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If I take a circuit, for instance an RC circuit, on a PCB while attached to some AC source or network analyzer and measure the voltage drop across one of the components, when I melt one of the connections, e.g., with a soldering iron (but with the component(s) still in place and in mechanical contact with the corresponding track(s)!), will then the circuit stop working?

I haven't tried it yet just in case something goes terribly wrong.

From a theoretical perspective, the junction I melt is then not in solid state, which means that all the quantum treatment of the valence-conduction bands is no longer valid. On the other hand, tap water, which is liquid, is a good conductor, so I don't know if electrons will be free to flow or not at all in the circuit through the melted junction.

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    \$\begingroup\$ Molten solder is a conductor - I once saw a bad PCB with an internal short where a polarity protection diode was floating on solder rendered liquid by the electrical heating. Note that many of your better soldering irons have grounded tips, which should be taking into account in any experiment contemplated on a live circuit. \$\endgroup\$ – Chris Stratton Nov 6 '15 at 5:10
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    \$\begingroup\$ Indeed, liquid solder conducts just fine. Contraiwise, tap water is a terrible conductor, unless you have really bad tap water with a high mineral content; and even then it's a terrible conductor as compared to solder, which is not all that great compared to copper. Deionized water, on the third hand, is a great insulator. \$\endgroup\$ – Ecnerwal Nov 6 '15 at 5:18
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    \$\begingroup\$ Solder is a conductor, it's valence and conduction bands always overlap. Heating it up, if anything, excites electrons and pushes them further into the conduction band. Tap water conducts because it has ions in it due to impurities, different principle entirely. \$\endgroup\$ – michaelyoyo Nov 6 '15 at 5:20
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    \$\begingroup\$ I once had a prototype CCFL power supply board that I was testing; it wasn't working very efficiently, and as I was probing the surface-mount rectifier diode... it fell off. It had been running hot enough to reflow its own solder connections. (As I recall, that product never got released...) Anyway, yes molten solder conducts. But that's not normal or recommended usage. \$\endgroup\$ – MarkU Nov 6 '15 at 5:58
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    \$\begingroup\$ The resistance of solder will go up as it is heated, but it will continue to conduct. \$\endgroup\$ – HighInBC Nov 6 '15 at 15:59

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