Use of Solder on Mains Voltage Lines Inside a Device

[helius]

There are listed (UL, ETL, CSA, etc.) devices that have solder joints carrying mains voltages or higher.

Of course there are. What you will not see (at least I've never seen it) is wire-to-wire connections made with solder (especially if the wire is movable), because it is not mechanically strong when used that way. As I mentioned three days ago, the Western Union splice was developed after years of failures in soldered telegraph wires, and is reasonably strong in tension. It still cannot handle flex.

For wire-to-board soldered connections, the joint is also unable to handle flex—that's why an acceptable method is to route the wires through snug holes in the FR4 before soldering them to the opposite side. And soldered cables must have wire supports and strain reliefs to avoid flexing the solder joints.