Soldering Tips

Good soldering skills are imperative to the proper operation of a circuit. This is intended as a quick and handy guide to show people how to solder.

Tools and Materials

I really can't overstate how important a good soldering iron is. If you're using a cheap or underpowered soldering iron you'll end up with a lot of unreliable cold solder joints, which will hold for a while but fail when moisture causes corrosion inside the joint or shock/vibration mechanically loosens it. I have loads of soldering irons, but I have two favorites that I use for almost all of my projects. For most projects I use a big Craftsman 100/140 watt dual heat soldering gun (Sears model no. 113.540450). This type of iron heats up quickly and the gun shape makes it easy to handle. Most importantly, it provides plenty of heat to melt pretty much any type of solder. It's perfect for projects where you'll be using physical wires, speakers, tube amps, connectors, and general point-to-point wiring. My other favorite is a little 15 watt soldering pencil from Radio Shack (Cat #64-2051B). This one is great for smaller conductors that might be burned away by too much heat like the hair-thin wires inside tonearms and the thin copper foil  on PCBs. A small iron like this is essential when working with surface mount components, especially small SOIC, SOP, and TSOP integrated circuits.

Since solder doesn't just melt, but forms what is called a eutectic bond with the surface metal, it's important to choose a solder that's compatible with the metal you're trying to bond. Common electric solder is composed of 60% Sn and 40% Pb, usually referred to as 60/40. This type of solder is OK, but my personal favorite is 62% Sn, 36% Pb, 2% Ag. Adding a noble metal like silver increases the electrical as well as thermal conductivity, which seems to make the solder melt and flow more evenly, especially over metals other than copper, and imparts some corrosion resistance. Electronics work calls for fine solder with a diameter of around 0.022" to 0.032". A good solder I use often is Radio Shack #64-013 (62/36/2, rosin core, 0.022" dia), but if you're worried about the lead, then Sears Craftsman #980061 (96/4, flux core,  0.032" dia) is a great lead-free substitute.

Flux "wets" the metal and helps the solder flow more easily over the surface. Most solders these days use a flux core, usually rosin. For large joints, or joints where a good quality connection is extremely important you should consider using a brush-on flux as well. Radio Shack #64-022 is a good choice. Crusty black stuff called dross builds up on the tip of a soldering iron when it's hot, so you should have something to clean it off. Radio Shack sells a compound specifically for this (Cat #64-020) that works very well, but it's kind of pricey. If you don't want to spend the money, a dampened cellulose sponge or paper towel will do in a pinch.

You will invariably screw up some time, and for that you will need de-soldering equipment. The only thing you'll really need for this is a little solder sucker like Radio Shack #64-2086. Don't bother with fancy spring operated pumps and desoldering irons, that's really not necessary. If you need to de-solder something really small, like an integrated circuit, you may also want to consider a de-soldering braid (Radio Shack Cat #64-2090).

You should invest in a good set of  small pliers. At minimum you should have one pair of needle nose pliers and one wire cutter. Sears sells sets of these, with varying lengths of needle nose pliers, wire cutters, and a wire nipper for around $20. My favorite is the little spring loaded Craftsman Professional deal pictured at right, which has a built in wire cutter.

For general wiring, I suggest solid core "hookup wire". Radio Shack sells 100' spools in a variety of colors. Stranded wire is harder to use, but comes in handy for applications where vibration or movement over time might cause solid wire to snap. The best size for general elecronics use is around 22 AWG, for both solid and stranded wire. When a smaller wire is needed, you can remove the conductors from phone or network cable.

Soldering

The tip of the iron should be "tinned" by wetting it with a small amount of flux and solder, or the tinning compountd mentioned earlier. Always keep a small amount of solder on the tip of the iron, but wipe it off whenever too much of it builds up. "Tin" any stranded conductors by coating them with a small amount of solder to keep the strands in place.
Make a good physical connection.  For wires, twist them together. For components with leads, use small needle-nose pliers to wrap the leads around what your connecting it to. For things with solder lugs, thread the wire through the hole in the lug and wrap it around the lug at least once. When mounting components to PCBs, bend the leads outward.
Heat up the joint for a few seconds by holding the soldering iron in the same spot, don't move it around, just let the metal conduct the heat. Try to touch both wires with the tip of the iron at the same time. For small surface mount ICs, solder pin 1 first, then solder the pin in the opposite corner, then solder each pin individually.
Feed the solder in where the tip of the iron meets the conductor. The solder will melt and help conduct heat into the joint, allowing more solder to flow. Be careful not to heat components like capacitors and ICs any longer than necessary. Use the right size iron: too small and you'll be heating the joint for an extended period of time, too large and you'll overheat the component - too much in either direction and you're likely to damage something. Things like tube sockets and wires are unlikely to be damaged by any amount of heat.
For larger joints, move the iron slowly down the joint while continuing to feed in solder at a steady rate. Only use enough solder to cover all of the conductors, you don't want big blobs hanging everywhere. Don't move any of the wires while the solder cools.
Once you're done, inspect the joint. Make sure there are no solder "bridges" between other nearby conductors, which happens quite easily with surface mount components. Use a solder sucker and/or de-soldering braid to remove excess solder if this happens.
The joint should be smooth and shiny, and the flow of solder should be evenly and seamlessly flowed around both conductors. If it's dull, you've moved it while it was cooling, and should re-melt it to ensure a good bond.
If it's formed a rounded bubble, you've not applied enough heat to melt the solder over both wires and should re-melt it to flow the solder evenly onto both conductors.

De-soldering

This isn't as hard as it sounds. Heat the solder joint until it melts, then use a solder sucker, which should remove most of the solder, leaving only a thin plating still on the part. You can then use a small pair of needle nose pliers to unwrap the lead. When removing through-hole components from PCBs, you may need to continue heating the part as you pull the leads out. Removing surface mount components is a little trickier, and there's no good way to do it without destroying the part you're removing. Use needle nose wire cutters to snip off each lead, being careful not to tear the foil off of the PCB, and remove the IC. Use a de-soldering braid to remove the solder and pin remnants still stuck to the board.