I spent the morning reading material Safety Data Sheets, because someone on the internet was wrong!Zach (@zakqwy), who I have immense respect for, posted this thread on twitter (and is not wrong):I 100% agree here, on all accounts (read through his thread): lead-free solder is better. I switched to lead-free almost a decade ago, and went from being a crappy solderer to pretty decent around the same time. Before that, going back years, I was a kid playing with lead and had no information on proper handling (or on proper soldering skills, but that’s another issue). I’m not going to write much about leaded vs lead-free solder skills or ease of use, because that’s less important to me than staying away from things that can damage my health, my kids’ health, or the environment. Suffice to say that lead-free soldering is a skill you can learn, and is plenty suitable for first-time solderers.There is some misinformation being spread that lead-free solder is somehow more toxic/worse for you than leaded solder, because of the flux fumes. Here’s an incorrect statement:[…] Sure, you get less heavy metal poisoning, but lead-free flux is exceptionally bad for you when compared to what is used in lead solder.I’ve heard this repeated enough anecdotally, that I wanted to know the truth of it. I’ll reserve from direct quoting a lot of folks defending their leaded solder. I respect a lot of these folks, and many have been doing this for longer than I’ve been alive. But that won’t stop me from correcting factually wrong statements saying lead-free solder is more dangerous than leaded solder. Saying “lead-free solder is more dangerous” on the internet adds to global misinformation that can actually cause harm.So I spent a morning reading material safety data sheets (SDS). An SDS is “a document that lists information relating to occupational safety and health for the use of various substances and products. SDSs are a widely used system for cataloging information on chemicals, chemical compounds, and chemical mixtures. SDS information may include instructions for the safe use and potential hazards associated with a particular material or product […]” [wikipedia]I picked up the everlasting spool of Kester lead-free solder I bought in 2011 and looked up the SDS for it. It turns out Kester makes leaded and lead-free versions of the same formulation. Indeed, much of the choice in solder seems to be around flux first, and leaded vs lead-free second.Here is the SDS for the lead-free version, the same stuff I personally use. [PDF]And here is the SDS for the leaded version of the same. [PDF]I’ll save you the trouble. Yes, rosin-core solder creates fumes that are not great to breathe. It contains this top-level hazard statement and precautionary statements classified by GHS labels:H317 May cause an allergic skin reaction.P261 Avoid breathing dust/fume/gas/mist/vapors/spray P280 Wear protective gloves. P302+P352 If on skin: Wash with plenty of water. P333+P313 If skin irritation or rash occurs: Get medical advice/attention.P501 Dispose of contents/container in accordance with local/regional/national/international regulations.Sounds pretty bad right? Indeed, you should properly ventilate and/or use a soldering air filter.Compare that to the leaded version with the same rosin formulation. This also creates fumes that are not great to breathe (the same fumes), plus a bunch of really bad stuff because of the lead:I think most of this speaks for itself, but I’d like to point out a subtle upgrade from P261 to P260 for the leaded stuff. It goes from “avoid” to “don’t” and I think this this is worth calling out specifically. Remember leaded paint? The stuff we banned in ‘78? The primary cause of lead-poisoning isn’t eating paint chips, it is breathing lead laced paint dust. “But solder dust? ‘Cmon, really? How can that even be?” I can hear someone typing already… Have you seen one of these?So its 100% better in every way right?Well of course it isn’t. You need to run an iron hotter, lead-free solder wire tends to have more flux, and naturally that vaporizes a little more flux. There is a lot of FUD put out by what seems to be manufacturers of fume extraction equipment. I’m not going to argue against more fume extraction. You probably need more. However, I haven’t seen any hard data on rosin flux at higher temperatures, just vague generalities. I’d like to see any data on how much of a difference this makes, but all I have to go on is the SDS, which is clearly not worse for lead-free.You are still vaporizing the same material. Let’s not muddy the water here. Use adequate ventilation and/or fume extraction.Not All Flux is Equal – Don’t Conflate Flux With MetalThere are a lot of flux options out there. Manufacturers have formulations for all sorts of things, including reflow soldering, wave soldering, and even robotic soldering options. There’s stuff intended for exotic manufacturing processes that don’t involve regular air, or processes sensitive to halogen.You can find solder wire with various formulations, including non-rosin or no-clean fluxes. These tend to be more hazardous than the rosin-based solder wire. The flux used in solder paste is different, and there are formulations with nastier stuff. Most seem to be skin and eye irritants, though some formulations even have carcinogens and more serious hazards. Again, browsing through leaded and lead-free options, this seems to be about the flux, and the leaded options have the same hazards, plus those for lead. In general, you really don’t want to be breathing the smoke from paste or no-clean products.Read through those SDS documents!Tips and RecommendationsNot all rosin-core solder wire is equal. The first lead-free solder wire I used was some stuff I got at SparkFun, over a decade ago. It was pretty terrible to work with. The flux would sputter and turn black quickly. If you haven’t had luck switching to lead-free, it’s worth giving another type of solder a try.I use Kester #48 flux solder wire, with the Sn96.5Ag03Cu.5 alloy, in 0.6mm size. I like it quite a bit. I got a roll back in 2011, and there seems to be no end to the stuff.For solder paste, I use ChipQuik SAC305 no-clean stuff. It’s full of skin and eye irritants, like all no-clean paste, but lacks the really nasty stuff intended for more exotic manufacturing processes. When I need lead-free solder to work like buttery magic, for drag soldering or removing bridges, I use some extra flux. The ChipQuik SMD291NL stuff is a tacky flux that is made of pure magic. On the SDS side of things, it has most of the same warnings: skin and eye irritants, avoid breathing, etc.Prove Me Wrong (or at Least Give Me Data to Look At)I’m not a materials scientist, and I don’t know everything. I didn’t know most of this stuff before I spent all morning reading those SDS. If you know more, drop a comment or email.