Steps of welding parts with soldering iron
1) First, remove the parts to be replaced with a soldering iron
2) Add tin to the pad of the PCB to be welded to saturation
3) The pad of main board is cleaned by electrostatic brush with a small amount of cleaning agent
4) Add a small amount of tin to the fixed resistor, capacitor and other parts. 
Then use the soldering iron head to paste the sample. When the time is less than 3 seconds, the new parts must be replaced
5) After the parts are welded properly, a small amount of tin should be added to the soldering iron head to prevent oxidation