Electronic – Hobbyist Leaded Solder

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.
  Lead = "Pb" crosses x-axis at 600K on lower graph here. These are interesting and useful graphs of the vapor pressure with temperatures of many elements. (By comparison, Zinc has about 1,000,000 times as high a vapor pressure at the same temperature, and Cadmium (which should definitely be avoided) 10,000,000 times as high. Atmospheric pressure is ~ 760 mm or Hg so lead vapor pressure at a VERY hot iron temperature is about 1 part in 10^11 or one part per 100 billion.

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 inhallation. 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.

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

$$log_{10}p(mm) = -\frac{10372}{T} - log_{10}T - 11.35$$

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

For more on soldering in general see Better soldering

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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 anyhing 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.
Washing of hands, not smoking while soldering and not licking lead has been noted as sensible.

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 bothe 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.

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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 airborn 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 have many decades of personal soldering experience experience and a reasonable awareness of industry experience. Dusty rooms we all know about, but that has no link to whether solder does or doesn't produce lead dust. Soldering can produce small lead particles, but these appear to be metallic alloyed lead. "Lead" dust from paint is liable to contain lead oxide or occasionally other lead based substances. Such dust may indeed be subject to aerial transmission if finely enough divided, but this provides no information about how metallic lead performs in dust production.

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 measureable quantities.

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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.

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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.
I'd GUESS it's probably OK BUT I don't know. I suspect that the effects are limited due to insolubility of lead - BUT lead poisoning from finely divided lead such as in paint is a significant poisoning path.

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
Here is Mayo Clinic's page on Lead poisoning symptoms & causes.
And Here is their page on diagnosis and treatment.
Mayo Clinic is one of the better sources for medical advice but, even then, it certainly does not replace proper medical advice.

42/58 Tin / Bismuth is not unknown as a low temperature solder but has issues.

While widely used for some very serious applications (see below) it is not a mainstream industry contender for general use. It is not obvious why not given its substantial use by eg IBM.

Identical to the Bi58Sn42 solder you cite is:

• Indalloy 281, Indalloy 138, Cerrothru.

  Reasonable shear strength and fatigue properties.

  Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.

  Low-temperature eutectic solder with high strength.

  Particularly strong, very brittle.

  Used extensively in through-hole technology assemblies in IBM mainframe computers where low soldering temperature was required.

  Can be used as a coating of copper particles to facilitate their bonding under pressure/heat and creating a conductive metallurgical joint.

  Sensitive to shear rate.

  Good for electronics. Used in thermoelectric applications.

  Good thermal fatigue performance.

  Established history of use.

  Expands slightly on casting, then undergoes very low further shrinkage or expansion, unlike many other low-temperature alloys which continue changing dimensions for some hours after solidification.

Above attributes from the fabulous Wikipedia - link below.

According to other references it has low thermal conductivity, low electrical conductivity, thermal embrittlement issues and potential for mechanical embrittlement.

SO - it MAY work for you, but I'd be very very very cautious about relying on it without very substantial testing in a wide range of applications.

It is well enough known, has obvious low temperature advantages, has been widely used in some niche applications (eg IBM mainframes) and yet has not been welcomed with open arms by industry in general, suggesting that it's disadvantages outweigh advantages except perhaps in areas where the low temperature aspect is overwhelmingly valuable.

Note that the chart below suggests that flux cored versions seem to be specifically unavailable either as wire or as preforms.

Comparison chart:

The above chart is from this superb report which however does not provide detailed comment on the above issues.

Wikipedia notes

• Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder. Such joints are also prone to cracking. Alloys with more than 47% Bi expand upon cooling, which may be used to offset thermal expansion mismatch stresses. Retards growth of tin whiskers. Relatively expensive, limited availability.

Motorola's patented Indalloy 282 is Bi57Sn42Ag1 . Wikipedia says

• Indalloy 282. Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola.

Useful lead free solder report - 1995 - nothing to add on above subject.