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Au/Sn – Different Solder Forms

  • Solder Alloys
  • Flux
  • AuSn

  • In previous posts (one, two , three) we saw the beneficial properties of 80Au/Sn and how surface finishes can be selected to interact with the alloy. Now we look at the different forms in which it can be manufactured.

    Au/Sn is available in all the forms you would expect from conventional solder alloys. That is to say, preforms, ribbon, wire, and powders (usually for pastes).

    Manufacturing competence of Au/Sn solders is an important consideration when selecting suppliers. Au/Sn’s ability to behave as a both a braze and a solder can present challenges in its preparation. As we saw earlier in our second post (Physical Properties) the alloy has a tensile strength ~10x greater than regular alloys. This makes it correspondingly harder and more difficult to process. Hard enough, in fact, that not all soft solder manufacturers can process the alloy. Conversely, the hardness means that, with care and precision, the alloy can be produced in exceptionally thin sections.

    Producing preforms presents similar challenges but, again, the hardness and strength of the alloy allows shapes that would be impossible to handle in softer alloys at corresponding thicknesses. The alloy’s hardness can be a benefit as well as a hindrance. We’ll go into this area in more detail when considering processing.

    A few suppliers can make wire and, of those, fewer still can make wire in very fine diameters. But, 80Au/Sn flux-cored wire is not possible at any size. Flux, if desired, has to be added at time of use.

    Which flux? This has to be addressed also when looking at the possibility of solder pastes. We saw in my first post on this topic that 80Au/Sn can be considered as either a low-temperature braze or a high-temperature solder, and that brazing fluxes are different from soldering fluxes.

    Given the higher processing temperatures of Au/Sn, flux considerations are more complex when compared to standard fluxes. Most organic compounds thermally degrade at about 300°C, and some char. This is certainly true of gum (natural) rosin and a large number of the available activators in standard fluxes. Thus, flux technology for Au/Sn will need to incorporate specialized heat-stable rosins/resins/ water-soluble organic acid fluxes. Post-solder cleaning can also be an issue if the cleaning procedure and parameters are not optimized. In practice, Au/Sn is normally only used with flux in solder pastes where the normal process benefits of paste outweigh possible difficulties of restricted flux choice and cleaning issues. For solid shapes, preforms, ribbons, etc., Au/Sn is soldered fluxlessly in a controlled inert atmosphere.

    Next time we’ll look at reflow and its impact on product specifications.