FAQ - How To Solder for Model Railways.

[Chrissaf]

Many posts on this forum have these words or similar:

"I can't or don't know how to solder"

or

"I tried soldering, but made a complete 'dogs dinner' of it"

With regard the first statement. Anybody can learn to solder. It really isn't difficult. And this is what this 'How To' is all about.

With regard the second statement. You must have the right tools for the job. Even I would make a 'dogs dinner' of it, if I was trying to use a 30 watt soldering iron with a '10mm wide' soldering bit to solder a 0.5mm wire on a DCC decoder pad.

I've been involved with soldering electrical and electronics since I was 15, now officially considered an O.A.P by HMRC. Once mastered, it can be an invaluable skill to have. My electrical apprenticeship covered all types of soldering, from closing lead sheathed cable joints with stick solder and a blow-torch. To soldering tiny components on Printed Circuit Boards.

As with most things, it is all about having the right tool for the job in hand. The tools below form the basic 'Soldering Toolbox' (in my opinion) needed for delicate electrical work on things like loco wiring repairs and decoder fitting (where DCC sockets don't exist). Also suitable for attaching droppers to track-work and any general need to solder wires together.

I'm sure that other's here will have their own preferences that they will swear by, but these are mine and arrived at after a lifetime of experience.

Obviously the first thing you need is a 'Soldering Iron'. The market leaders are Antex and Weller. My personal preference being Antex. Don't buy cheap. Cheap irons are more likely to have steel soldering bits as opposed to soft iron or copper. The mistake most soldering newbies make is to use a Soldering Iron that is too hot. For Model Railway work a 15 watt iron is more than adequate. Do avoid the trigger gun (instant heat) type Soldering iron. Their tips are (in my opinion) just not small enough for Printed Circuit Board work. Antex produce a wide range of 'bits' for their irons. For track-work and general wire joining, a 2 or 3mm chisel bit is ideal. For wiring on decoders, a bit that is less than 2mm diameter would be preferred.

Example of an Antex 15 watt C15 soldering iron.

Example Antex Fine Tip soldering bits.

Next you need Solder.

Even if you ignore the advice above and buy a cheap iron. If you do nothing else, do not buy cheap 'Lead Free' solder. Cheap lead free solder requires a much higher temperature to melt and does not flow as easily as the more expensive 'Lead Based' solder.

The ideal solder for Model Railway soldering is 60/40 Multi-core of 1.2mm diameter or less. For this type of work, solder with a diameter larger than 1.2mm is probably too thick. Multi-core means that it contains 'flux' in little tubes that run the length of the solder. Flux is an important ingredient needed to make solder flow over the joint when molten. When soldering something that does not solder easily, then additional flux may be needed to supplement the flux in the 'Multi-core'. The 60/40 is the % ratio of Tin vs Lead in the solder.

Materials that solder easily are:

• Copper
• Brass
• Nickel Silver (i.e modern track rails)
• Tin or items that have been coated in tin.

Example of 60/40 Multi-core solder 1.2mm diameter.

The key to successful soldering is cleanliness. The two items being soldered together must be free of tarnish (oxidation) and any other foreign matter. The shinier they are, the easier it will be to solder them together. Fine 'wet n dry' or a small wire brush can clean things, but one easy less labour intensive way of preparing the surfaces to be soldered is to 'tin' them first.

Tinning involves baring a piece of wire, a freshly bared piece of wire i.e insulation removed should already be clean and shiny enough for tinning. However, if the wire looks dark or greyed then it has probably been subjected to heat and become heavily tarnished (oxidised). The oxidisation needs to be removed before tinning will be successful. Drawing the wire through a folded piece of fine 'wet n dry' or rubbing with 'wire wool' will clean it up. Once clean and shiny, touch the iron and the solder to the wire at the same time and the flux in the Multi-core should make the solder flow over the wire. Speed is of the essence, if the insulation starts to creep back up the wire, then you are holding the iron against the wire for too long.

Most manufactured solder pads on Printed Circuit Boards are tinned at the factory, but even if already tinned, then I still advise using the iron and the solder to add a little more solder to the pad. The reason why will become evident shortly. If the solder pad is not factory tinned and is still the colour of copper. Then use a 'glass fibre' cleaning pen to to make the copper shiny before applying the solder and iron to 'tin' it.

Example of a Glass Fibre Cleaning pen.

It is not just the two items being soldered together that need to be clean and tinned. The soldering iron bit tip needs to be clean, shiny and freshly tinned too. The constant heat in the iron will burn any flux residue on the tip and turn it black. The black burned residue must be removed before you use the iron to perform the soldering task. Some use the cheaper option of a damp (not wet) iron cleaning sponge. This does work well, but I much prefer the more efficient and quicker method of stabbing the iron tip into a pot of 'brass wool'. You need to get into the habit of every time you pick up the Soldering iron to perform a soldering task, that you stab (plunge) the iron bit into the brass wool pot 2 or 3 times to clean off the black burnt flux.

Example Soldering Iron Cleaning Brass Wool Pot.

Once the two items to be soldered together are cleaned and tinned. Then touch them together and hold the soldering bit tip against them till you see the solder flow. In the case of the Printed Circuit Board pad. If the tinning process has deposited a sufficient load of solder on the pad, the the wire should solder to it with just the soldering iron without the need to add any additional Multi-core solder.

However, apply the heat of the iron sparingly. Do not cook the joint. If the solder does not start to flow straight away, then remove the iron heat and add a little more solder using the tinning process and try again. Too much heat can lift the solder pad away from delicate Printed Circuit Boards or damage nearby electronic components.

If just joining two wires together and once bared, the wires are shiny and clean. Then the tinning process can optionally be ignored. Just twist the two wires together and then apply the iron and solder to the twisted joint till the the solder flows. Again, the same caveat as above, do not cook the joint. If it is prepared correctly and you are not using 'Lead free' solder, the solder should flow easily.

In my opinion the best method for insulating 'in-line' wire to wire joints, is to use 'Heat Shrink Sleeving'. Heat shrink can be purchased in a wide range of diameters. Normally, the stated diameter is the diameter before shrinking. Most heat shrink will have a 2:1 shrinking ratio. So, for example, a 2mm Heat shrink will shrink down to 1mm.

My personal preference for shrinking down 'heat shrink' is to use a 'hot air gun' that is marketed and designed for ceramic jewellery makers and craft works. 300w hot air gun on eBay.

Example below:

So in conclusion for successful soldering.

1. The right tools for the job, soldering iron, bit size and cleaning tools products.
2. 60/40 Multi-core solder and not 'Lead Free' or synthetic solder ingredient blends.
3. Bright and shiny preparation of surfaces to be soldered.
4. Do not cook the joint, use the heat sparingly.