Soldering Made Easy: Part 1.

This article by John de la Lande on soldering has been published in a number of railway modelling magazines and updated as new solders and fluxes have become available.

Introduction

Soldering appears to be a complicated, messy and difficult if not arcane artform, to many modellers. In practice it is not that difficult but does require a basic knowledge of what soldering is, good work habits and techniques, and the patience to put the effort in to master the techniques involved. Why then do we need to use soldering in railway modelling? The answer is that there is often the need to join thin strips, bars, tubes of metal (normally brass and nickel-silver) together to form a strong joint that can withstand stress and/or shock. Examples are trackwork; mechanisms for operating points and signals; chassis and underframe construction. Glues specifically designed for metals and others such as "Superglue" are continually becoming available and are improving in quality yearly. However, they still cannot be used in place of soldering for many applications.

This is the first of a number of self-contained articles that will cater for the soldering beginner with basic techniques right through to the latest techniques with resistance soldering. These articles will be based on my experience over many years. As well I will draw on material from the many excellent articles and chapters on soldering that have been published over the years (a list of some of these references are included at the end of this article).

BASIC SOLDERING TECHNIQUES

Let us assume that we are in the position that all modellers find themselves in at one time or another. We have just purchased an etched brass kit of a particular prototype which we just had to have, have unpacked it, read the instructions, and looked with increasing horror at the one, two or more etched brass sheets in the kit plus sundry other components. How do we start? What do we need? It is essential that you use the right solder, tools and techniques if you are to build a successful model so I will discuss each item that you will require before we get down to actual soldering.

The Solder

Clearly we need some solder, but what type? Is the melting point critical? The answer is that we do not use the multicore solder that is readily available for electronics work or the solder plumbers use. These usually melt at a temperature much too high for kit building (ie greater then 200-250C) and at these temperatures you run a pretty good chance of distorting or buckling the components you are soldering as well as burning your fingers! What you need for most soldering is a solder that melts at about 140-180C although solders that melt somewhere in the range of 70C to 190C are extremely useful on occasions (this is discussed in a later article). Note that this is the temperature at which the solder begins to melt and you will need to apply more heat to solder successfully. Where do you get solder that melts at about between 140 and 180C? This is of course the perennial question that faces all modellers. I have had difficulty in the past obtaining solders for modelling and Brunel Models can now can supply solder and flux (made in Australia) which is sold by Brunel Hobbies and your local Hobby Shop.

The Flux

This is an essential component of the soldering process, which is often overlooked or its purpose is not understood. Fluxes perform two essential tasks, they clean up oxides which form when heated metal is exposed to the air and reduce the surface tension of the hot molten solder so that it will flow across the surface of the metal and between surfaces that are to be joined. The flux can be either a paste or a liquid and is normally acidic and hence corrosive. For modelling, fluxes which are based on phosphoric acid are the most useful and one should certainly not use Baker's flux which is a paste used by plumbers. I use either Brunel Model's Number 1 or 2 flux (the equivalent of Carr's Red and Green flux). You could make up your own by diluting phosphoric acid from between 5% to 15% (see ref 1)- remember though that extreme care should be taken if decide to do this.

Soldering Paste or Cream

Solder paste or cream is a thick liquid consisting of a flux in which is mixed tiny particles of solder. Although Carr's have produced a solder paste for many years it was not satisfactory due to its tendency to spatter and leave a residue that needed to be cleaned up to avoid corrosion of the soldered joint. It also did not keep well and would eventually dry up. However, technology has come to the rescue and Brunel Models now supply a solder cream in a syringe that is "no clean" and "no residue" and melts at about 179C and another at 138C. This type of solder was developed for the electronics industry and space applications and is ideal for constructing brass kits.

The Heat

I was going to call this section "The Soldering Iron" but on reflection soldering is about the application of just the right amount of heat to the joint to be soldered and this can be done in many ways. The most common way is, of course, by using a soldering iron but other methods such as resistance soldering are also available (some of these will be discussed at a future date). In practice a soldering iron is an essential tool that will be required even if other methods are used. If you already own an iron it should be rated at about 40W, have readily exchangeable tips and be light and small and easy to handle. If not you need another iron! What should you buy if you don't have one? There are a number of suitable irons available from electronics shops, such as Dick Smith, ranging in price from about $40 to $160. I use a Digital Temperature Controlled Soldering Station, which has the advantage (as the name implies) of being able to control the temperature of the soldering iron tip to within a few degrees within the range of about 100 - 400C as well as being able to pre-set three temperatures. Being able to control the temperature of the iron can be an advantage and there are a number of simple devices that can be used to do this which are connected between the soldering iron and the wall socket power point.

