A group of welding processes that produces coalescence of materials by heating them to the soldering temperature and by using afiller metal having a liquidus not exceeding 450°C (840°F) and below the solidus of the base metals. The filler metal is distributed between closely fitted faying surfaces of the joint by capillary action. .
The soldered joint is generally considered to be a metallurgical bond between the solder filler metal and the base metals being joined. Strength of the joint can be enhanced by mechanical configuration of the joint. Some solder joints do not have a metallurgical bond, but are held together by adhesion properties of the interface.
The metallurgical solder joint is produced by reaction of the base metals and the filler metal. The solder alloy is applied as a liquid metal that wets and spreads in the joint, and generally forms a layer of an intermetallic compound with a small amount of the base metal. On solidification, the joint is held together by the same attraction between adjacent atoms that holds a piece of solid metal together.
A sound soldered joint is achieved by the selection and use of specific materials and processes. There are many soldering filler metals, processes, methods, procedures, and types of equipment, and many metal alloys that can be joined. Specific applications require consideration of all these factors to obtain the optimum manufacturing and service results. Filler metal selection, joint design, metal cleaning, heating methods, fluxes, and joint properties are variables. Temperature ranges of commonly used soldering alloys are compared with base metal melting points in Figure S-18.
Advantages of Soldering- A major factor in the popularity of soldering is that it is a low-temperature process and therefore has minimum effect on base metal properties. The low temperature used for joining requires little energy input and allows precise control of the process. A wide range of heating methods can be adopted, giving flexibility in design and manufacturing procedures. Modem automation produces large numbers of joints in electrical and electronic circuits. Highly reliable joints can be obtained with carefully controlled procedures. The occasional defective soldered joint can easily be repaired.
Chemistry, physics, and metallurgy are the main disciplines involved in soldering. Wetting and spreading solder filler metals on metallic surfaces are conditioned by the surface tension properties of the materials involved and the degree of alloying taking
place during the soldering action. Soldering normally requires the presence of a flux. The flux cleans the metal to be joined and lowers the surface tension between the molten metal and the solid substrate. The flux improves the wetting and spreading of the solder
Wetting takes place when the solder leaves a continuous permanent film on the base metal surface. Alloying depends on the solubility of the base metal in molten solder metal. A high level of alloying between the base metal and solder metal can retard spreading, therefore, good solder filler metals usually dissolve only a moderate amount of metal. Intermetallic compounds may form, depending of the metal systems involved.
Many solder joints are designed with gaps that require capillarity between the solder and base metal. Capillary action is improved by lowering surface tension, narrowing the gap in the joint, and using a highly compatible displacement flux.
Surfaces of the materials to be joined must be cleaned of dirt, oxides, or other contaminants. One function of a flux is to provide a final cleaning by chemical reaction with the metal surface. This attack should be slight but effective. Covering the surface
with flux is no substitute for prior cleaning.
When heated, the flux is activated; it cleans contacted surfaces and protects the cleaned areas from oxidation during soldering. The solder filler metal is applied when the joint has been heated to the soldering temperature. The surfaces are protected by the acti-
vated flux during soldering action. When soldered joints have been cooled, some residual flux may be present that needs to be removed to prevent early joint deterioration.
Physical problems affecting wetting, spreading, and capillary action can result in unsatisfactory joints. They generally result from poor surface condition or improper flux. Some metals, for example, chromium, cannot be readily wet by most known solder filler metals. De-wetting is the retraction of solder on an already wetted surface which leaves areas of incomplete coverage. Inadequate cleaning, poor flux selection, and wrong solder composition are the main causes of dewetting.
Basic Steps
Base Metal Selection. Base metals are usually selected for specific properties that are needed for the component or part design. These include strength, ductility, electrical conductivity, weight, and corrosion resistance. The solderability of the base materials must also be considered because the selection of flux and surface preparation will depend on the base materials.
Solder Selection. The solder is selected to provide good flow, penetration and wetting capability in the soldering operation, and the desired joint properties in the finished product.
Flux Selection. Flux is intended to enhance the wetting of base materials by the solder by removing tarnish films from precleaned surfaces, and by preventing oxidation during the soldering operation. The selection of the type of flux usually depends on the solderability
of the base materials. Rosin fluxes are used with base metals in electrical and electronic applications, or with metals that are precoated with a solderable finish. Inorganic fluxes are often used in industrial soldering such as plumbing and vehicle radiators. The flux requirements for soldering a number of alloys and metals are indicated in Table S-4.
Joint Design- Joints should be designed to fulfill the requirements of the finished assembly and to permit application of the flux and solder by the soldering process that will be used. Joints should be designed so that proper clearance is maintained during heating. Special fixtures may be necessary, or the components can be crimped, clinched, wrapped, or otherwise held together.
Precleaning- All metal surfaces to be soldered should be cleaned before assembly to facilitate wetting of the base metal by the solder. Flux should not be considered as a substitute for precleaning. Precoating may be necessary for base materials that are difficult to solder.
Soldering Process- The soldering process should be selected to provide the proper soldering temperature, heat distribution, and rate of heating and cooling required for the product being assembled. Application of the solder and flux will be dictated by the selection of the soldering process.
Flux Residue Treatment- Flux residue should be removed after soldering unless the flux is specifically designed to be consumed during the process.
Solders
Solders have melting points or melting ranges gen- erally below 425°C (SOOOF). A wide range of solder filler metals designed for use with most industrial metals and alloys are commercially available. These generally flow satisfactorily with the appropriate fluxes to produce good surface wetting, and result in joints with satisfactory properties. Tin-lead alloys are the most widely used solder filler metals.
