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Rosin In Soldering Fluxes: A Complex Material

  • Solder Flux
  • Liquid Flux
  • Flux
  • What's in a rosin?  Would refined pine sap by another name...

    IPC J-STD-004B Amendment 1 defines rosin flux as: primarily composed of natural rosin (colophony), extracted from the oleoresin of pine trees and refined. The rosins used shall have a minimum acid value of 130 as determined per ASTM D-465.

    It further defines resin as: primarily composed of synthetic resins and/or natural resins other than rosin types.

    The above definitions give flux manufacturers a tremendous amount of flexibility in how we define our fluxes and, from a practical standpoint, we really have come to believe that whether something is classified as RO or RE makes little difference because, given today’s technologies, the RO or RE classification tells one little about its flux properties.

    All rosins are resins, but not all resins are rosins. We owe the fact that we classify fluxes as rosin to pine tree sap having fluxing properties while still being a dielectric and water-insoluble. This fact did not escape military electronics manufacturers who liked the properties of rosin so much that they went to great lengths to define the best rosin as Grade WW gum rosin, and its properties, from which military grade hardware was to be constructed under contract issued prior to the J-STD. Even though the J-STD was issued in 1995, we still see some legacy MIL contracts requiring MIL-SPEC fluxes. Grade WW gum rosin (water white, defining its color) found its way into both MIL-F14256 (flux) and QQ-S-571 (cored solder, paste, and alloys) and it stuck.

    What is Grade WW gum rosin made of? Gum rosin is refined sap that comes from live pine, and related, trees. Wood rosin, by contrast, comes from ground-up stumps and logs. I once saw a chemical analysis of the Grade WW gum rosin that came from Georgia pine trees… somewhere over the years, I've lost track of it. The complete chemical analysis had between 40 and 50 different components from the following families of chemicals, the primary of which is abietic acid and including various isomer, dimers, polymerized forms of each acid:

    • abietic acid
    • neoabietic acid
    • dehydroabietic acid
    • palustric acid
    • levopimaric acid
    • pimaric acid
    • isopimaric acids
    • terpenes

    While rosin is, in and of itself, a complex mixture of chemicals, to complicate things further, not only does rosin come from all over the world and have differing ratios of acid components, but it is also possible to single out many of the individual rosin fractions and refine them to be a single pure rosin chemical. It is in this way that flux companies can manufacture rosin fluxes that have rosins (or resins) that have different melting or softening points. Refined gun rosin that comes out of the tree has a softening point of about 77°C, which means that this type of rosin will be softening during an 85°C surface insulation resistance (SIR) test. A better choice for high reliability under high temperature and humidity conditions might be a rosin fraction that softens at 95°C or 120°C. The choice of rosin affects both wetting and reliability and it even affects how the residue performs under other tests, such as Foresite’s C3 test. But, that is the subject for another day.

    Indium Corporation formulates its fluxes, solder pastes, and cored wire with the best material for the required job. Generally, if the material is Grade WW rosin or a rosin fraction that is refined from rosin, we will label the flux as RO. In the flux industry today, how fluxes are classified is something of a matter of choice, as long as the definition is close to that provided above by the IPC. Hopefully this is not confusing for the flux user. Whether something is classified as RO or RE matters little.