Preventing dissolution of thin copper wire during soldering

[Joe Gwinn]

There have been a number of threads on the matter of opens caused by soldering of #42 or thinner magnet wire, this being a particular problem with solderable polyurethane insulation. Anyway, turns out that Bell Labs had the same problem, and did some research on it, and published an article, which is attached. Turns out that tin is the main culprit.


Lead alloys for high-temperature soldering of magnet wire.pdf

[David King]

Seems like any solder with a bit of Cu added helps a bit. Has anyone seen 97.5Pb, 1Sn, 1.5Ag solder?

[Joe Gwinn]

Seems like any solder with a bit of Cu added helps a bit. Has anyone seen 97.5Pb, 1Sn, 1.5Ag solder?

High-lead solders are going to be harder and harder to find in small quantities these days, due to RoHS.

The typical zero-lead solder alloys are mostly tin, with some silver. So, for pickup use, the copper-bearing solder alloys are going to be more practical.

henk_multicore_99c_tds.pdf

[David King]

Well we don't really mind the extra temp since we need to melt off the poly/nylon anyway.
Looks like the opposite of eutectic is "pasty" -scientific terminology?

I also hadn't realized until I saw that graph that eutectics also have the lowest possible melt temps. That makes sense I guess.

[tedmich]

Tin gets another knock on top of its dreaded whiskers!
Avionics Magazine :: System Design: Death by Tin Whiskers

[David King]

Tin has superior "wetting" ability to lead so it's not all bad.

[salvarsan]

The Henkel PDF on solder alloys is instructive, suggests you might want solder with copper in it to reduce dissolution from fine wire. Kester (alloys .pdf) has no equivalent offering.

Yup, tin is the copper solvent or 'culprit', but without a little dissolution, air creeps in and copper oxide eventually forms an insulator in the connection. Adding a percent of copper or silver to solder controls the dissolution.

@David: Yeh, eutectic is a minimum melting mixture. Liquids have a similar class of mixtures called azeotropes in which no amount of distillation can increase the purity. 68% nitric acid and 190 proof alcohol are frequent examples.

I hadn't noticed that the Henkel conglomerate bought Ersin (now called Multicore) but the Multicore trademark on the PDF is distinctive.

[Joe Gwinn]

Looks like the opposite of eutectic is "pasty" -scientific terminology?

Actually, yes. A eutectic alloy has the lowest melting point, and solidifies all at once, like a pure metal. If the alloy is not eutectic, then as one cools for the fully liquid state, one alloy component starts to freeze out first, so there is a range of temperatures where one finds solid particles of that component floating in a liquid consisting of what's left. As one continues cooling, the solid to liquid ratio continues changing, until everything has frozen. In the temperature range between all liquid and all solid, we have a wet powder. In other words, a paste.

All this is captured in the phase diagram.

This is the reason for the advice not to move a solder joint until it has fully cooled. With non-eutectic solders, there can be a wide range of temperatures where the solder is a weak paste, which can be disrupted. With eutectic solders, there is no paste range, so bad solder joints are far less likely.

[David King]

Wouldn't thixotropic be the fancy word for paste? It has more letters than eutectic and a higher Scrabble score too.

[Joe Gwinn]

Wouldn't thixotropic be the fancy word for paste? It has more letters than eutectic and a higher Scrabble score too.

No, the terms are independent. Not all thixotropes are pastes (some are goos, like epoxy resin with silica fibers), and not all pastes are thixotropic (like too-wet cooky dough).

[salvarsan]

thixotropes

Honey = thins when you stir it.
The opposite is goo that thickens in response to shear, is a dilatant.

The video shows kids running across a tub of corn starch goo but sinking when they stand still.
A pool filled with non-newtonian fluid - YouTube

[David King]

And I've been called a dilatant my whole life. Who knew?
As an example I'm playing with a new waterbased finish from general finishes that shows this property. It's quite thick in the can but yet it sprays well undiluted. The nice thing about it is that once it hits the guitar it stiffens up to prevent it from running and sagging. That allows you to spray thicker coats.