Lead-free solder has a much higher melting point than leaded. If you have a soldering iron and some leaded solder, set your iron to a temperature where it just melts the leaded stuff. Then, try heating up a component on the board at that temperature (make sure it isn't connected to a large copper pour).

If you're having trouble getting the solder to melt, there's a good chance that it's lead-free.

What type of solder is safest for home (hobbyist) use?

This advice is liable to be met with doubt and even derision by some - by all means do your own checks, but please at least think about what I write here:

I have cited a number of references below which give guidelines for soldering. These are as applicable for lead-free solders as for lead based solders. If you decide after reading the following not to trust lead based solders, despite my advice, then the guidelines will still prove useful.

It is widely know that the improper handling of metallic lead can cause health problems. However, it is widely understood currently and historically that use of tin-lead solder in normal actual soldering applications has essentially no negative health impact. Handling of the lead based solder, as opposed to the actual soldering, needs to be done sensibly but this is easily achieved with basic common sense procedures.

While some electrical workers do have mildly increased epidemiological incidences of some diseases, these appear to be related to electric field exposure - and even then the correlations are so small as to generally be statistically insignificant.

Lead metal has a very low vapor pressure and when exposed at room temperatures essentially none is inhaled. At soldering temperatures vapor levels are still essentially zero.

• Tin lead solder is essentially safe if used anything like sensibly.

  While some people express doubts about its use in any manner, these are not generally well founded in formal medical evidence or experience. While it IS possible to poison yourself with tin-lead solder, taking even very modest and sensible precautions renders the practice safe for the user and for others in their household.

• While you would not want to allow children to suck it, anything like reasonable precautions are going to result in its use not being an issue.

A significant proportion of lead which is "ingested" (taken orally or eaten) will be absorbed by the body.

BUT you will acquire essentially no ingested lead from soldering if you don't eat it, don't suck solder and wash your hands after soldering. Smoking while soldering is liable to be even unwiser than usual.

• It is widely accepted that inhaled lead from soldering is not at a dangerous level.

The majority of inhaled lead is absorbed by the body.

BUT the vapor pressure of lead at soldering temperatures is so low that there is essentially no lead vapor in the air while soldering. Sticking a soldering iron up your nose (hot or cold) is liable to damage your health but not due to the effects of lead. The vapor pressure of lead at 330 C (VERY hot for solder) / 600 Kelvin is about 10^-8 mm of mercury.

The major problems with lead are caused either by its release into the environment where it can be converted to more soluble forms and introduced into the food chain, or by its use in forms which are already soluble or which are liable to be ingested. So, lead paint on toys or nursery furniture, lead paint on houses which gets turned into sanding dust or paint flakes, lead as an additive in petrol which gets disseminated in gaseous and soluble forms or lead which ends up in land fills are all forms which cause real problems and which have led to bans on lead in many situations. Lead in solder is bad for the environment because of where it is liable to end up when it is disposed of. This general prohibition has lead to a large degree of misunderstanding about its use "at the front end".

If you insist on regularly vaporising lead in close proximity to your person by eg firing a handgun frequently, then you should take precautions re vapor inhalation. Otherwise, common sense is very likely to be good enough.

Washing your hands after soldering is a wise precaution but more likely to be useful for removal of trace solid lead particles.

Use of a fume extractor & filter is wise - but I'd be far more worried about the resin or flux smoke than of lead vapor.

Sean Breheney notes: " There IS a significant danger associated with inhaling the fumes of certain fluxes (including rosin) and therefore fume extraction or excellent ventilation is, in my opinion, essential for anyone doing soldering more often than, say, 1 hour per week. I generally have trained myself to inhale when the fumes are not being generated and exhale slowly while actually soldering - but that is only adequate for very small jobs and I try to remember to use a fume extractor for larger ones. (Added July 2021)

Note that there are MANY on we b documents which state that lead solder is hazardous. Few or none try to explain why this is said to be the case.

Soldering precautions sheet. They note:

• Potential exposure routes from soldering include ingestion of lead due to surface contamination. The digestive system is the primary means by which lead can be absorbed into the human body. Skin contact with lead is, in and of itself, harmless, but getting lead dust on your hands can result in it being ingested if you don’t wash your hands before eating, smoking, etc. An often overlooked danger is the habit of chewing fingernails. The spaces under the fingernails are great collectors of dirt and dust. Almost everything that is handled or touched may be found under the finger nails. Ingesting even a small amount of lead is dangerous because it is a cumulative poison which is not excreted by normal bodily function

Lead soldering safety guidelines

Standard advice Their comments on lead fumes are rubbish.

