The solder you used probably had a flux core. Older solder had rosin based flux. Newer solders tend towards less corrosive compositions (e.g. no clean).I gotta admit, I don’t understand flux. I’ve soldered for 25 years as a hobby-ist, just never used it.
I went for 30-40 years without buying any liquid flux. It's handy where you need more cleaning activity. If you've ever tried to solder things that seem to have an oily film, you'll appreciate having the extra flux. The solder wets better.
https://www.mclpcb.com/blog/prevent...,to the component for an optimal solder joint.