>> Don't overheat the joint, it only needs to be hot enough to let the solder run in, not red-hot.
The solder (the lead-free sort anyway) melts at 228°C, the flame on a blowtorch is 1970°C so you don't need to heat it for very long at all. When you see the flux turn liquid is about right.
Oh, and DON'T flux inside the joint, only brush the thinnest smear of it around the outside of the pipe.
The pipe will get too hot to touch for about 15 inches either side of the joint and stay that way for 10 minutes, here speaks the voice of experience ;-)
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