Indium Blog

Halogen-Free Solder Paste

For my first entry, I'd like to say a word about the differences between Halogen-Free and Halide-Free, for both fluxes and solder pastes. The halogens are the group 7a elements on the periodic table. Common halogens are Chlorine and Iodine, both of which are used in our daily lives. The others are Fluorine, Bromine and Astatine. (Astatine, per Wikipedia is a halogen, but is a highly radioactive element has a half-life of 125 nanoseconds to 8.3 hours, depending on the isotope. It is the rarest naturally occurring element with less than one ounce available in the earth's crust at any given time. I don't think you will find this halide or halogen in solder paste paste.) What makes halogens so special is that in their ionic forms (halides), they are all missing a single electron, which it wants REALLY BAD. Making them very active, chemically. But, they form 2 types of compounds, covalently-bonded halogen and ionic-bonded halides. So, solder pastes that my be described as "halide-free" may not be completely devoid of Fluorine, Chlorine, Bromine, and Iodine. But, only free of ionically bonded halides. The other side of the coin, is Halogen-Free solder pastes and flux. These are "truly" halide-free, and halogen-free. Which means, that under chemical testing, there is NO Fluorine, Chlorine, Bromine, Iodine or Astatine. Of course there are different ways to test for each, halide-free and halogen-free solder paste and flux, which include Titration, Ion Chromatography, and Oxygen (Parr) Bomb/Ion Chromatography. Here are some details about each (Thank you to Indium's Advanced Assembly Materials Product Manager, Tim Jensen for the details): Titration: Titration can be an effective method for assessing ionic halides within a flux. However, if a solder paste manufacturer decides to use halide activators, they will typically use a covalently bonded halides (which are better for SIR and long term reliability) for no cleans which are not detected through titration. Therefore, a statement such as "halide-free by titration" simply means that there are no ionic halides. In addition, there are many chemicals used in fluxes that can interfere with the test to cause a false positive (i.e. appear as if they are a halide). Ion Chromatography (IC): This is the currently recommended test method used by the IPC J-STD-004. However, it suffers similar challenges to titration in that it is only good at detecting ionic halides. Ion chromatography is also prone to interference, which can cause false positives. Oxygen Bomb / IC: This test method actually burns off all of the organics and Hydrogen parts of the flux. The halides remain in the ash. This ash is dissolved and run through ion chromatography. This method breaks the covalent bonds and minimizes any potential interference to allow IC to give you a true halide reading. This is Indium Corporation's standard method for testing halide-free and halogen-free content. More information may be found at our Online Help: Indium Knowledge Base.