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Can I solder wire onto a flat surface?

Yes. In fact, if your solder technique isn't being successful in getting the solder to stick solidly to a flat surface, I would argue that any twist and through-hole solders are gonna be iffy, too.

The important characteristic of a proper solder joint (whether it's being applied to a flat surface or any other kind of joint) is that the melted solder should bond to the surface of the metal quite solidly. So whether you are just touching two pieces of metal together, or mechanically entwining them (twists or through-hole), your bonds should be equally strong.

The secrets to getting a good solid bond with the surface are:

1. Proper application of a good liquid or gel flux.

2. Sufficiently heat the metal that you're bonding the solder to.

Certain kinds of large connectors (I used to have trouble with a certain kind of guitar cable plug) require that you heat up almost the entire connector in order to get the solder to start sticking. I don't think your RCA connector tab will be like that, though. You should be OK.
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Tony Fabris