Choosing the right solder for the job is just as important as selecting the correct iron. Solder used to be made primarily from lead, but as a result of health and safety issues most modern wire solder is made from tin alloy with, in some cases, small amounts of nickel or silver. You can still buy solder in bars but this is primarily for large soldering jobs outside of aeromodelling, such as plumbing.
My favourite everyday solder is Omega 18swg wire solder, a 63% tin / 37% lead composition from Warton Metals (tel. 01706 218888) that seems particularly well suited to the type of soldering we do in this hobby; the ‘free-flow’ flux it’s impregnated with (running along tiny cavities within the wire itself) is the best multicore flux I’ve used by far. Which brings us to another important aspect of soldering.
WHAT THE FLUX?
It seems to be taken for granted these days that most solder available on the market has flux within and is therefore perfectly suitable – end of story. In my opinion this is the biggest mistake made as, quite often, the flux in multicore solder isn’t particularly good, though this can be complemented by a decent dousing of proper flux paste such as the excellent Safflux.
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I’ve attempted solder jobs, as I’m sure you have, where the solder simply refuses to ‘take’ on one of the surfaces being soldered. I often find difficulty with certain brushed motor terminals, but in every case applying a small amount of Safflux sees the solder not only take to the stubborn terminal but completely engulf it, and the resulting solder joint is, frankly, beautiful!
Flux works by breaking down the surface tension on the work-piece and allowing the molten solder to flow properly. It also cleans the metal and inhibits tiny slag deposits and impurities that can ruin certain joints.
A decent flux is probably the most obvious missing ingredient in soldering; if the rest of the equipment you’re using is right for the job but you’re still not getting good results then you can bet the problems are flux related. My advice is to invest in a small pot of paste-type flux to complement the flux within your solder. You simply won’t believe the difference. Don’t use an acid-based flux as this will slowly corrode the joint, which is consequently sure to fail somewhere down the line!
It goes without saying that any area to be soldered should be dry, clean and free from grease. If you bare the insulation from the end of a piece of wire to be tinned and twist the strands round with your thumb and forefinger, even the sweat and grease from your finger ends can be enough to make tinning difficult.
Occasionally an item that’s difficult to tin or solder can be made to behave simply by being lightly sanded with fine abrasive paper, which also scores the metal surface slightly and gives the solder a good ‘key’.
Even with the right gear, a good technique is required to get consistent results. To do this we must understand the basic principle of electrical soldering and that is simply to raise the temperature of both pieces to be soldered to the point where solder will melt on contact; through the properties of the flux, the molten solder can then penetrate the joint sufficiently to produce a permanent bond that has low resistance to electricity.
A key point here is the reference to raising the temperature of both pieces to be soldered. I’ve seen people applying copious quantities of molten solder to a heated piece of wire, then attempt to join it to a cold piece of wire as if it were some type of metal ‘glue’. This is not how to solder and will result in a very poor joint.
Both parts, be they wire, terminals or connectors, first need to be ‘tinned’. This entails coating or penetrating each individually heated (to around 340 – 350ºC) component with an appropriate amount of solder. This is done by simply heating the part until it’s hot enough to allow the solder (applied to the work-piece, not the iron) to flow. When soldering two tinned pieces together it’s quite often the case that no further solder is needed, but if the joint looks a bit ‘dry’ (i.e. you can still see wire strands and the joint appears dull) then apply more solder. Don’t apply solder to the iron here, only to the work-piece.
It’s important to saturate the joint completely so that it ends up shiny with a visible layer of solder having solidified when viewed from any angle; over-saturation may add a few extra grams and result in an unsightly ‘blob’ underneath (due to gravity), but this can usually be filed or sanded off if necessary. Far better to slightly over-saturate a joint than to starve it, as long as any excess solder doesn’t come into contact with a nearby wire or terminal and risk a short circuit. This is something you have to be particularly careful with when, for instance, re-soldering the red, white and blue Rx wires to the PCB of a speed controller after they’ve been inadvertently yanked out of the unit in a crash!
Consider desoldering and you might think it’s just a case of applying heat with the iron and pulling the two pieces apart. The problem is that there’s (hopefully!) a significant amount of solder in this well-saturated joint, and this has to go somewhere! If you’re lucky it will remain on each side of the joint you’re desoldering, but in many cases an unwanted line or string of molten solder will remain, which is particularly bad news on a circuit board or an area of multiple connections.
The answer is to use a desoldering pump or desoldering braid. The pump is a simple, spring-loaded suction device; once the solder of the offending joint is molten, press the button on the pump and it literally sucks a significant amount of solder from the joint. These pumps are very good, especially where there’s a large quantity of solder to dispel.
The alternative is to use the aforementioned soldering braid, which is a strip of lightly fluxed woven wire mesh that’s particularly attractive to solder. Offer it to the heated joint and in most cases the solder will jump ship from the saturated joint to the solder-hungry braid, thus significantly reducing the amount of solder left holding the joint together. The downside of using desoldering braid is that if you’re not careful you can inadvertently saturate and solder the braid to the work-piece, ending up with an even bigger mess! The best way to avoid this is to desolder a little at a time, regularly using a fresh piece of braid when the previous piece becomes saturated, helping to ensure that the solder ends up only in the braid and no longer in the joint.
Whilst writing this article, a new branch of Maplins opened in my native Carlisle. I’ve used their soldering products for many years and thanks to Chris, their assistant manager, I was able to photograph some of my favourites. There are a number of Maplin stores throughout the UK and they also do mail order. There are a lot of cheap (and in some cases very nasty) soldering products on the market but Maplins have done their homework and whilst you will only ever get what you pay for, their soldering products are well above average quality and value.
I hope this article will help improve your soldering and as a quick reference, here’s a 3-step summary as a reminder:
Follow these simple steps and you’ll hopefully achieve the type of solder joints you’ve always wished for, and that your ruinously expensive models deserve!
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