Circuits Assembly - January 2008 - (Page 42)

Pb-Free Lessons Learned Halogens and Halides A gearhead’s guide to some of the chemicals that make up our boards. here’s been a lot of buzz about halogen-free and halide-free products. But what do halogen-free and halide-free actually mean? What exactly are halogens and halides? Why do we want to get rid of them? And why do we use them in the first place? I wanted to find out more about these families of chemicals getting so much attention. I’m a mechanical engineer, and chemistry was never my strongest suit. When I hear the term “halogen,” I think of automobile headlights with brighter, more focused beams, not flame-retardants that contain bromine. But with the talk about halogen-free and halide-free, I felt compelled to ask a couple of chemists exactly what this meant. Of course, I didn’t understand their first explanation. So I asked them to rephrase their explanation in more basic terms, and they did. We went through this cycle a couple times. Eventually, I got it. It’s actually quite simple. Still, I figured I’m not the only assembly engineer who has forgotten more than they remember about basic chemistry, so this month’s lesson learned is more or less a gearhead’s guide to halogens and halides, with a quick flashback to your high school chem class. Halogen or halide? A halogen is an element in the seventh column of the periodic table, which includes fluorine, chlorine and bromine. Iodine and astatine complete the group, but are not the subjects of primary consideration for electronics manufacturers. Elemental halogens have seven valence electrons and are very reactive. In their quest to obtain an eighth electron, halogens can be very strong oxidizing compounds. Once a halogen does obtain that eighth electron, it picks up a net negative charge and becomes a halide. Having the net negative charge makes the halide compound ionic. A common example is table salt: Na+Cl-. Some readers also may be familiar with the similar term halite, spelled with a “t” as opposed to a “d.” Halite is the mineral form of NaCl that’s known as rock salt. It’s regularly used in winter climates to melt snow and ice from walkways and roadways. Where and why are they used? In broad terms, halogens are associated with PWBs and components, while halides are associated with soldering operations. There are exceptions. For the most part, halogenated compounds are found in PCBs, solder masks, mold compounds, connectors, cable insulation and wiring conduit. They are used because they offer flame retardant properties to otherwise flammable plastics. The 1979 ban on polychlorinated bisphenols (the “other” PCB) gave way to the use of brominated organic compounds. Now some of these brominated flame-retardants (BFRs) are on the hit list too. Some halogenated compounds have already been replaced with halogen-free products; others are slated for removal and T Chrys Shea is an R&D applications engineering manager at Cookson Electronics (cooksonelectronics. com); chrysshea@ cooksonelectronics. com. Her column appears monthly. replacement over the next several years. Bear in mind not all halogenated compounds are toxic. Consider table salt: Ingestion of excessive quantities may not be advisable, but it is certainly not considered a toxic substance. Here’s one of the exceptions I mentioned: Non-ionic halogenated compounds can also be used as raw materials in soldering fluxes to help keep activators working as process temperatures increase, but they are used in far smaller quantities than in boards or components, and their inclusion is not an absolute necessity. Wave soldering fluxes designed to withstand extended thermal cycles may likely contain halogens, but some of today’s most successful Pb-free wave fluxes and pastes do not. Inclusion of halogens in flux materials is determined by the desired performance characteristics of the flux itself, and is usually fixed early in the product development cycle. If a flux is to be halogen-free, its basic composition must consider the absence of halogens, and other ingredients in the formulation must be selected accordingly. Although halogens are sometimes included in fluxes, halides are the compounds most commonly associated with them. While halogens can be added to improve a flux’s thermal robustness, halides are added because they can rapidly reduce metal oxides and, in combination with a variety of organic acids, make ideal activator systems. But they have been cited as possible sources of post-soldering corrosion, so manufacturers of high performance and harsh environment electronics traditionally avoid them if they run no-clean processes. On the other side of the assembly spectrum, manufacturers of more cost-sensitive systems often prefer fluxes that include halides. The halide-containing fluxes provide better wetting during soldering – and therefore, more mechanically reliable solder joints – on lower-cost components and boards more likely to exhibit solderability problems. Because halides can be associated with post-solder corrosion, no-clean fluxes containing halides will always contain rosin to encapsulate any leftover, un-reacted ionic material and ensure electrochemical reliability. Here’s the other exception I mentioned: Halides are often used in the etching, plating and final finishing processes of PWB fabrication. They sometimes can be found as residual contaminants on bare boards if not cleaned properly. If completed assemblies demonstrate ionic contamination issues, it is wise to check the bare boards to understand whether the contamination source is in the assembly process itself, or if the contaminants were left from fabrication. Why do we want to eliminate them? A quick Web search revealed some convincing reasons to limit use of specific halogens. If incinerated under certain conditions, they can form dioxins, which are known carcinogens. In structural fires, they can emit toxic gases that prevent victims from 42 Circuits Assembly JANUARY 2008 circuitsassembly.com http://www.cooksonelectronics.com http://www.cooksonelectronics.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - January 2008

Circuits Assembly - January 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing Walking on Water Flux Selection for Lead-Free Wave Soldering An A-to-Z Guide to X-Ray Inspection, Part II Evolution in Action Research Priorities for the Electronics Industry Selective Soldering Process Doctor Pb-Free Lessons Learned Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - January 2008

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