These are alloys of two or more metals used for joining other metals together by surface adhesion without melting the base metals as in welding and without requiring as high a temperature as in brazing. However, there is often no definite temperature line between soldering alloys and brazing filler metals. A requirement for a true solder is that it have a lower melting point than the metals being joined and an affinity for, or be capable of uniting with, the metals to be joined.

Types and Forms

The most common solder is called half-and-half, plumbers’ solder, or ASTM (American Society for Testing and Materials) solder class 50A, and is composed of equal parts of lead and tin. It melts at 182°C. The density of this solder is 8802 kg/m3, the tensile strength is 39 MPa, and the electrical conductivity is 11% that of copper. SAE (Society of Automotive Engineers) solder No. 1 has 49.5 to 50.0% tin, 50% lead, 0.12% max antimony, and 0.08% max copper. It melts at 181°C. Much commercial half-and-half, however, usually contains larger proportions of lead and some antimony, with less tin. These mixtures have higher melting points, and solders with less than 50% tin have a wide melting range and do not solidify quickly. Sometimes a wide melting range is desired, in which case a wiping solder with 38 to 45% of tin is used. A narrow-melting-range solder, melting at 183 to 185°C, ASTM solder class 60A, contains 60% tin and 40% lead. A 42% tin and 58% lead solder has a melting range of 183 to 231°C. Slicker solder is the best quality of plumbers’ solder, containing 63 to 66% tin and the balance lead.

Solder alloys are available in a wide range of sizes and shapes, enabling users to select that one that best suits their application. Among these shapes are pig, slab, cake or ingot, bar, paste, ribbon or tape, segment or drop, powder, foil, sheet, solid wire, flux cored wire, and preforms. There are 11 major groups of solder alloys:

Tin-antimony. Useful at moderately elevated operating temperatures, around 149°C, these solders have higher electrical conductivity than the tin-lead solders. They are recommended for use where lead contamination must be avoided. A 95% Sn-5% Sb alloy has a soli-dus of 235°C, a liquidus of 240°C, and a resulting pasty range of -11.1°C.

Tin-lead. Constituting the largest group of all solders in use today, the tin-lead solders are used for joining a large variety of metals. Most are not satisfactory for use above 149°C under sustained load.

Tin-antimony-lead. These may normally be used for the same applications as tin-lead alloys with the following exceptions: aluminum, zinc, or galvanized iron. In the presence of zinc, these solders form a brittle intermetallic compound of zinc and antimony.

Tin-silver. These have advantages and limitations similar to those of tin-antimony solders. The tin silvers, however, are easier to apply with a rosin flux. Relatively high cost confines these solders to fine instrument work. Two standard compositions: 96.5% Sn-3.5% Ag, the eutectic; 95% Sn-5% Ag, with a solidus of 221°C and liquidus of 245°C.

Tin-zinc. These are principally for soldering aluminum since they tend to minimize galvanic corrosion.

Lead-silver. Tensile, creep, and shear strengths of these solders are usually satisfactory up to 177°C. Flow characteristics are rather poor and these solders are susceptible to humid atmospheric corrosion in storage. The use of a zinc chloride base flux is recommended to produce a good joint on metals uncoated with solder.

Cadmium-silver. The primary use of cadmium-silver solder is in applications where service temperature will be higher than permissible with lower melting solder. Improper use may lead to health hazards. The solder has a composition of 95% Cd-5% Ag. Solidus is 338°C and liquidus is 393°C.

Indium-lead alloys. These are alkali-resistant solders. A solder with 50% lead and 50% indium melts at 182°C, and is very resistant to alkalies, but lead-tin solders with as little as 25% indium are resistant to alkaline solutions, have better wetting characteristics, and are strong. Indium solders are expensive. Adding 0.85% silver to a 40% tin soft solder gives equivalent wetting on copper alloys to a 63% tin solder, but the addition is not effective on low-tin solders. A gold-copper solder used for making high-vacuum seals and for brazing difficult metals such as iron-cobalt alloys contains 37.5% gold and 62.5% copper.

Palladium-nickel. A palladium-nickel alloy with 40% nickel has a melting point about 1237°C. The brazing filler metals containing palladium are useful for a wide range of metals and metal to ceramic joints.

The remaining four groups of Zn-Al, Cd-Zn, and solders containing bismuth and indium were covered earlier.

Silver Solder

Silver solder is high-melting-point solder employed for soldering joints where more than ordinary strength and, sometimes, electric conductivity are required. Most silver solders are copper-zinc brazing filler metals with the addition of silver. They may contain from 9 to 80% silver, and the color varies from brass yellow to silver white. Cadmium may also be added to lower the melting point. Silver solders do not necessarily contain zinc, and may be braze filler metals of silver and copper in proportions arranged to obtain the desired melting point and strength. A silver braze filler metal with a relatively low melting point contains 65% silver, 20% copper, and 15% zinc. It melts at 693°C, has a tensile strength of 447 MPa, and elongation 34%. The electrical conductivity is 21% that of pure copper. A solder melting at 760°C contains 20% silver, 45% copper, and 35% zinc. ASTM silver solder No. 3 is this solder with 5% cadmium replacing an equal amount of the zinc. It is general-purpose solder. ASTM silver solder No. 5 contains 50% silver, 34% copper, and 16% zinc. It melts at 693°C, and is used for soldering electrical work and refrigeration equipment.

