The real issue isn’t lead/lead-free solder, the issue is the flux, rosin or rosin free…
Lead/lead free is down to personal preference, most old-school folks will argue for lead based solder on grounds of it being easier to work with which is debatable, normally problems with soldering lead-free come down to needing to have a decent digital temperature controlled soldering iron which has more fine grain control over temperature, set to the temperature in the manual for the solder.
Control
Some other factors that affect this perception are that it does handle substantially differently, so the way it wicks, how the metal goes shiny when set, etc are different and that makes it hard to switch between them. For me having used lead-free solder most of my life I find lead solder more of a pain, so I think this is mainly in the realms of experience of what you are used to.
Whiskars
However another issue which is more genuine is tin whiskers, over a period of time affected by altitude tin based solder will grow ultra-small conductive tin whiskers that reach out and cause shorts on a circuit board. This is why lead is added to solder in the first place. Lead free solder is typically tin with alloys to slow this down, depending on the life expectancy of the product you might use different alloys, the best being Sn/Pb (lead which is restricted from manufacture) or NiPdAu (which is expensive).
Inhalation
Although this doesn’t affect hand soldering it has create a fear around using lead solder.
Lead based solder is normally 60/40 Pb/Sn however the boiling point of lead (Pb) is 1749ºC so there is no possibility of inhaling it in a vapour, similarly tin (Sn) boils at 2603ºC so again no where near the melting points of these metals (200-300ºC) used for soldering.
RoHS
@Dermot is correct the main move away from lead solder is for RoHS (Restriction Of Hazardous Substances) which is an EU directive to remove amongst other things lead from production of electronic and electrical device production, and distribution.
Rosin
Having used lead and lead free for years, I’ve found the issue is the fumes, and that I probably have developed on top of my asthma, occupational asthma from the rosin flux fumes, this is the most serious concern for Makerspace in my view, as it is cumulative.
At work we use:
Warton Metals Omega II SAC3 Fast Flow Flux 2% Solder Wire
We’ve completed COSHH on using this chemical with students and although we recommend they use a fume diffuser box and wash their hands as part of the process there isn’t any real need to.
As I have still suffered with my asthma with this rosin-free solder we did air monitoring in my office with just a fume diffuser and found that after 1 hour multiplied up to a 7 hour day of continuous/intensive soldering in a poorly ventilated environment that we were an order of magnitude away from the WELs for the inhaled fumes.
Another way of saying this is that the rosin free component of this was product valuable as it doesn’t cause occupational asthma, and the fumes inhaled were low after an intensive day of soldering.
The actual solder metal however will develop whiskers over long term > 5 years. Thats fine for most projects at Makerspace however.
My view is whatever we choose we should aim for something rosin free because of the lack of ventilation and poor LEV (extraction) options available to us, not only are the folks soldering affected by this but all users in the space as they will breath the diffused solder fumes containing rosin.
I would strongly advocate we find something rosin free, and provide lead and lead free for folks that want it.
