Indium Corporation - Soldering 101 - A Basic Overview

Reprinted courtesy of Indium Corporation

This Application Note provides a basic understanding of the soldering process to individuals new to the field of oldering. It is not meant to be an exhaustive treatment of the subject, but simply a brief overview.

Metallurgical Bonding Processes

The attachment of one metal to another can be accomplished three basic ways: 1) Welding, 2) Brazing and 3) Soldering. The most significant difference between these methods is the process temperature.

  1. Welding typically involves the bonding of highmelting temperature metals such as steel to steel. The process can be performed with or without the addition of a filler metal. Ideally, the filler metal has properties that match the properties of the adjoining pieces. This process requires very high heat, typically provided by an acetylene torch or DC arc welder. Both metallic surfaces being joined are melted locally to effect the bond.
  2. Brazing, unlike welding, is a lower-temperature process. It does not involve melting of the substrate surfaces, but rather depends on the formation of intermetallics to provide adhesion. The process uses a filler alloy (braze) that has a melting temperature above 350°C but lower than the melting temperature of the metals being bonded. Brazing fluxes are required to remove oxides from the filler material and mating pieces, and to promote good flow of the molten filler. Typically, brazing fluxes contain borates and fluorides and are considered corrosive for electronics use.
  3. Soldering, like brazing, uses a filler metal, and in most cases, an appropriate flux. The filler metals are typically alloys (there are some pure metal solders) that have liquidus temperatures below 350°C. The elemental metals typically alloyed in the filler metals or solders are: tin, lead, antimony, bismuth, indium, gold, silver, cadmium, zinc and copper. The fluxes often contain rosin, acids (organic or mineral) and/or halides, depending on the desired flux strength. These ingredients reduce the oxides on the solder and mating pieces.

The Physical Forms of Solder

The common forms of solder are: 1) Paste, 2) Preforms, 3) Spheres, 4) Wire, 5) Ribbon and Foil, 6) Shot and Ingot (Bar)...

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