Hello: I have soldering my electronic stuff since the 1950s.
What is the application current, power voltage, eg 12V auto, 10A 100 W amp, etc?
1/ Beware that thick stranded wires often are hard to solder or tin due to oxidation or contamination of the plastic insulation. In that case we scrape the strands with xacto knife before soldering.
2/ The PCB pads are a poor and unreliable method to attach such heavy guage wires, suggest thinner gauge wire or use a PCB with screw terminal connectors.
3/ Best irons for large power work is Metcal. Our preffered iros are the old Hakko, 936 etc.
4/ Use only high quality rosin core eutetic solder 63 tin/37 lead and NEVER use 60/40 or 50/50 or lead free. We use Kester.
5/ A good mechanical bond to keep the wire from moving during cooldown is best, rather than just touching the wire and depending on the solder to hold it.
THE SLIGHTEST MOVEMENT DURING COOLDOWN WILL CRYSTALIZE AND RUIN THE JOINT.
Just the ramblings of an old retire EE
Kind Regards,
Jon