Any tin present in silver solders makes them brittle; lead and iron make the solders difficult to work. Silver solders are malleable and ductile and have high strength. They are also corrosion resistant and are especially valuable for use in food machinery and apparatus where lead is objectionable. Small additions of lithium to silver solders increase fluidity and wetting properties, especially for brazing stainless steels or titanium. Sil-Fos is a phosphor-silver brazing solder with a melting point of 704°C. It contains 15% silver, 80% copper, and 5% phosphorus. Lap joints brazed with Sil-Fos have a tensile strength of 206 MPa. The phosphorus in the alloy acts as a deoxidizer, and the solder requires little or no flux. It is used for brazing brass, bronze, and nickel alloys.

Another grade, Easy solder, contains 65% silver, melts at 718°C, and is a color match for sterling silver. TL silver solder has only 9% silver and melts at 871 °C. It is brass yellow in color, and is used for brazing nonferrous metals. Sterling silver solder, for brazing sterling silver, contains 92.8% silver, 7% copper, and 0.2% lithium. Flow temperature is 899°C.

A lead-silver solder to replace tin solder contains 96% lead, 3% silver, and 1% indium. It melts at 310°C, spreads better than ordinary lead-silver solders, and gives a joint strength of 34 MPa. Silver-palladium alloys for high-temperature brazing contain from 5 to 30% palladium. With 30%, the melting point is about 1232°C. These alloys have exceptional melting and flow qualities and are used in electronic and spacecraft applications.

Cold Solders

Cold solder, used for filling cracks in metals, may be a mixture of a metal powder in a pyroxylin cement with or without a mineral filler, but the strong cold solders are made with synthetic resins, usually epoxies, cured with catalysts, and with no solvents to cause shrinkage. The metal content may be as high as 80%. Devcon F, for repairing holes in castings, has 80% aluminum powder and 20% epoxy resin. It is heat-cured at 66°C, giving high adhesion. Epoxyn solder is aluminum powder in an epoxy resin in the form of a putty for filling cracks or holes in sheet metal. It cures with a catalyst. The metal-epoxy mixtures give a shrinkage of less than 0.2%, and they can be machined and polished smooth.

Lead-Free Solder Replacements

In spite of the sustained efforts of researchers and technology leaders in packaging, to date there is no lead-free solder alloy that is a drop-in replacement for tin-lead solder in assembly processes. Because tin-lead solder has been used for so long and is so much a part of the typical process engineer’s thinking, the quest for an affordable lead-free alloy replacement is facing mounting pessimism that the effort will be successful.

On the other hand, optimism that adhesive-type solders will prove to be a serious alternative to metallurgical materials continues to grow. These polymer-based conductive adhe-sives are being used in various applications previously "reserved" for tin-lead solders. Regular production equipment and traditional assembly processes are producing high-quality assemblies with demonstrated long-term reliability using the new solders. And for some products, polymers are considered an enabling technology. One major market is the polyester-based flexible circuit market, particularly those built using polymer thick film.

Polymer Solders

Polymer solders are also known as "conductive adhesives," or materials that provide the dual functions of electrical connection and mechanical bond. The adhesive components of a typical material are some form of polymer, i.e., long-chain molecules widely used to produce structural products, which are also known for their excellent dielectric properties. Already used extensively as electrical insulators, as solders their necessary conductivity is accomplished by adding highly conductive fillers to the polymer binders.

The most common polymer solders are silver-filled thermosetting epoxies supplied as one-part thixotropic pastes. Silver is used not only because it is usually cost-effective, but also for its unique conductive oxide. A blend of silver powder and flakes achieves high conductivity while maintaining good printability. Because the mechanical strength of the joint is provided by the polymer, the challenge in a formulation is to use the maximum metal loading without sacrificing the required strength. (Some polymer solders contain more than 80% metal filler by weight.)

Polymer solders do not typically form metallurgical interfaces in the usual sense. Electrical integrity requires that the metal filler particles be in close contact to form a conductive path between the circuit trace and the component lead. Ideally, the silver flakes will overlap and smaller particles will fill in the gaps to form a conductive chain.

In the past, polymer solders were successful only on circuits using precious-metal conductors. This was because junction resistance was seen to increase to unacceptable levels when ordinary printed circuit boards and components were joined with these materials. To solve the instability problem it was necessary to create stable, nonmetallurgical junctions with oxidiz-able surfaces. For example, one polymer-solder formulation, used on flexible circuits and recently optimized for rigid boards, provides junction stability between solder-coated and bare-copper surfaces via polymer shrinkage during curing to force irregular particles through the interface oxides.

Advantages in using polymer solders include compatibility with a range of surfaces including some nonsolderable substrates; low-temperature processing, resulting in lower thermal stress; no pre- or postclean requirements, thereby reducing equipment needs and cycle times, and lowering or eliminating the release of volatile organic compounds.

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