How to Solder: Great Tips on Soldering for Beginners

Learn the basics of soldering to help with projects around the farm.

| November 15, 2012

Almost anyone can learn the craft of welding, Andrew Pearce argues in his straightforward and handy guide to do-it-yourself metal work, Farm and Workshop Welding (Fox Chapel Publishing, 2012). In this excerpt from chapter 7, Pearce gives detailed instructions on how to get started in soldering, complete with tips on how to overcome common problems with welding techniques. 

Buy this book from the MOTHER EARTH NEWS store: Farm and Workshop Welding.

Soldering is a lower-temperature version of bronze brazing, using filler alloys that are less physically strong and melt at lower temperatures. As with bronze work, the bond between metals is not made by fusion. Instead it’s formed partly by the filler hooking into the tiny hills and valleys of the joint metal surfaces, but mainly from the solder dissolving (not melting) into a very shallow surface layer of the joint. So for soldering to work, it must have free access to ultra-clean parent metals. For this reason flux is always used as a chemical backup to mechanical surface cleaning.

Silver soldering suits dissimilar metals and is generally used with capillary joints, leaving little or no external build-up of filler. The filler itself is a fairly expensive mix of copper, zinc and silver which, being tougher than soft solder yet still electrically conductive, is useful where a joint must stand moderate heat and vibration. Electrical heating elements are often silver-soldered. The temperatures needed for silver soldering can only be achieved by a flame (or arc), not an iron. If the work is not too big it can be laid on a firebrick hearth and heated with a small butane torch.

How to Solder: About Solder Flux

Buy the flux when you buy the silver solder. Take great care over preparation. Pre-flux the joint, then dip the rod in flux. Work quickly so the flame does not tarnish the cleaned surfaces. Let the silver solder melt with the heat of the joint metal, not directly by the flame. After soldering the flux residue can be washed off with hot water. 

Soft soldering is the much more common form, used on heat-sensitive items or on joints that don’t need much mechanical strength. Filler metal melts at 482ºF (250ºC) or below, so the necessary heat can be transferred from an iron. The alloy is a lead-tin mix, often spiced with antimony. Soft-soldered joints are everywhere — in vehicle wiring, radiator header tanks, plumbing joints in copper pipe. Different metals can be joined too, like copper to steel.

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