Mechanical Soldering

by Martin Gregorie

This article originally appeared on the Free Flight Mailing List and was subsequently re-published in Free Flight Down Under.

There seems to be a mystique that soldering (and the closely related silver soldering and brazing) are difficult, arcane tasks. Well, they're not. They are quite easy provided you use the correct tools and materials and are scrupulous about cleaning the workpiece before making the joint.

The art of making a good solder joint is to heat the parts to be joined above the solder's melting point so that the solder will wet the parts and will flow into and around the joint by capillary action.

The parts to be joined must be clean because dirt stops the solder wetting them. The job must be done quickly before either solder or metal parts have time to oxidise at the high temperatures because oxidised surfaces or oxidised solder will also prevent the solder from wetting the metal parts.

A low wattage electronics iron has virtually no heat capacity and insufficient energy input to heat the parts to be joined.

I do all model size mechanical soldering with a 2 oz copper iron, the sort that you heat up over a gas ring, Bakers Soldering fluid (acid flux) and a bar of plumbers solder.

The key to successful mechanical soldering is an iron with a copper bit that holds enough heat energy to get the work thoroughly hot and to do it in a second or two. An iron that has a copper bit with a square cross-section and a fine, tapered point at the business end is about right. The 2 oz iron is barely big enough for good heat capacity while a 4oz iron is getting a bit big and clumsy for fine work. If you don't have gas or can't find the sort of iron I've described, look in your local DIY for a 100 watt plumbers iron. While you're there, get the acid flux and plumbers solder. Acid flux (Bakers Fluid or local equivalent) is a clear fluid with the consistency of water. It acts primarily as a degreaser when it boils off though it may also help to stop clean surfaces from oxidising.

Avoid soldering guns like the plague - in my opinion they are nearly useless. They have no heat capacity at all in that thin copper wire bit and at the same time they are too big and clumsy for electronic work. On top of this they can wreck semiconductors with the eddy currents they are liable to induce in nearby wiring. Theres a couple of hundred amps or more flowing through that wire loop when the trigger is pulled!

Resin-cored solder should be avoided for mechanical work as it always has too much resin in it and the resulting globs of melted resin can weaken the joint.

The technique is to get the iron nice and hot while you thoroughly clean the surfaces to be joined with fine emery cloth and don't touch them once they are bright and shiny.

Now tin the iron. Use an old file to knock off the black oxides etc. that build up on the bit, dip it into the flux and then dab it onto the bar of solder. If the iron is hot enough you'll get a 'skuwush' noise and a cloud of steam from the flux and the solder will melt immediately and then wet out the iron surface. If the end of the iron still has black bits etc attack it with the file to remove them and re-tin it. A properly tinned iron has a thin, even, clean film of bright solder over the parts that will come in contact with your joint.

If theres little noise from the flux and the solder doesn't melt immediately the iron is too cold. If the solder doesn't wet the iron the bit is dirty and needs more clean-up with the file before tinning it again. If you're using a gas-heated iron put it back on the ring to keep hot. A cold iron does not make a good joint. Another mark of a good hot flame heated iron is that the flame should show green (copper) colours after the iron has been tinned and put back on the heat.

Now you're ready to tackle the joint. Position or clamp the work so that the (clean) surfaces to be joined are where you want them and use a match stick or pipe cleaner to wick some flux into the joint. Take the iron, stick it into the end of the solder bar to pick up some solder and place it on the work. Again you should see steam from the flux and the fluid solder should wick into the joint. If the iron is big enough for the job you are asking it to do the joint should form literally within a second or two. Soldering something like the seam of a brass shim fuel tank will take under 5 seconds. You should not be left with convex blobs of solder on the outside. If you get wicking and blobs the iron is cold or you used too much solder. If you get blobs that don't wet the surface of the work you haven't cleaned the joint properly or you're trying to solder the unsolderable, like alloy.

Once the job is done and cooled enough to inspect and handle, wash it thoroughly under a cold tap to get rid of the flux, which will cause corrosion if left on the joint.

Remember that:

The idea with wire to wire joints is that the (copper) binding wire provides the strength while the solder merely forms a matrix that locks everything together. The binding should be tight before you solder that join. If you can only see the solder and not the binding, theres too much solder on the joint; the ideal is that just enough solder to wet everything involved in the joint is used.

If you are soldering brass tubes together you can usually get away without binding the joint (brass and copper work extremely well with tin/lead solder). However, to join brass tube to piano wire is a different story. If you're soldering the wire inside the tube just do it because the solder should wick into the joint and the acid flux should clean the surfaces enough to get some sort of joint, but if the wire is being soldered to the outside of the tube it will need to be bound first unless the wire is tightly wound at least half way round the tube.

Apologies for going on so long to all of you who can make that perfect joint every time, but its my experience that generally the only people who know how to do mechanical soldering, as opposed to electrical soldering which is quite a different animal, are skilled manual workers such as long-time model fliers, plumbers, mechanics, fitters, etc., together with the relatively few other people who did workshop classes at school or night school. I was lucky enough to do this as a supervised Saturday morning activity when I was at boarding school and so learnt the rudiments properly before I had a chance to pick up bad habits.