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Soldering Basics
How to Avoid the Molten Lead Blues
Copr. 1996,2006 by Jack A. Orman

The art of making a good solder joint is essential to anyone who wants to make electronic equipment, and it is an art form that is not too hard to master, once the basics are learned. Without good solder joints, your project will not perform up to its potential.

To start with, you need the proper equipment. This means, at the minimum, a soldering iron and solder. For working with components, IC chips, transistors, resistors, etc., you need a low wattage iron. Typically a 20 to 35 watt iron is good for this purpose. If you are working with connectors, cables and other devices that have more mass to absorb heat, then a higher wattage iron may be necessary.

Many soldering irons are available with interchangeable heating elements which may be changed to suit the job at hand. Having a 33w element and one in the 45w to 50w range would be ideal. But if that is too costly, the 33w unit is a good general purpose element. Many electronics suppliers stock good quality irons and an acceptable iron is stocked by Radio Shack.

Weller, Ungar, and many electronics suppliers also carry soldering stations which have a control that allows one to vary the temperature of the soldering iron tip. This is a good way to control heat and allows an iron with one element to be used for any soldering job by varying the heat to suit the size of the components being joined.

Iron-clad tips are the best however solid copper ones will serve the purpose but require a little more care. The iron tips do not corrode as rapidly after being used and typically last longer. Many soldering irons have tips that may be changd to suit the type of part you are soldering. A fine point tip will be used on IC leads and a larger chisel-shaped tip may be put on to transfer more heat to heavier components like jacks and cables.

Any tip should be tinned the first time it is heated up. That is, a light coat of solder is applied to the hot tip to protect it from oxidation and corrosion. This coating may be removed and reapplied as required. The thinnest layer that can be adhered to the tip is the best. A damp sponge is often kept by the soldering station to wipe the hot tip on between uses. This will remove flux and excess solder. A plain sponge from the hardware store will suffice. Be sure it is not the type that is saturated in soap or cleaning products. Cut a few parallel slits in the sponge, then when you wipe the tip in the slits, it will clean the entire tip in one pass.

A copper tip that has become corroded or cover with oxidized solder can be renewed with a metal file. The oxidation and solder are removed and the tip reshaped to a fine point. Then the shiny copper tip should be retinned to protect it. This technique should not be used on an iron-clad tip since it will remove the iron coating. Between treatments with the file, I will scrape the tip with the edge of a knife to remove light corrosion or solder build-up, again, re-tinning after cleaning.

In case you haven't noticed, there are more than one type of solder. In fact, there are quite a few types. Basic electronics solder is an alloy of tin and lead. A 60/40 solder has a composition of 60% tin and 40% lead. This will work well for almost any project. A 63/37 solder or "eutectic solder" is also available which has a lower melting point and hardens to a more uniform surface.

Wire solder is sold in coils or on spools and comes in a variety of diameters; 0.016", 0.020", 0.032", and larger or smaller sizes. I tend to use a small diameter solder since it is easy to control the amount being applied, but any size in that range is acceptable.

Wire solder is usually hollow with the core filled with rosin flux. This flux material removes metal oxides from the surface of the work and allows the solder to adhere better. Always use a solder containing flux. Never use acid-core solder, or paste flux. Sal ammoniac blocks are sold for tip cleaning but they should be avoided as well since the chloride salt residue from the block will cause corrosion on pc boards and connectors. Remember, rosin flux only.

It is a believed by many builders of high end audio equipment that silver solder gives a better, more conductive joint and therefore better sound. Typical silver solders contain 2% to 4% silver along with the tin and lead, and are available readily from the same sources as regular solder.

Lead-free solders that contain only tin and silver are also sold, but they are more difficult to use since the melting point of the alloy is higher and it is easier to make "cold joints" with it. I stay away from it for electronics use.

The trick to making a good solder joint is that less is more. Less heat; less solder. You want just enough heat for the solder to wet the surface, but not enough to damage components or raise the copper pc trace. It is easy to tell when this occurs. Solder applied to a too cold surface will ball up or refuse to adhere. As more heat is applied, the solder will flow over the heated surfaces and form a smooth shiny coating. It is important to hold the pieces of the connection completely still while the solder cools. Any movement can cause a poor solder joint. It only takes a few moments for the iron tip to heat the components so that the solder flows evenly over the metal. Excess solder can cause bridging of cicuit traces and malfunction of the project, or drips of solder can fall on other parts of the circuit.

A thin shiny coating with a good mechanical connection is what you are trying to achieve. Heat must be applied to both of the surfaces being joined so that the solder can make that connection properly.

The copper traces on the pc board should be very clean before you mount any components. A steel wool pad (without soap impregnated in it) or fine grade sandpaper (400 or 600 grit) may be used to remove surface oxidation from the copper. Lightly buff the pc board traces until shiny, then wash the board with acetone or isopropyl alcohol on a clean cloth to remove any sanding residue. Allow the board to dry thoroughly before soldering on it since both of these solvents are flammable.

The leads that extend from many components also need cleaning to make a good connection with the board. I have found that capacitor leads are tinned to protect them but the solder coating has oxidized after manufacture. Resistor leads are likewise subject to oxidation. There is also a waxy residue found on component leads from the tape rolls on which they are handled or stored. A quick wipe with a piece of fine sandpaper or a light scrape with the edge of a knife blade will remove the oxidized or waxy coating and make soldering easier.

For the beginner, a good way to practice is with a piece of perfboard and some surplus components. Use perfboard that has copper pads etched on it and build up your technique by soldering components to the pads. Apply heat to the pad and the component lead simultaneously. After a few seconds apply solder to the pad and lead, not to the iron. If it does not melt, wait a few moments and try again. It will not take long before you are able to make round shiny joints on the first try.

Soldering leaves behind some of the rosin flux after you have made the joint. The flux residue should be removed before using the circuit since a clean board will perform better and flux can even act as a small-value capacitor when it is left between adjacent traces or connectors. Circuit board cleaners and flux removers are availble at most places that sell solder and irons. Use it sparingly to remove flux or other residue from the board. TF-113 was used for many years and was the ideal solvent for this purpose, however, it is no longer available since it is a CFC and has ozone-depleting potential. Unfortunatey, many of the replacements are flammable and more irritating if inhaled so care must be exercised in their use. As with any chemical, handle with care, and wash your hands after using, especially before eating or smoking.

Remember a cold solder joint is dull and often had a crusty or crystalline appearance. A good solder joint is shiny, smooth and mechanically strong. Practice makes perfect, but making good solder joints is a task easily mastered.

Selected Resources

Soldering Equipment - Small Bear Elec.

Soldering Supplies - Circuit Specialists

Inexpensive Soldering Accessories

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