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Old 27-06-2015, 11:33 AM   #1
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Default How to solder electrical connections

As this question gets asked many times, I though I'd put a posting on the forum explaining the technique that I was trained to use and have been using for around 50 or so years now!

The basics of soldering are:- A soldering iron of suitable wattage and tip size for the work being undertaken fitted with a clean bit that’s in first class condition, 60/40 lead content Rosin cored solder and clean connections. Lets make a start…. For everyday soldering a minimum 25 watt iron with a small to medium sized bit is required. Larger bits sized irons have their place, but are not always needed for most electrical joints. Larger wattage irons are equally fine to use if the irons tip is of a suitable size. A larger wattage iron is often more desirable as the heat transfer into the item being soldered will be much quicker.

To make a good quality soldered joint, heat the iron for at least five minutes. Don’t rush, the irons tip must be up to full temperature. Have to hand a damp soldering iron's tip cleaning sponge pad. If you own a soldering iron stand its likely it came with a sponge. If not, then cut a piece of ordinary non synthetic sponge and use that. Remember to keep the sponge damp.
Once the iron is hot, wipe the tip onto the sponge to remove all previous oxidisation and old solder residue. Assuming the tip is in a good condition and it must be! Apply a little of the rosin cored solder to the tip.

On electrical joints never use solid stick type solder nor most paste or liquid types of flux, as these all contain a mild acid which over a long period of time causes high resistance problems within the soldered joint. Solid solders and liquid fluxes are normally the reserve of the solid sheet metal soldering jobs – Loco building, plumbing etc.
If you must use flux for electrical joints in addition to what is inbuilt in the rosin cored solder, then ONLY EVER use a special flux designed for electrical soldered joints. It does not contain any harmful acids that would need washing away with water. Examples are:- Carrs Orange label, DCC Concepts Sapphire flux or fluxes sold under the Reflow banner.

For jointing two or more wires together….With the irons tip coated in liquid solder (wetted) and having previously dry assembled the joint - It must be cleaned too, use a fibre glass pencil or scrape the surfaces of both components clean, unless its freshly stripped wire where the sheathing keeps the surface of the wire nice and clean. Place the wetted irons tip directly onto the connection. Wait a few seconds for the heat of the tip to transfer into the wires and then apply a little more rosin corded solder onto the heated joint, not onto the irons tip. You should see the solder start to melt and flow into and around the joint. Once sufficient solder has been applied to coat the whole joint remove both the iron and corded solder. NOW DO NOT TOUCH or MOVE the joint. Wait at least 10 seconds after removing the heat to allow the joint to cool and the solder to set. What you should end up with is a solid, clean joint. Sometimes the PVC sheath on the wire/s being soldered will shrink back a little. This is a nuisance at times and is due to a) The wires PVC sheathing having a low temperature range or b) Too much heat applied to the joint for too long a period of time.

Finally, before you go onto solder another joint or you have completely finished and before you disconnect the iron, clean the tip again on the damp sponge. You will get a many years of use from a soldering iron if you keep its tip clean!

Soldering wires to the bottom or outside of the rail is the same principle, but here I find pre tinning both the end of the wire and the pre cleaned place on the rail where the wire is to connect to be the best method. Pre clean the rail with the aid of a fibre pencil or other means - file or scraper etc. Tin rail with a little solder, both the place on the rail and the wires end. To Tin, apply a small amount of solder to the clean irons tip, then touch the iron onto the area to be tinned. Heat/Solder will flow from the tip onto the area. This should only take a few seconds, keep the irons tip in place and feed some of the cored solder into the area. Remove iron and wipe the tip on the damp sponge. Once every item has been tinned, place the stripped wire end, which if necessary has been pre bent to a small ‘L’ shape, up to the solder on the rail. Apply a little solder to the irons tip and place the iron on top of the wire and lightly press down towards the rail. The hot solder on the irons tip will cause both the wires solder and the rails solder to melt into one. If necessary apply a little more cored solder onto the wire with the iron still in place should there not be enough on the rail to make a solid connection. Carefully remove the iron and ensure the wire maintains in contact with the rail and doesn’t move, waiting for 5 to 10 seconds to allow the soldered joint to cool. The use of a small screwdriver blade or even tweezers to hold the wire in place until the solder solidifies and prevent your fingers burning is an option I often use.

The use of crocodile clips or any similar sprung clamps fixed onto the rails just either side of the soldering work area are advisable, as these act as ‘mini heat shunts’ and help prevent the rail being overheated away from the soldering area which can, if the heat is allowed to be transmitted along the rail, subsequently causing the plastic sleeper fixings to melt.

Don’t forget to wipe the tip on the damp sponge after finishing all the work and before turning off the iron.

One thing that I have found for the beginner to soldering, is to obtain a spare scrap piece of track and some pieces of wire and practise, practice and practice until you feel comfortable and at ease with soldering.
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