A solder seal butt connector for 10-12 gauge wire. Solder seal butt connectors are made of a clear, heat shrink tube, with a low temperature solder ring in the middle.

Then, you slide the solder seal connector over the join such that the solder bead is over stripped wire to be soldered.

Brilliant. Here you can see the solder seal connectors first shrinking, then melting, then soldering. In practice these soldering joints aren’t quite so magical, but they are pretty darn close.

[Tom] doesn’t much like breadboarding. He prefers to wire up prototypes with perfboard and solder point-to-point with enameled magnet wire. That may sound troublesome to some of you, but [Tom… ...

Then again, both American Autowire's Michael Manning and Haywire's Ken Logue endorse soldering the tip of a crimped terminal to ensure the integrity of the seal.