to get parts to line up is to gently tap the circuit board, this helps the surface tension pull the parts into alignment. You can perfectly align hundreds of parts with one gentle tap with your tweezers or spudger.

With surface mount repair, dont hesitate to bring in a second iron, it make things go much faster when you heat both leads at the same time, and you get very nice looking results, and that matters to anyone that has to inspect your work.

Step 5: Lets Watch a Movie!

Heres a nice video that covers the basics very well. This video is
over 30 years old, it doesnt really cover lead-free soldering, but it does a very good job of illustrating and explaining good soldering techniques, and information, even if its a bit out of date, and has weird music.

After re-reading this instructable, and looking over related instructables, It appears that I should probably put a lot more detail and content in. That will take more time than I want to spend right now. I will probably start a new document when I have more time for it.