Tools

Apart from the normal tools needed for kit building (and bashing) I will mention the ones that are particularly useful for good soldering. You will need a number of needle files (flat, half round and round I find the most useful), a fibre glass brush (these look like a ball point pen and are available from Brunel Hobbies), fine tweezers, small clamps (aluminium hair clips will do nicely), a surface to solder on that won't melt (I use a metal plate), small metal bars and other items such as wooden blocks, card, etc that could be used for holding the components (more later).

THE BASIC RULES OF SOLDERING

The first and most important rule is cleanliness. Dirt is one of the most common causes of soldering problems. If the surfaces to be joined are not clean, and I mean shiny clean, then you are starting with a severe handicap. As well the soldering iron tip must be kept clean and not contaminated with other solder or environmental dirt. At the very least you should keep a tip for etched kit construction and one for electrical work, track construction, etc. I keep one for each different type of solder I use for kit construction (type defined by the melting point of solder). Mixing solders of different melting points or constituents is not recommended as it can result in a solder with completely different characteristics than either of the ones that were mixed (eg 140C solder mixed with solder with a melting point of 70C will produce an extremely brittle solder).

The second rule is use the right solder for the job in hand. In most cases use about 170oC solder as discussed above (exceptions will be mentioned later).

Thirdly, only apply sufficient heat to produce the required joint. Too much heat can distort the components being joined and even unsolder parts that have already been joined! Too little heat will not allow the solder to flow and will result in a weak brittle joint (sometimes called a dry joint).

SIMPLE JOINTS

Before starting on that kit it is most useful to put into practice all that has been discussed above. Let us take a small piece of brass (say 2cm by 4cm) and a small strip of brass (say 0.5 cm by 3 cm) and solder the small strip so that it overlaps the larger piece by about 1 cm. How do we proceed?

First, clean the brass, not just where the joint is to be made but the whole piece! You can use a heavy duty washing and cleaning compound called Flashkleen (or equivalent) available from car accessory shops or a good wash with a strong detergent and a small stiff brush or cleaning pad. All grease, dirt, grime and even marks left by sticky tape should be removed in this process. Clean the area on both pieces where the joint is to be made with the fibre glass brush. Now clamp the larger piece horizontally. Don't clamp it directly in a vice or on a large piece of metal which will drain all the heat out of the brass when you apply the soldering iron. Use wood or cardboard to insulate it. I often use a super strong rare earth magnet on a metal sheet with a piece of cardboard between the brass and the metal sheet.

Using a small paint brush apply flux to the area to be soldered. Next melt some solder onto the tip of the hot soldering iron and apply the tip to the area where the flux was applied. The solder should flow from the tip over the fluxed area to form a thin layer of solder. if there is insufficient solder on the tip of the iron, touch the solder to the tip where it is held against the brass. Do this carefully! The idea is to apply just enough solder to form a coating over the brass. This is called tinning. I always tin the pieces to be joined if at all possible unless I am using solder cream. It makes a much neater joint. Do the same with the smaller strip of brass.

Now to join both pieces together. Either hold the smaller piece in the correct position for soldering or clamp it in the right position insulating it with wood or cardboard. I always clamp or use a jig made from wood or card when soldering two pieces together. Holding the piece to be soldered can be tricky and result in the misalignment of the pieces. Apply some flux along the edges of the small piece of brass - it will flow along the edges of the small piece of brass into the joint. Apply the tip of the soldering iron to the edge of the small piece of brass and move it along slowly.

The flux will transmit the heat into the joint and the solder from the tinning will melt and flow. As soon as this occurs remove the iron and wait 10 to 15 seconds before moving the joint. You should now have a neat, solid joint! The same technique can be used to join the smaller piece at right angles to the edge of the larger piece although in this case it is possible to add extra solder to the inside of the joint for extra strength if it can't be seen.

If you are using solder cream place a small amount on the area to be soldered then solder as described above. No flux is required.

References

• Locomotve Kit Chassis Construction in 4mm. Iain Rice. Wild Swan. 1993.
• Carr's Soldering Handbook. Carr's
• Model Railway Kit Building. T.J. Booth. PSL. 1989.
• The 4mm Engine. R Guy Williams. Wild Swan. 1988.

Material Suppliers

As well as directly from Brunel Hobbies you can also purchase solder and flux from a number of Brunel Models retailers