a. SOLDERING weldingtheory
a. General. Soldering is the process of using fusible alloys for joining metals. The kind of solder used depends on the metals being joined. Hard solders are called spelter, and hard soldering is called silver solder brazing. This process gives greater strength and will withstand more heat than soft solder. Soft soldering is used for joining most common metals with an alloy that melts at a temperature below that of the base metal, and always below 800°F (427°C). In many respects, this is similar to brazing, in that the base is not melted, but merely tinned on the surface by the solder filler metal. For its strength, the soldered joint depends on the penetration of the solder into the pores of the base metal and. the formation of a base metal-alloy solder.
b. Solders of the tin-lead alloy system constitute the largest portion of all solders in use. They are used for joining most metals and have good corrosion resistance to most materials. Most cleaning and soldering processes may be used with the tin-lead solders. Other solders are: tin-antimony; tin-antimony lead; tin-silver; tin-lead-silver; tin-zinc; cadmium-silver; cadmium-zinc; zinc-aluminum; bismuth (fusible) solder; and indium solders. These are described below. Fluxes of all types can also be used; the choice depends on the base metal to be joined.
(1) Tin-antinmony solder. The 95 percent tin-5 percent antimony solder provides a narrow melting range at a temperature higher than the tin-lead eutectic. the solder is used many plumbing, refrigeration, and air conditioning applications because of its good creep strength.
(2) Tin-antimony-lead solders. Antimony may be added to a tin-lead solder as a substitute for some of the tin. The addition of antimony up to 6 percent of the tin content increases the mechanical properties of the solder with only slight impairment to the soldering characteristics. All standard methods of cleaning, fluxing, and heating may be used. weldingtheory
(3) Tin-silver and tin-lead-silver solders. The 96 percent tin-4 percent silver solder is free of lead and is often used to join stainless steel for food handling equipment. It has good shape and creep strengths, and excellent flow characteristics. The 62 percent tin-38 percent lead-2 percent silver solder is used when soldering silver-coated surfaces for electronic applications. The silver addition retards the dissolution of the silver coating during the soldering operation. The addition of silver also increases creep strength. The high lead solders containing tin and silver provide higher temperature solders or many applications. They exhibit good tensile, shear, and creep strengths and are recommended for cryogenic applications. Because of their high melting range, only inorganic fluxes are recommended for use with these solders.
(4) Tin-zinc solders. A large number of tin-zinc solders have come into use for joining aluminum. Galvanic corrosion of soldered joints in aluminum is minimized if the metals in the joint are close to each other in the electrochemical series. Alloys containing 70 to 80 percent tin with the balance zinc are recommended for soldering aluminum. The addition of 1 to 2 percent aluminum, or an increase of the zinc content to as high as 40 percent, improves corrosion resistance. However, the liquidus temperature rises correspondingly, and these solders are therefore more difficult to apply. The 91/9 and 60/40 tin-zinc solders may be used for high temperature applications (above 300°F (149°C)), while the 80/20 and the 70/30 tin-zinc solders are generally used to coat parts before soldering.
(5) Cadmium-silver solder. The 95 percent cadmium-5 percent silver solder is in applications where service temperatures will be higher than permissible with lower melting solders. At room temperature, butt joints in copper can be made to produce tensile strengths of 170 MPa (25,000 psi). At 425°F (218°C), a tensile strength of 18 MPa (2600 psi) can be obtained. Joining aluminum to itself or to other metals is possible with this solder. Improper use of solders containing cadmium may lead to health hazards. Therefore, care should be taken in their application, particularly with respect to fume inhalation.
(6) Cadmium-zinc solders. These solders are also useful for soldering aluminum. The cadmium zinc solders develop joints with intermediate strength and corrosion resistance when used with the proper flux. The 40 percent cadmium-60 percent zinc solder has found considerable use in the soldering of aluminum lamp bases. Improper use of this solder may lead to health hazards, particularly with respect to fume inhalation.
(7) Zinc-aluminum solder. This solder is specifically for use on aluminum. It develops joints with high strength and good corrosion resistance. The solidus temperature is high, which limits its use to applications where soldering temperature is in excess of 700°F (371°C) can be tolerated. A major application is in dip soldering the return bends of aluminum air conditioner coils. Ultrasonic solder pots are employed without the use of flux. In manual operations, the heated aluminum surface is rubbed with the solder stick to promote wetting without a flux.
(8) Fusible alloys. Bismuth-containing solders, the fusible alloys, are useful for soldering operations where soldering temperatures helm 361°F (183°C) are required. The low melting temperature solders have applications in cases such as soldering heat treated surfaces wherehigher soldering temperatures would result in the softening of the part; soldering joints where adjacent material is very sensitive to temperature and would deteriorate at higher soldering temperatures; step soldering operations where a low soldering temperature is necessary to avoid destroying a nearby joint that has been made with a higher melting temperature solder; and on temperature-sensing devices, such as fire sprinkler systems, where the device is activated when the fusible alloy melts at relatively low temperature. Many of these solders, particularly those containing a high percentage of bismuth, are very difficult to use successfully in high-speed soldering operations. Particular attention must be paid to the cleanliness of metal surfaces. Strong, corrosive fluxes must be used to make satisfactory joints on uncoated surfaces of metals, such as copper or steel. If the surface can be plated for soldering with such metals as tin or tin lead, noncorrosive rosin fluxes may be satisfactory; however, they are not effective below 350°F (177°C).
(9) Indium solders. These solders possess certain properties which make them valuable for some special applications. Their usefulness for any particular application should be checked with the supplier. A 50 percent indium-50 percent tin alloy adheres to glass readily and may be used for glass-to-metal and glass-to-glass soldering. The low vapor pressure of this alloy makes it useful for seals in vacuum systems. Iridium solders do not require special techniques during use. All of the soldering methods, fluxes, and techniques used with the tin-lead solders are applicable to iridium solders.