Halogen Test Methods For Fluxes
Silver Chromate Paper Test (J-STD-004; TM-650 #2.3.33): This is a qualitative test in which a sample of flux is applied to Silver Chromate Paper and allowed to remain for 1-minute. After removing the flux, if the paper changes colors then it indicates the presence of Cl or Br. This test only identifies the halogen in the ionic form and is prone to false positives from chemicals such as amines, cyanides, and isocyanates. It is also provides no indication to the total halogen present.
Titration (J-STD-004; TM-650 #2.3.35): This is a quantitative test that assesses the total halides present in a flux as a Cl equivilant. A flux or flux extract is titrated to its endpoint following the J-STD procedure. This test method is an improvement over Silver Chromate in that it provides a value for how much halide is present. However, it also is only effected by ionic halides. There are also a wide variety of organic chemicals that can falsely be identified as halides.
Ion Chromatography (J-STD-004; TM-650 #2.3.28): This is a quantitative test that can identify the total quantity of halides present in a flux. Based on the retention time in the ion exchange column, a chromatogram is developed and peaks are identified as various ions based on previously developed standards. This test method allows you to quantify how much halides are present and which particular halide. The challenge with ion chromatography is that it only identifies the ionic species and the covalently bonded halides are not detected. In addition, there are chemicals that have similar retention times to Cl- and Br- which can result in non-halides being misidentified as a halide. There is a growing practice of running ion chromatography on reflowed flux residue. This technique can probably more accurately detect halides in the flux because halides won’t volatilize and some of the covalent bonds may have been broken. However, unless all of the covalent bonds are broken, the testing of the flux residue will still under assess the amount of halides present.
Oxygen Bomb Combustion Followed by Ion Chromatography (EN 14582): This is quantitative test method that is recommended by Indium Corporation and is growing in popularity throughout the electronics industry. This test method involves subjecting a sample of flux to an oxygen bomb combustion in which the all of the organic material is burnt off at very high temperatures. The remaining ash consists of the halogens and other inorganic materials. That ash is the run through ion chromatography in which a true reading of halide content can be determined. Any covalently bonded halides have those bonds broken through the oxygen bomb process.
Since most halogen restrictions are based on the finished circuit board assembly, there has been discussion on whether the oxygen bomb & IC test should be run on the flux residue rather than raw flux. This presents a challenge as the scraping of flux residue can potentially remove some of the circuit board. In addition there may be some difference between the residue near the solder joint and residue that has spread a long distance from the joint (where the residue is scraped can make a difference). Both of these challenges make the testing of the residue less than ideal. To assess the halogen content of the flux residue, Indium Corporation recommends testing the flux through a TGA using a simulated reflow profile. This will provide an accurate value for the amount of residue remaining after reflow. Then, after testing the raw flux through oxygen bomb & IC, a simple conversion can be done using the safe assumption that no halogen will volatilize. For example, if the oxygen bomb & IC results show 450 ppm of Cl- and the TGA shows that the flux volatilizes 50% during reflow, you can determine that there will be 900 ppm Cl- in the flux residue.
