SIlver Soldering Techniques and Tips
Silver soldering, also known as 'hard' soldering or silver brazing, is a process in which two or more parts are joined by melting and flowing filler metal into the joint. The melting point of the filler metal is above 420°C and flows into the joint by capillary action. A silver soldered joint is a sandwich of different layers, each metallurgically linked to the surface of the joined parts, and is very strong - much stronger than soft soldering. It is called 'silver' soldering because the filler material often contains silver.
In contrast, welding is a process in which the base metal is melted during the joining process, the two halves of the molten joint flow together and allowed to freeze. only silver/hard soldering is discussed below.
In hard soldering, the metallurgy of the surface is changed and the joints are metallurgically linked, and becomes part of the underlying metal surface. This is why a silver soldered joint is very much stronger than a soft soldered one, and sometimes stronger than the base materials joined.
- Silver soldering produces fumes, and you should avoid breathing these. Solder only in a ventilated area (but not so ventilated that breeze cools the job!). Stand back from the work, not over it.
- Safety glasses and copper tongs are essential. Parts get hot! Handling hot pieces of metal with tongs at all times.
- Pay attention! It is very easy to concentrate on the job in hand in front of you and wave the flame elsewhere.
- Be careful around the sodium bisulfate pickling solution. It is corrosive and can burn skin and eyes. Dispose of exhausted pickle solution per UVM's laboratory waste disposal procedures.
How To Silver Solder
The basic steps in making a silver soldered joint are:-
- Make it
- Clean it
- Flux it
- Apply heat and solder
- Cool it
- Clean and inspect it
Silver soldering is NOT a gap filling process, and requires a gap of only a few microns (0.001mm) or 1 or 2 thou (0.001") for proper capillary action during joining of parts.
Clean parts are absolutely essential. There should be no sign of oil or grease, and should preferably be scrubbed mechanically with fine emery cloth.
Flux is a material that protects the surface of the metals by preventing oxygen from interacting with the metals to be joined, and prevent further oxidisation, during the heating process. Flux is an essential part of silver soldering, and usually applied as a paste. Too much flux will cause problems; it prevents the capillary action of the solder into the joint, or fails to remove the oxides properly when heated.
Flux contains fluorides.
Allow the joined parts to cool naturally before carefully placing in pickling solution using copper tongs.
After soldering, clean up any accumulated flux from the job, and clean by pickling or emery paper. Check from both sides where possible that all joints are soldered correctly. If not, it is usually possible to re-flux the work and repeat the soldering process after cleaning. Make sure the solder has fully penetrated the joint, and there is a small fillet of solder in the corners.
Tips & Hints
It is important that the job is heated evenly to ensure that the metals will be at the right temperature for the solder to flow easily. If thicker and thinner parts are to be joined, heat the thicker parts first, even to the extent of heating only the thicker parts from the back until they are at temperature, then heating only briefly the thinner bits - they will be hot already from being (indirectly) in the flame. See diagram below. A bead of silver solder, held in place by the flux, will melt through the joint when the thicker base is at the correct soldering temperature.
Small amounts of silver solder can be placed along the joint. It is put into place BEFORE heating and lies there held by the surface tension of the flux until it melts. Note that silver solder always flows towards the heat source.
If the assembly is likely to move during the soldering process, wire the parts together with iron binding wire to hold them in place. Useful also when a second soldering operation is necessary on the same assembly. Just remove and discard after soldering. If the part is more complicated, then some thought needs to be given to the sequence of build.
The flux must melt and be active by the time the silver solder melts. Fluxes like most things have a finite life and can become exhausted, depending on how hot and for how long they are heated. Apply flux with small brush when the work has cold.
After the part cools down it then goes into a pickle bath until clean. Once clean, wash the part off in running water.
Don't get the assembly too hot - brass fittings tend to melt instantly if too hot! This is one reason to indirectly heat tthe whole piece. The solder only melts when everything is at the right temperature (rather than melting first in the flame), and goes into the joint. Remember, even heating is better than concentrated heat. As a guide to temperature, the state of the flux or the colour of the metal may be used. The colour of the metal also provides a guide to its temperature, and a little practice on scrap pieces is advisable for beginners.
When heating, it is better to continually move the flame about over the job than to just keep it fixed in one place. And concentrate on heating the whole job, not just one part.
Quenching from the hot state is unnecessary and can be dangerous as steam generates in hollow parts of the work and can be ejected at high velocity, if acid is being used things are worse as fumes are often given off as well. Quenching also produces thermal shock causing uneven stresses and may ultimately damage to the work. Better to let the part cool naturally to room temperature.
After pickling rinse copiously with clean cold running water.