Workshop practice

The Chemistry of Soldering

Bob Alderman

Article from Gazette Volume 20 number 3 - May 2017

This is an update to a note I wrote way back in 2002 to help modellers when I was frequently demonstrating soldering for etched kit construction at various shows. I had annual tutoring visits to the Missenden Modelling Weekends where I taught basic soldering techniques for 20 years.

The soldered joint was developed to join items of lead pipework together. Plumbing derives its name for the use of lead (plumbum, chemical symbol Pb) in the architectural context. Lead has long been used for water pipes (the Romans) and roofing including the drain hoppers and down pipes. The rapid expansion of domestic plumbing in the late 19th century resulted in improved means of jointing the water pipes.

Wiping molten lead onto the joint areas to join and seal them created early joints. These can still be observed in the older toilet plumbing where they show as a bulge in the pipes. The addition of tin to strengthen the joint and other alloying materials like bismuth, cadmium and silver to change the melting point and flow characteristics, have resulted in the solders we use in our modelling. The removal of lead from solders has not helped our cause. A burgeoning electronics industry has resulted in many specialist solders with properties tuned to particular applications. When a joint is made the solder alloys on the surface of the metal it is on. This is the physical and chemical attachment of the solder to the metal. In the modelling situation we are generally using brass in one of its several alloy forms. Nickel silver is a brass; there is a percentages of nickel but no silver in addition to the copper and zinc.

The working part of the joint is the chemical combination of the tin in the solder with the copper in the brass. If a joint is examined under high magnification there are zones of alloys through the joint. The Intermetallic zones are only microns thick. These can be illustrated as shown:

It can be seen that there is still a component of the joint that is pure solder. The thinner this can be made will generally mean the stronger the joint is. The solder above is 188 otherwise 60/40 lead/tin.

How to make the joint:

The basic ingredients are heat and flux. The heat melts the solder and provides the energy to create the chemical bonds. The flux is used to chemically clean the surface and prevent oxidation of the metal to be joined under the heating.

It is at this point that modellers seem to get in a mess! Just remember, soldering is a basic skill, not mysterious art. So, to make a good joint the following is required:

When all elements are correctly brought together, the solder will run like mercury following the tip, flow into the joint and form a small meniscus on both surfaces.

An iron that is too hot will still make the joint but the solder oxidises on the tip, requiring frequent cleaning and re tinning. Often the copper of the tip is exposed and in conjunction with the effect of the flux and high temperature oxidises and requires frequent filing to brighten the copper before tinning again; thus the bit gets smaller and smaller and the end become hotter as it gets nearer the heat source; a self-destructive cycle.

Note 1: My crude Rule of Thumb for determining a usable temperature for the tip is 2.5 times the solder melting point. i.e. 145 x 2.5 = 362.5 C, or 360 deg C.

Note 2: Flux: several choices all effective, phosphoric acid, plumbers paste flux or the new no-clean varieties. Choose the one that suits you.