FWIW - the vapor pressure of lead is given by

Quoted from The Vapor Pressures of Metals; a New Experimental Method

Wikipedia - Vapor pressure

For more on soldering in general see Better soldering

Lead spatter and inhalation & ingestion

It's been suggested that the statement:

• "The majority of inhaled lead is absorbed by the body. BUT the vapor pressure of lead at soldering temperatures is so low that there is essentially no lead vapor in the air while soldering."

is not relevant, as it's suggested that

• Vapor pressure isn't important if the lead is being atomized into droplets that you can then inhale. Look around the soldering iron and there's lead dust everywhere.

In response:

"Inhalation" there referred to lead rendered gaseous - usually by chemical combination. eg the use of Tetraethyl lead in petrol resulted in gaseous lead compounds not direcly from the TEL itself but from Wikipedia Tetraethyllead page:

• The Pb and PbO would quickly over-accumulate and destroy an engine. For this reason, the lead scavengers 1,2-dibromoethane and 1,2-dichloroethane are used in conjunction with TEL—these agents form volatile lead(II) bromide and lead(II) chloride, respectively, which are flushed from the engine and into the air.

In engines this process occurs at far higher temperatures than exist in soldering and there is no intentional process which produces volatile lead compounds. (The exceedingly unfortunate may discover a flux which contains substances like the above lead scavenging halides, but by the very nature of flux this seems vanishingly unlikely in the real world.).

Lead in metallic droplets t soldering temperatures does not come close to being melted or vaporised at anything like significant partial pressures (see comments and references above) and if any enters the body it counts as 'ingested', not inhaled.

Basic precautions against ingestion are widely recommended, as mentioned above.

For lead "spatter" to qualify for direct ingestion it would need to ballistically enter the mouth or nose while soldering. It's conceivable that some may do this but if any does the quantity is very small. It's generally recognised both historically and currently that the actual soldering process is not what's hazardous.

A significant number of webpages do state that lead from solder is vaporized by soldering and that dangerous quantities of lead can be inhaled. On EVERY such page I have looked at there are no references to anything like reputable sources and in almost every such case there are no references at all. The general ROHS prohibitions and the undoubted dangers that lead poses in appropriate circumstances has lead to a cachet of urban legend and spurious comments without any traceable foundations.

And again ...

It was suggested that:

• Anyone who's sneezed in a dusty room knows that it doesn't have to enter the nose or mouth "ballistically". Any time solder splatters or flux pops, it creates tiny droplets of lead that solidify to dust. Small enough particles of dust can be airborne and small exposures over years accumulate in the body. "Lead dust can form when lead-based paint is dry scraped, dry sanded, or heated. Lead chips and dust can get on surfaces and objects that people touch. Settled lead dust can re-enter the air when people vacuum, sweep or walk through it."

In response:

A quality reference, or a few, that indicated that air borne dust can be produced in significant quantity by soldering would go a long way to establishing the assertions. Finding negative evidence is, as ever, harder.

There is no question about the dangers from lead based paints, whether form airborne dust from sanding, children sucking lead painted objects or surface dust produced - all these are extremely well documented.

Lead in a metallic alloy for soldering is an entirely different animal.

I am unaware of discernible "Lead dust" occurring from 'popping flux', and I'm unaware of any mechanism that would allow mechanically small lead droplets to achieve a low enough density to float in air in the normal sense. Brownian motion could loft metallic lead particles of a small enough size. I've not seen any evidence (or found any references, that suggest that small enough particles are formed in measurable quantities.

Interestingly - this answer had 2 downvotes - now it has one. Somebody changed their mind. Thanks. Somebody didn't. Maybe they'd like to tell me why? The aim is to be balanced and objective and as factual as possible. If it falls short please advise.

___________________________________________________________

Added 2020: SUCKING SOLDER?

I remember biting solder when I was a kid and for about 2 years I wouldn't wash my hands after soldering. Will the effects show up in the future??

I can only give you a layman's opinion. I'm not qualified to give medical advice.

You can be tested for lead in the blood very easily (it requires one drop of blood) and it's probably worth doing.

Internet diagnosis is, as I'm sure you know, a very poor substitute for proper medical advice. That said

A great question, and since a textbook could probably be written to answer it, there's probably not going to be any single answer. I want to provide a general answer tailored to hobbyists, and hope that people more knowledgeable can come in and tie up specifics.

