Products might be “halide-free” at room temperature but, once heated, contain halogens far outside ppm limits.

So far, there is no official, government-mandated requirement forcing the production of halogen-free products and, therefore, the use of halogen-free materials, but the electronics industry is arguably moving in that direction. The drive to be halogen-free is being fueled primarily by increasingly environmentally-conscious customers, pressure from non-governmental environmental organizations and the desire by suppliers to be proactive.

What is lost on many in the electronics industry, however, is the underlying motivation of the halogen-free effort and the difference between halogens and halides. As part of electronics devices, halogens do not cause harm to humans or to the environment. The concern regarding halogenated materials has to do with the harmful byproducts that can be released into the environment at the end of a product’s life during the incineration process. Incineration of certain halogen-based materials releases dioxins into the atmosphere and these byproducts can be harmful to humans. Even though the majority of electronics products contain none of the halogens that cause harm, fear of having the term “halogen” associated with any electronics materials is pushing the industry to exclude halogens from electronics manufacturing.

Remarkably, many electronics professionals are unaware of the distinction between halides and halogens and this lack of knowledge can prove problematic. The below may help distill some of the differences.

Halides

• Ionic and have a charge (example Cl^-, Br^- , and F^-).
• Added to solder paste to improve activity, encourage cleaning.
• Generally hydroscopic and can cause stability challenges.
• No clean fluxes normally encapsulate remaining halide constituents post reflow in the rosin to ensure post assembly reliability.

Halogens

• Covalently bonded group VIIA element without a net ionic charge (example Cl₂, Br₂, and F₂); not available for fluxing activity at room temperature but only at high temperature.
• Primary source in electronics assemblies is in BFRs in laminates.
• Certain halogens – PBBs and PBDEs – present potential toxicity problems when incinerated (causing dioxins).
• Used in solder paste to enhance activity without impact to reliability

Halides and halogen materials have different reactions both at room temperature and at elevated temperatures. Ionic halide bonds are typically easily broken, which allows free halide to react with moisture and cause corrosion at room temperatures. On the other hand, covalently bonded halides are much more stable at room temperature and the bonds are not easily broken. At elevated temperatures – such as those consistent with soldering – the covalent bonds are broken and the halide can react with the oxide to clean and encourage wetting.

The test requirements for halide-free and halogen-free materials are completely different as well. While international standards have been set for halide-free, high reliability solder interconnects and the test procedures have been well-established, the same cannot be said for halogen-free. Table 1 further illustrates these differences.

As the chart shows, the three primary standards for halogen-free do not equate to 0 ppm halogen. Therefore, the majority of today’s so-called “halogen-free” materials are not truly free of halogens. To complicate matters further, the testing regimen for halogen-free materials can also be challenging. As halogens have a neutral charge and cannot be detected by standard ion chromatography, they must be converted into ionic halides through a combustion process. This means that many so-called halide-free materials may, in fact, contain hidden halides. Under the current analytical methods, testing occurs during a state in which chemical bonds are not broken, but are kept under isolation in a neutral charge format. When these chemicals are heated – as happens during the solder reflow process – the bonds are broken and halides can then be detected. So, many products that claim to be “halide-free” might be so at room temperature but, once changed during the manufacturing process, can contain halogens far outside of the current recommended ppm limits.

Because covalently bonded halogen delivers many benefits for the soldering process, many materials developers use small amounts of halogen – just below the ppm limit – in their formulations. While this is legitimate, the danger is that excess halogenated materials can be added during the manufacturing process. The only way to ensure a truly halogen-free product is to have zero deliberately added halogens in the formulation and products constructed of the purest raw materials available. The situation is made more complex by the testing challenges halogen-free introduces, with manufacturers largely relying on their suppliers to confirm the purity of the materials they are receiving.

While the jury is still out as to whether or not halogen-free government mandates will come to pass, manufacturers that understand the halogen-free/halide-free distinction will be in a far better position to navigate any legislation that may be imposed.

Jie Bai is a chemist at Henkel Electronics Group (henkel.com); jie.bai@us.henkel.com.