Historical Background
Soldering is a technology that has been in continuous development from ancient times. Many artifacts discovered in archeological excavations were joined by soldering. The technology seems to have existed for several thousand years, with changes as metallurgical knowledge and new metals were discovered.
Copper and lead alloys were the first to be joined. Early metallurgists learned to identify eutectics in binary systems. The use of eutectic alloys permitted soldering to join simple shapes into complex items of jewelry and utensils. The industrial revolution promoted widespread use of soldered joints. Advancements in alloy joining, processing techniques, and applications continue today. Soldering is now used in industrial applications, satellite communications, computers, and the space program.
The following is excerpted from the Welding Encyclopedia, First Edition, edited by L. B. Mackenzie, Welding Engineer Publishing Company, Chicago, 1921.
Fusible alloys are used to join metals by soldering. The types of soldering are distinguished by self-descriptive names: hard, soft, silver, gold, aluminum, copper, tin, pewter, and spelter. The kind of solder used depends on the metals to be joined; in all cases, the solder should be more fusible than the metals to be joined.
Hard solders are called spelter, and hard soldering is called brazing. Brazing produces greater strength than soldering with the soft solders. Hard solders will also withstand more heat than soft solders. Hard solders contain metals such as copper, zinc, or silver, and require a red heat to melt them.
Soft solders are made of such metals as lead, tin, or bismuth. They are used for applications in which the articles to be soldered must be air- or water-tight, but are not exposed to high temperatures, and when strength is not a factor. It is a much simpler operation to join metals with soft solder than with hard solder, and soft soldering is used when possible in place of brazing.
The ordinary good grade of solder is made of tin and lead in equal parts. Fine solder: two parts tin, one part lead; cheap solder: one part tin, two parts lead.
Mixing Solders- By varying the proportions and adding bismuth, a solder can be made that will melt in boiling water. In mixing solders, the least fusible metal should be melted first and the more easily fusible metals added. Mixing soft solders should be done under melted tallow and agitated by thrusting a stick of green wood or raw potato under the molten metal. The escaping steam stirs and mixes the metals very thoroughly. They can be then run out in molds. A small channel of angle iron will serve in mixing hard solders. They should be melted under a coating of powdered charcoal or borax. Hard solders may be reduced to granulated form (the most convenient form for use) by casting into small strips or ingots and filing with a coarse file.
Silver solder should be rolled or hammered into thin strips or sheets and cut to suitable size. A silver coin hammered thin makes a very satisfactory solder for iron, copper and hard brass. Copper to which is added 10% of silver is suitable for soldering sheet steel.
Soldering
To prepare for soldering, the surfaces must be cleaned, either by scraping or filing, or by using a suitable acid, or a combination of both. In the case of sheet iron covered with scale, one method is to scrape or file the surface, or scratch it with a wire brush and coat it with strong muriatic (hydrochloric) acid, letting the acid act for 5 or 10 minutes, and then wiping it dry and applying cut acid (hydrochloric acid to which an excess of zinc has been slowly added).
In the soldering process, the metals must be heated above the melting point of the solder, and since metals readily oxidize when heated, a flux is necessary to coat the surfaces after they are cleaned, to prevent their oxidation. Cast iron may be soldered by using a flux made by adding zinc chloride to melted tallow and heating until it foams and turns a reddish brown. Zinc chloride solution also is satisfactory. It is very important, however, to clean the surface very thoroughly and solder immediately after cleaning.
Selection of Flux- For hard soldering, borax is used as a flux. For silver soldering, finely powdered borax mixed with water to the consistency of paste is very effective. However, this flux should be allowed to dry after applying. For soldering galvanized iron, raw hydrochloric acid is used as a flux.
For soldering copper, brass, or gunmetal, a flux of zinc chloride, ammonium chloride, or rosin is used. For soldering zinc, galvanized iron and steel, hydrochloric acid or ammonium chloride (Sal-ammoniac) is used. For soldering tinware, pewter or lead, a flux of rosin, turpentine, or Russian tallow is used.
Jewelry Soldering
If acids are used to clean the soldered joint, it should be thoroughly washed to remove all excess acid to prevent subsequent corrosion. When soldering jewelry, zinc chloride should be used. For soldering small pieces, tin foil cut to size and moistened with a solution of Sal-ammoniac placed between the pieces to be soldered may be used. The pieces should be made flat and smooth at the joint, the tin foil inserted and the pieces gently heated. For soldering gold articles, a solder made of two grams silver, one gram copper, and one pennyweight gold may be used with success. A good solder for general use contains 18 parts gold, 4 parts silver, six parts copper, and two parts zinc. Antimony, arsenic, tin and lead should not be used in soldering gold.
Burnt Borax Flux- The flux used is usually borax, but it should be properly prepared. This is done by covering the bottom of a pan with a thin, even layer of ordinary commercial borax and heating it over a slow fire until it will crumble in the fingers to a fine dry powder. An hour’s heating should be sufficient.
Brazing Solders- The brazing solder and the burnt borax are thorough mixed in suitable proportions, and water is added to bring the supply, for later use, to the consistency of putty. This is mixed with more water, as required, to a mixture like grout in cement work, and applied to the parts to be soldered. For brass and steel tubes, a proportion of 10parts solder to one part burnt borax is used. Other mixtures are used for other metals. The melting points of the metals being soldered must be taken into account in selecting the grade of solder to be used.”