Flux before confidence

Most bad solder joints start as a time-saving decision. The pad looks wet enough. The wire stopped wiggling. You move on. Three weeks later the intermittent fault teaches you humility.

Wetting is the tell

Good wetting looks like the solder wanted to be there. Bad wetting looks like a ball perched on a pad. If you need to interpret it charitably, reflow it. Charity is not a manufacturing process.

Flux is not optional seasoning

Flux cleans the moment of contact. Skipping it because the tip was already shiny is how mystery wires are born. Put fresh flux on, heat the joint, feed solder to the pad — not the iron — and let the iron leave before the solder freezes.

Good enough is a lie with a schedule

If the project is a weekend toy, a slightly ugly fillet can be fine. If the project will leave the bench, inspect under light, tug gently, and refuse to ship a joint you would not sign. The bench lead's rule: no mystery wires, no mystery joints.