Why does molten solder squeak when you move the iron in it?

[eti]

Why does molten solder squeak when you move the iron in it? If you dragged a cold soldering iron across a cold solder joint it would not squeak in the same way. Is this because the state of the solder is liquid causing some sort of mechanical change? This has always intrigued me.

[tom66]

Could be the rosin (solid but flexible) flux in the solder expanding.  A good experiment is whether this happens with solid core (flux-free) solder.

[Psi]

How old is this solder?

[T3sl4co1l]

Intermetallics.

Tim

[eti]

How old is this solder?

All the solder I've ever used at home or at work, does it.

[mag_therm]

I am familiar with it, especially when tinning a large pad when the iron is not quite hot enough.
Read about "stiction", a mech eng term, can cause vibration.

I do a some hobby brazing. Sometimes if the filler is fed when the parent metal is not quite hot enough, I have felt the filler rod vibrate as it melts.
A metallurgist could probably tell us what is happening when alloy is forming from eutectic filler.

[Kleinstein]

With lead free solder and only partially molten one could get the tin-cry:
https://en.wikipedia.org/wiki/Tin_cry

[coppice]

Why does molten solder squeak when you move the iron in it? If you dragged a cold soldering iron across a cold solder joint it would not squeak in the same way. Is this because the state of the solder is liquid causing some sort of mechanical change? This has always intrigued me.

I think it only makes noises when its on the edge of melting, as the molten and not quite molten parts of the blob interact. Freely flowing solder seems quiet.

[Infraviolet]

I've occasionally noticed a little silent outgassing from flux when you feed a very big solder (60/40 lead and tin, rosin cored) joint (motor terminals) with a lot of solder fast on to a particularly hot iron, but never heard a squeak. How big were the joints where you heard this? And which sort of solder were you using? You weren't be any chance soldering on a recently solvent cleaned PCB which might have still had liqudi residue to boil off in the presence of the iron's heat were you?

I have heard of the tin cry, but I thought the whole reason for lead, or other non-tin metals in solder alloys was to prevent pure tin's irriating behaviours (tin whiskers and such) and that this suppressed the effects which caused the tin cry at the same time.

[T3sl4co1l]

With lead free solder and only partially molten one could get the tin-cry:
https://en.wikipedia.org/wiki/Tin_cry

Hm, that would be interesting, but I'd bet it goes away at much lower temperatures -- where diffusion relieves stress before the "cry" mechanism can snap over.

I wonder over what temperatures it happens, actually.  Don't know offhand.

Tim

[tooki]

… but I thought the whole reason for lead, or other non-tin metals in solder alloys was to prevent pure tin's irriating behaviours (tin whiskers and such) and that this suppressed the effects which caused the tin cry at the same time.

And lowering the melting point. The melting point of e.g. 63/37 tin-lead solder (183°C) is significantly lower than that of pure tin (231.9°C) or pure lead (327.5°C).

[eti]

Why does molten solder squeak when you move the iron in it? If you dragged a cold soldering iron across a cold solder joint it would not squeak in the same way. Is this because the state of the solder is liquid causing some sort of mechanical change? This has always intrigued me.

I think it only makes noises when its on the edge of melting, as the molten and not quite molten parts of the blob interact. Freely flowing solder seems quiet.

Yes precisely right!