Afterthought on Lead-Free Solder

In this posting, I had written about lead-free solder and its similarity to the older idea of silver-tin-copper mixture. What I had written was my earlier assumption, that a fourth element had been added to the mixture to bring its melting-point down a bit.

From what I have read, what is in fact used is Sn96.5/Ag3.0/Cu0.5 with a melting-point of 217°C, as opposed to the melting-point of 189°C which the old-fashioned solder has. What this means, is that the modern, lead-free solder, is in fact silver-sod, and not a silver-sod-like solder.

Also, I had run into a problem in my use of this solder-type, which was the formation of a resin-droplet on the soldering joint. To my naked eye, this tiny spot of brown looked a bit like copper-wire that might not have been whet by the solder, but two inspections with the magnifying lens revealed that it was a translucent droplet on the surface of the soldering joint.

The silver-sod in wire-form which I had bought, had a rosin core. This is a type of electrical flux, where several types are possible. But apparently, other references exist, of resin-droplets being left on the soldering joints. As I had imagined, such deposits are likely to be hardened, and require a wire-brush to scrape off. I was avoiding the use of such force, to avoid possibly compromising the soldering joint I had just made.

Contrarily to the example I just cited, mine was a brown deposit and not black.

But my afterthought would be, that it was probably a mistake of mine, not to get out the wire-brush and clean the joint. Yet, since mine was an example of rosin resin, I also do not fear much that damage will result from having left a droplet of it on the tin.


(Edit 12/01/2016 : ) Rereading the above article had made me so unsure of my first soldering joint, that I just decided this evening to redo it.

The key point: Residues of flux can cause oxidation, and can thus lead to a failure of the connection later down the road.

I removed the electrical tape and re-inspected. The old droplet of brown, had turned green.

So this time around, I re-soldered the same connection, and after that, used a wire-brush to remove any excess residue. Also, this time I took extra care, to use ample solder.


Modern Lead-Free Solder

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.


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