Soldering is a method of joining two metal parts using a third metal that has a lower melting point. Before you can join two similar or dissimilar metals with the alloy called solder, make sure the following considerations are observed.
- The metal pieces to be joined must be clean.
- The correct soldering device must be used and must be in good condition.
- The correct solder and soldering agent, called flux, must be carefully chosen.
- The proper amount of heat must be applied.
For most soldering, you will need the following pieces of equipment:
- Soldering copper. This can be purchased in pairs. The two pound pair weighing a pound a piece is an average size for regular sheet metal work. It is also available in several sizes according to weight, from three ounces to three pounds per pair.
- Ordinary soldering furnace or a blowtorch. Use this to heat the soldering copper.
- Electric soldering copper. This is more convenient because it will maintain uniform heat and can be used wherever there is an electric circuit. Electric copper can be identified by its wattage. It ranges from 50 to 300 watts.
Different kinds of solder are most commonly used for different applications. Among them are:
- Soft solder. A mixture composed of tin and lead. The most common, called half and half, consists of 50 percent tin and 50 percent lead. It melts at about 420°F. Solder containing larger amounts of tin, such as 60-40, melts at a lower temperature and is more free flowing. Thos with more lead requires more heat. This is available in bar, solid wire-filled with a flux core, or ribbon.
- Silver solder. These are composed principally of silver, copper, and zinc in varying amounts. They are used for high-strength joints or where high temperatures are encountered. They have higher melting points than lead-tin solders.
- Soldering irons. These are made of copper. They are sometimes called soldering coppers. Copper is used because of its resistance to oxidation and corrosion. It readily absorbs and gives out heat. Soldering copper are made in different shapes and varying weights.
Fluxes
When metal is exposed to air, an oxide or film of rust and tarnish forms on it. The process of oxidation increases greatly when metal is heated. This must be prevented because the oxide film teds to keep the metal from reaching soldering temperature. Because of the film, the solder cannot meld with the metal. A flux is needed in order to remove the oxide from the metal and prevent the formation of new oxide. The flux also reduces surface tension, so that the molten solder will flow easily. It therefore assists the alloying action of the solder. There are two types of flux: corrosive or acid and the noncorrosive.
The corrosive or acid flux is more effective. It is frequently used on electrical connections. The acid flux must be removed from the article after soldering to prevent corrosion by washing it in hot water.
An acid flux can be prepared in the shop by adding small pieces of zinc to muriatic (hydrochloric) acid until the zinc no longer dissolves. This solution is called zinc chloride flux. Flux must be mixed half and half with water before using. Mix the flux in a well-ventilated room. Salt ammonia in power or bar form is also satisfactory to use. There are many types of liquid, powder, or crystal fluxes available under various commercial names. They come in the form of bar, wire, or hollow core.
Noncorrosive flux is suitable for all electrical and electronic works. The better commercial kind can be used on tin plate, copper, brass, and other alloys of copper. Powdered resin is a very good flux. Many variations of this in liquid and paste form can be purchased commercially in hardware stores.
Tinning Soldering Copper
The tip of a soldering copper becomes covered with oxide after it has been used for some time. If it is overheated, the tinned point becomes corroded and must be tinned again. The oxide coating keeps the heat from flowing to the metal. To correct this, the point must be cleaned and covered evenly with solder. This process is called tinning a soldering copper. The following are steps in tinning a soldering copper:
- File the point of the soldering copper with a mill file until it is clean and bright.
- Heat the soldering copper until it turns yellow to light brown.
- Apply acid or resin-core solder to the point, then rub the point on a bar of sal ammoniac. Apply a few drops of solder.
- Dip the point in liquid flux and rub it with a bar of solder until the point is tinned.
- Wipe the point with a clean cloth to remove the excess molten solder.
The following steps explain how to solder a seam or joint:
- Place the two pieces to be soldered on a table that has a nonconductor top, such as asbestos or firebrick.
- Make sure that the seam is properly held together. If necessary, clamp or write it in place.
- Apply a thin coat of flux with a swab or brush. Use resin flux on most metals. A raw-acid flux is needed for galvanized iron. If wire-core solder is used, the area will be fluxed as it is soldered.
- Heat the soldering copper to the correct temperature. You can tell if the temperature is right by touching the solder to the point. If it melts quickly, the soldering copper is almost correctly heated. Never allow the copper to become red-hot because the tinned point will be burned off. Always keep the soldering copper at the proper temperature.
- Track the seam in several places by applying the point of the soldering copper to it. Allow the point to remain there until the flux sizzles. Then immediately apply a small amount of solder directly to the copper, since this merely makes the solder run without joining the two pieces.
- Start at one end of the seam, hold the copper with the tapered side flat on the metal until the solder melts.
- Move the soldering copper along slowly in one direction only, never in back and forth motion. Apply more solder in front of the point as needed. If necessary, press the freshly soldered portion of the seam together with a file until the solder hardens.
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- Check your by answering the following questions:
- Is the soldered seam smooth?
- Are the metals firmly soldered together?
- Did I use just enough solder?