A friend of mine asked me about soldering and I thought the conversation may be useful to others. Here is is.
My friends email:
Finally I have soldered all the connections to my new digital speedo on the bike!
Soldering takes me forever ‘cos I am crap at it BUT I think soldering iron is shyte & maybe the solder too!
I can never seem to find a higher power iron than the usual DIY shop type soldering irons & don’t know if solder is solder is solder or if some is better than others!
“The Two Governing Factors of Soldering.
Regardless of what is being soldered or how large the joint is; the most important two points to concentrate on when soldering are cleanliness and temperature.
Solder these days is all pretty much the same. That said, you need to use stuff that has flux built into the solder itself to ensure the metals to join are clean. There is a new EU directive that says we should all be using lead-free solder. For my Liquiphonics solder, not only is it lead free but it also has a certain amount of silver in it. Soldering with it is not fun as it feels a bit like dried up toothpaste and has problems flowing. It skins over quickly and needs lots of flux which means you have to work fast with it and get the job done first time. But I doubt you are using this kind of solder and the older, lead type is not a problem by comparison.
So you will probably have solder which contains flux. But the flux can only do so much so you need to make sure both metal parts to be joined are perfectly clean. Do not rely on the fact that a piece of wire is free from oxidation just because it is inside insulation sleeving – it may have been cut at that point a while ago and moisture could have crept up the sleeving and corroded the wire. Clean any wire that looks dirty or corroded, using fine sandpaper, a pair of pliers or the side of a match box etc. Only if you can see bright metal is it clean enough to join and even then, the flux will be needed.
Both wires should be ‘tinned’ before soldering. This is where the iron heats up the wire, solder is introduced and it flows all over the wire end covering it in a thin layer of solder (tin). You make sure both parts are tinned and only then should they be soldered together. Many wires come tinned already but if they look dull, they may need a quick scrape/sand first.
If the joint is dirty or corroded, the time to solder increases and that is not good as talked about in 2.
You need to use an iron that is capable of producing enough heat energy to quickly heat up the two metal parts. But that is fixed and generally speaking, cannot be varied unless you use an iron with temperature control. I use Wellar soldering irons and change the tip in order to increase the temperature of the joint. The only reason I might change from a number 7 tip (e.g. for soldering components on PCBs) to a number 8 tip is if the joint involves thicker wire as the more metal there is, the more heat energy is needed from the iron. If the joint has too much metal for the iron it acts like a heatsink and will wick the heat away from the joint; the joint will take a long time to heat up. So not only will it start to corrode and get dirty as you try to heat it because the flux has all been burnt off, but the heat will travel along the metal and damage other parts.
Soldering iron temperature aside, the way to control the joint temperature is through time; how long you keep the iron on the joint. If the iron is not hot enough, the heat will have time to travel along the metal (e.g. wire) and start to damage something that is not meant to have so much heat (e.g. insulating sleeving, PCB copper track or an electronic component). What you need to make a good joint is a quick soldering action.
Most important – to ensure the metal parts to be soldered heat up quickly, add a small amount of solder to the tip of your iron just before you solder the joint – ‘wet’ the tip. This point does not matter so much when you are soldering lots of quick joints on say a PCB because the joints are made so fast the iron’s tip always has a small amount of solder on it and if anything, the excess needs to be wiped off when it builds up too much. Of course, the first joint in that example would need the tip wetted first. Wetting the tip increases the contact area between the iron and the parts to be soldered. With poor contact, the joint takes too long to solder, burning off the flux and that makes the joint difficult to solder.
Wet the soldering iron’s tip.
If you have two different size metal parts to solder, put the iron’s wetted tip on the larger part first and then solder the joint. If you put it on the smaller part first, the bigger part will not get hot enough fast enough so smaller parts are damaged, the flux burns off, the joint is difficult to solder.
Ensure the parts to be soldered are fixed in position as you cannot make a quick joint if bits keep moving around – have everything ‘ready to go’ for the jointing to work well.
- Make sure all parts to be soldered are clean and fixed in place,
- wet the tip of your iron,
- immediately apply the tip to the joint to heat it,
- press the solder lead up to the wires of the joint, keep tapping it on the joint, when the solder starts to melt, push it into the joint so that if flows all over, around and into any gaps,
- if the smoke from the flux stops but the joint is not made, push more solder in to add more flux – if that goes on for long, you may have to remove some solder,
- when all the joint is properly wet with no gaps, immediately take the iron away and allow the joint to cool.
Never move a joint while it is cooling as the solder may still be liquid or semi-solid. You can see it go solid, don’t move yourself or anything else – just freeze – until the solder is definitely solid. If the joint moves before the solder has solidified – redo it.
Never blow on a joint to cool it quicker; you will introduce stress into the joint with larger and smaller crystals and that could be a future dry joint. Just let the joint do its thing and cool as fast as it wants to.
When soldering multi-stranded wires, always twist the fine individual wires of the multi core wire together before soldering. If you don’t, the individual wire strands splay apart with the applied heat, as they expand. They will push their way through the insulation, the insulation will melt into the joint, the joint is then dirty with burnt plastic, that needs more heat to clean it off with – it’s a difficult joint to make and the insulation will not go right up to the joint leaving exposed wires.
I might put a video of soldering on my blog some day,