Indium Blog

80Au/20Sn - solder or braze? Excellent question.

  • AuSn
  • Gold Solder
  • Solder Alloys

  • You may have heard the 80% gold 20% tin alloy referred to as a solder. Or, you may have heard it called a braze. I’m going to explain how both statements are correct.

    As we all know, solders are generally comprised of “soft” metal elements like tin, lead, bismuth, and indium. The hundreds of documented useful alloy combinations these elements produce all melt below about 340°C. Brazes are made from “harder” and stronger elements like silver, copper, aluminium, and zinc. They melt at temperatures above 300°C, sometimes as high as 900°C.

    There are other differences. For example, for solders, a eutectic or near-eutectic composition is usually considered desirable, whereas brazes tend to have a wide pasty range (a large difference between the alloy’s solidus and liquidus temperatures).

    In addition, solder fluxing systems differ greatly from those used for brazes. Solder fluxes generally operate below 350°C for relatively short periods of time and are usually not removed after processing. Brazing fluxes operate at much higher temperatures, protect the joint for a much greater period of time, and are always cleaned off. A large percentage of brazing is performed under an inert or near-inert atmosphere. Conversely, only a small portion of soldering operations experience this. Additionally, solder reflow profiles are very different than braze profiles.

    Here is where the eutectic alloy, 80Au/20Sn, gets interesting; it performs like a braze but processes like a solder.

    80Au/20Sn delivers superior properties and performance, provides very high strength joints, and makes high-reliability connections, all in a lead-free, corrosion resistant formulation, like a braze. It does so via (in most ways) a soldering process – including both flux and reflow profile considerations.

    So, the answer to the question is...either or both! Over the next few posts we’ll take a look at what all this actually means, and how you can take advantage of this unique alloy. Meanwhile, tell me how YOU use 80Au/20Sn alloy.