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.


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