This you tube video, despite all it's faults, clearly shows what I have experienced. You can see how the poor quality solder does not melt at the contact between hte solder and the joint. It turns to mush unless the heat goes way up. A sure sign of impurities and definitely not 60/40 solder. I like how you can see the heat flowing up the solder wire and discolouring it. Presumably that is the flux being burned off.
Does extra flux help? Yes, and no. Some of the brands I have tested almost get a recommendation with extra flux. But only a couple of brands met with my approval.