Summary

Science Content

Flux Cores

Lead Free

Tutorials

In general, if you got it at the electronics hobby shop, it's good to use for hobbyist purposes.

Sn96.5/Ag3.0/Cu0.5 solder's liquidus is 217 °C, and good old 63/37's eutectic point is 189° C. So yes, you have the temperature turned up much too hot. Most parts are rated for 260 °C for 10 seconds max. Soldering at the temperatures you have been using can potentially damage components, has been weakening the glue under every pad you solder (the copper is glued to the FR4), producing significantly more hazardous fumes, something that is already more hazardous with lead-free solders, and generally does nothing good.

I think I know why you have your iron turned up that high. If it is, and no offense intended, a crappy iron or even a good iron with a crappy tip, you will need to turn it up to a much higher temperature than normal to overcome the crappy tip's crappy thermal resistance to start a joint. However, once the joint is solidly wetting the tip, the thermal resistance drops significantly, and the joint will experience those too-high temperatures in full force. So even if it seems like that is the temperature that is needed to begin melting solder in a reasonable amount of time, once it is melting, it may get much too hot.

In fact, I had a Hakko iron with a crappy fake tip and I constantly had it set on the exact temperature range you are using for lead-free soldering, at least until I realized what was going on. Sometimes I even bumped it up to 400° C. Which I now know was a bad idea.

There is a very simple way to tell if your iron's tip is a good one or one you shouldn't be using for real electronics work. Remove it from the iron and use a magnet to see how magnetic it is. It should not be magnetic at all, or maybe very very weakly magnetic. Good tips have copper cores, and maybe a thin steel jacket or shim inside, but otherwise no significant amount of ferromagnetic material. Bad tips will use steel/iron, as it is much less expensive. Unfortunately, it also will result in a tip with on the order of 6 times the thermal resistance, which is really not OK.

OK, now I can finally answer your question!

Those dark splotches are leftovers from the resin-based flux. No-clean fluxes are often made out of water-soluble resins (vs rosin) and during heating, most of it will evaporate away. Usually, there are some inert solids left over however, and high temperatures will cause them to oxidize (or something) and turn brown to black. These solids are very much the opposite of what the solder wants to wet, so it will be clumped and pushed to the surface of the molten solder joint. It should come off but probably only with some rough scraping, like with a wire brush. As far as I know, it should not have a meaningful impact on the joint, but if you want to avoid this, I suggest lowering your iron temperature (and possibly getting a new tip and/or iron depending, so soldering is effective at said lower temperature), though this may not totally resolve the issue. Sometimes, there are just some inert impurities in a particular batch of solder flux. It's ok as long as it is pushed out of the joint to the surface, which it almost always is.

Oh, one last thing: no clean flux isn't. It ought to be called "no urgent cleaning", but most fluxes labled no clean will certainly leave a film or residue behind, and while they are not going to completely eat away the copper of a trace if left on the board for a few days or more, I have heard that they aren't always as inert as people seem to think. It could still cause problems, albeit on a much longer (months) time scale. But, most of what is in this paragraph is simply things I've heard from other engineers I trust, but I don't have any actual data to back it up, so bring a bag of salt etc.

But we always had trouble using the lead free solder.

Yes, nobody likes it. It's somewhat harder to work with overall, even to this day.

but then, the copper tracks on the pcb became easier to peel off due to this

This was a big problem when RoHS was new and not nearly as much of a problem nowadays. Though naturally it depends a lot of the PCB quality - some PCBs meant for mass production are simply not designed to have components re-soldered multiple times, RoHS or not.

Further more our iron tips got corroded way faster.

Higher temperature means shorter tip life. For RoHS solder you typically use around 350°C as professional setting, whereas for leaded you would do around 320°C.

I haven't noticed any major quality improvement of solder tips since the introduction of RoHS.

But how is the latest lead free solder these days? Does it still need a higher temperature to work with?

Yes. Approximately 30°C higher temperature.

Is there any latest technology in lead free solder that maybe allow us to use at similar temperature as the leaded one?

As mentioned in comments, there is specialised solder containing bismuth with lower melting points. I have no personal experience of using it, but from what I hear it is slightly more brittle and should preferably only be used for special jobs with heat-sensitive components. There are some versions of it with more silver etc that are better, but then it turns more expensive instead. Also, flux core wire might not be available. See Any drawbacks to "low temp" lead-free solder paste?