It was a traditional show, that lead, tin, arsenic. antimony and ultimately bismuth could be combined into a metallic mixture, that would melt in your hand. Further, without the antimony or bismuth, traditional solder was formed.
Dumping waste lead into the environment is now frowned upon, so that alternatives to this have been sought for some time.
For a long time, a type of alloy was known, by the name of silversod, or in German, “Kaltlot”. This was a Silver-Tin-Copper mixture which household soldering irons were not hot enough to liquefy. For the same reason – a significantly higher melting point – silversod was not used in electronics, because higher temperatures can easily damage them.
But the quest continued. From what I can tell, a lead-free solder is available commonly today, which does contain silver.
But because this solder contains a fourth element – which I do not know – it has a lower melting point than, and cannot be named silversod.
(Edit 12/02/2016 : ) It has come to my attention, that modern solder only contains tin and lead, not a third element, the latter of which would bring down the eutectic point further.
Similarly, since we live in a society that likes to simplify, the possibility exists that the silversod I was used to as a child, which my soldering iron could not liquefy, merely consisted of tin and silver – no copper.
When we use a soldering gun, this is already unsuitable for delicate electronics. One reason is the fact that the tip of a soldering gun produces a strong magnetic field, which will induce currents in circuit-boards, strong enough to damage transistors and chips. Thus, soldering guns are mainly meant for heavier, less-sensitive soldering. Here, the lead-free solder is used.
If you ever need to solder a modern circuit board, first of all, may God Help You. The chips today are mainly SMD packages that need to be soldered by a robot. Yet, there might still be discreet components, with leads that pass all the way through the PC board, the old-fashioned way. And here, the use of a 25-35 Watt soldering iron is applicable, although even-lower wattages, that are harder for amateurs to use, are also available in soldering pencils.
For electronics, the old, lead-bearing solder is still used, precisely because even the latest lead-free mixture has a slightly higher melting point yet, that can damage chips and transistors.
For the project I just started, I plan to use a soldering gun, and therefore also the lead-free, silver-bearing solder.
(Edit 12/02/2016 : ) When I wrote above, that a eutectic mixture of lead, tin, arsenic, antimony and bismuth would achieve a remarkably low melting-point, my attention went first to the fact that these elements belong to groups 4a and 5a of the periodic table.
What I only tend to see after a second look, is that Sn and Pb are also the bottom two elements of group 4a, thus being the most clearly-metallic and the heaviest.
It would make sense that to alloy those with Sb and/or Bi produces better results, than with As. The reason is the fact that the element arsenic is much lighter than lead or tin, and is itself a metalloid rather than a metal.
And so eutectic mixtures devoid of arsenic always made sense.
Yet, if we were too skittish to dump lead into the environment, we had best also not dump antimony or bismuth, since like arsenic, they are all quite toxic.
The choice of elements to add to lead-free solder, is constrained by their toxicity more than their metallurgic suitability. It has been known since ancient times, that a combination of tin, lead and copper would lead to pewter, which was also easy to work due to its low melting-point. But the actual toxicity of pewter, due to its lead-content, was only known much later.
Also, it seems to follow common sense that when we added antimony and bismuth to the original series, their ideal concentrations become progressively lower, than those of the elements which preceded them. Likewise, we would need to observe that modern silversod only contains about 0.5% copper.
If a fourth element was to be added, its concentration would also become lower than that. And then, the effect which an element would have on the alloy, if present to less than 0.5%, would start to become negligible for that reason.
And so it would seem to follow that in the case of silversod, there is no suitable fourth element.