Soldering Processes

Nov. 15, 2002
Soldering uses alloys that melt below 840°F to join metals.

Soldering uses alloys that melt below 840°F to join metals. Molten solder fills the space between surfaces to be joined, adheres to the surfaces, and solidifies.

Typical soldering steps include:

  • Cleaning the joint
  • Cleaning the metal
  • Applying flux, and
  • Applying heat and solder.

Electrical and electronic applications account for much soldering. Sealing, such as tin-can and radiator seams, is the second most common use.

Automated soldering equipment produces high-quality joints at a relatively low cost per joint.

Conditions for choosing a heating method involve:

  • Cost of providing the heat to bring an assembly to soldering temperature.
  • Efficiency in bringing joint to soldering temperature.
  • Sensitivity of the assembly to heat.
  • Ability to automate.

Solder alloys:The largest group of soldering alloys is tin-lead alloys. These solders are compatible with all base-metal cleaners, fluxes, and heating methods, and can join most metals. Joint clearances of 0.002 ± 0.0005 in. are recommended.

The highest-melting-temperature solder in this group, 5A, is suitable where operating temperature of the assembly may reach 300°F. General-purpose tin-lead solders have classifications 35A through 50A. They provide optimum soldering properties and good strength at operating temperatures below 250°F. The 60A solder is particularly suitable for delicate heat-sensitive electronic components. Solders with higher tin contents, such as 70A, can be used for soldering zinc. Where moisture might reach the assembly, 95% zinc/5% aluminum solder should be used.

Tin-lead-antimony alloys are used for the same types of applications as tin-lead solders. They are not recommended for use on aluminum, zinc, or galvanized steel. Addition of antimony (up to a maximum of 6%) in place of some of the tin improves strength and does not affect wettability or flow characteristics. Joint-clearance recommendations are the same as tin-lead solders.

Tin-antimony solder (95% tin) has the best electrical properties of any common solder alloy. It has high strength at temperatures up to 300°F and provides excellent flow characteristics.

Tin-silver alloys have the same characteristics as tin-antimony solders and are used for delicate instrument work and high-strength applications.

Tin-zinc alloys are used mainly for soldering aluminum -- primarily where a low soldering temperature is required.

Lead-silver solders have high tensile, creep, and shear strengths to 350°F. Fatigue properties are better than those of nonsilver solder alloys. Lead-silver solders have poor wetting characteristics, however, and corrode in humid atmospheres. The addition of 1% tin, replacing silver, increases wetting and flow, and reduces susceptibility to corrosion. When tin is added, silver content should be limited to 1.5% to avoid segregation.

Cadmium-zinc solders are used to join aluminum to itself and to other metals. They are suitable for applications where service temperatures may reach 400°F or higher. Zinc-aluminum solders are specifically designed for soldering aluminum. They provide high joint strength and good corrosion resistance. A 95% zinc/5% aluminum solder is used without flux for ultrasonic soldering of heat exchangers.

Indium solders are used for special applications, such as cryogenic products. The 50% indium/50% tin alloy is suitable for glass-to-glass and glass-to-metal soldering.

Fusible alloys are bismuth-containing solders, used where soldering must be done at temperatures below 361°F. These solders creep during longtime loading above room temperature. Higher-bismuth-content solders are not easily adaptable to high-speed soldering, nor do they easily wet base metals.

Nonferrous materials: Most nonferrous metals and their alloys can be soldered. Some require special solders or fluxes and carefully controlled production methods.

Aluminum soldering differs from other common metals in several ways. Aluminum forms a tenacious, refractory oxide; therefore, use of reaction fluxes is mandatory, except for ultrasonic soldering.

Aluminum soldering requires special techniques to produce flow into certain joints. Solder composition is more important with aluminum than with many other metals, because galvanic corrosion can cause catastrophic failure.

Commonly soldered wrought-aluminum alloys are 1060, 1100, 1145, 3003, 5005, 6061, 7072, and 8112. Aluminum casting alloys are not solderable. Solders with high zinc content are used on wrought-aluminum assemblies. Tin-lead solders are not recommended because of their poor corrosion resistance.

Copper is one of the easiest metals to solder. Nearly all solders and fluxes can be used; the choice depends primarily on the application.

Most copper-base alloys have good solderability. However, alloys containing beryllium, silicon, or aluminum require acid fluxes. Also, copper-zinc alloys (brasses) should not be soldered with tin-antimony or tin-lead-antimony solders containing more than 1% antimony or 0.02% arsenic.

Nickel and high-nickel alloys can be soldered either to themselves or to other solderable metals. Pure nickel and such alloys as Monel, K Monel, Permanickel, and Duranickel are solderable with inorganic fluxes. Alloys such as Inconel, Incoloy, Inconel X, and Nimonic 75 are difficult to solder.

Age-hardenable materials should be soldered after aging. The temperature involved in soldering will not soften such metals as Duranickel, K Monel, and Inconel X, which have been fully age hardened. Relatively high-tin solders such as 60% tin/40% lead or 50% tin/50% lead are used for joining high-nickel alloys.

Zinc soldering is not difficult. Zinc-chloride fluxes containing some hydrochloric acid are generally used. Zinc die castings normally contain aluminum and other alloys that make them very difficult to solder. These must be treated as though they were an aluminum alloy.

Ferrous materials:Solderability of steel is influenced by its alloying constituents. Low-alloy or mild steels are most easily soldered. Mildly corrosive fluxes are necessary. Medium-carbon steels (0.30 to 0.45% C) solder less readily than low-carbon steels (0.30% C or less). Consequently, surface preparation is more critical. High-carbon steels (over 0.45% C) are seldom soldered. A hot alkaline cleaner, followed by an acid dip, generally produces good soldering surfaces. To facilitate fabrication, these metals can be precoated with solder to provide corrosion protection and improve on-site joining.

Stainless steels can be joined by commercially available lead-tin solders, but corrosive stainless-steel fluxes must be used. If the soldered joint is inaccessible and flux cannot be removed after assembly, pretinning of parts is imperative. Cast iron should not be soldered.

In the case of critical applications, including those for nuclear products, designers should make sure there are no detrimental reactions between the base metals and the solder used.

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