This is the best way, which I still use after all these years. If you just count the seconds
you are definitely counting too fast...
Always remove the iron from the board track and part after four seconds – if you keep burning
at the same spot you will destroy the track and it will come off the printed circuit board (PCB).
Work on a different part of the board to give the components time to cool down so they are not harmed.
If you keep going you can burn a hole right through the board...
Now look at your joint with a magnifying glass – if the solder looks like a ball it's not fused to the track on the PCB.
If a join looks doubtful lift the board up and using gravity and the tip of the iron pull the
solder off the joint. If the solder simply pulls away leaving the track and part lead exposed then it never fused.
If the solder says behind and looks shiny you have a good join.
Although I have been doing this for over 30 years I'm sure there are better ways, so please be so kind as to
let me know if there is a better way...