For most of the history of industrial electronics, solder has been pretty boring. Mix some lead with a little tin, figure out how to wrap it around a thread of rosin, and that’s pretty much it. Sure, flux formulations changed a bit, the ratio of lead to tin was tweaked for certain applications, and sometimes manufacturers would add something exotic like a little silver. But solder was pretty mundane stuff.
Then in 2003, the dull gray world of solder got turned on its head when the European Union adopted a directive called Restriction of Hazardous Substances, or RoHS. We’ve all seen the little RoHS logos on electronics gear, and while the directive covers ten substances including mercury, cadmium, and hexavalent chromium, it has been most commonly associated with lead solder. RoHS, intended in part to reduce the toxicity of an electronic waste stream that amounts to something like 50 million tons a year worldwide, marked the end of the 60:40 alloy’s reign as the king of electrical connections, at least for any products intended for the European market, when it went into effect in 2006.
Whether the intention of EU regulators when adopting RoHS was to completely shake up the electronics industry is a moot point, because that’s pretty much what happened. The industry initially dug in its heels and made arguments against switching to lead-free solders, some of which were valid enough chemically and electronically to earn exemptions from RoHS. But lead-free solders, mostly tin with copper and silver mixed in, became the law of the land for most consumer electronic products.
Despite the industry’s early predictions of doom from lead-free solders, manufacturers adapted to the changes well. Processes for wave soldering and reflow were changed, new flux chemistries were explored, and in general the predictions of a world gone haywire from the short-inducing whiskers that were sure to grow like weeds from tin-based solders failed to happen. Exemptions were granted for applications likely to suffer from whiskering issues, but for consumer products, lead-free solders became mainstream pretty seamlessly.
But that doesn’t mean that everyone is sold on a lead-free future. That’s what Zach Fredin, Superconference regular, badge hacker, and open-source hardware entrepreneur, discovered when he recently tweeted his feelings about going 100% lead-free in his soldering. Zach got a fair amount of push back from his followers, some of whom tweeted back that they avoid lead-free solders because the flux fumes from them are more toxic.
The battle was then joined by Ben Hencke, another lead-free fan. He noticed Zach’s tweet and the oppositions to it, and instead of going with his gut, he went to the literature. He wrote up his findings in an interesting blog post, wherein he details what he found when looking over the Safety Data Sheets (SDSs) for two Kester solders, one leaded and one lead-free. Basically, his read is that both solders contain the same flux, from which he concludes that flux toxicity is a specious argument for sticking with leaded solder.
It’s All About the Flux
But is that true? While these two particular solders have the same flux cores, I don’t think you can make the argument that the flux fumes are the same, which is of course what counts. Lead-free fluxes typically require more heat than their non-RoHS cousins, and greater heat can vaporize more of the flux, resulting in more fumes. What’s more, hotter temperatures could potentially alter the chemistry of the vaporized flux, making it more toxic. Something like that happens with grilled meats, where cooking over an open flame leads to the creation of heterocyclic amines and polycyclic aromatic hydrocarbons in the smoke, potential carcinogens which waft up onto the meat in the smoke.
Could this be behind the claims of lead-free fluxes being worse for you than the fumes from traditional fluxes? I don’t know, but I do know that it gives me pause, and makes me rethink the whole lead versus lead-free debate. I think both Zach and Ben have valid points, and I applaud Ben for taking the time to actually read the SDSs and reporting on what he found. It’s persuasive, but I’m not willing to switch to 100% lead-free based solely on the that.
Here’s where we turn it over to you, dear readers, for we’re fairly certain you have strong and varied opinions on this issue. What do you think about the whole lead versus lead-free debate? Do you feel like there’s more risk from the metals in the solder or from the associated flux fumes? Have you seen any studies that analyze the composition of fumes from different solders? Or is this all a red herring as long as proper fume extraction and common sense safety measures are employed? Sound off in the comments below.