Your solder looks like not having a flux core. For electronics, you need flux core solder. The flux will make the solder wick into the tube on the PCB, all the way to the other side of the PCB and then your pin has a strong mechanical connection with the pcb.
You are right that @3dlukas needs flux, which it seems he does not use.
However, I disagree on whether there should be flux in the solder core or not.
I like to use solder without flux in the center, which gives me the opportunity to add as much flux as needed.
As a rule, I add significantly more flux, usually I lubricate the parts to be joined so that they almost bathe in it :). But everything is soldered then quickly and reliably.
Flux is needed when soldering not only to remove oxides from the parts, but also to evenly transfer heat from the soldering iron to the soldering point. Therefore, there should be a lot of flux.
Addition @3dlukas I don't recommend you to use a lead-free solder until you learn how to solder well.
Yes, that is the wrong solder to use. You don't want Bismuth in your solder unless you have a special reason!
Also it pays to get a quality solder brand, a lot of cheap solder is just rubbish, plus you can get ripped off with amount significantly less than claimed (e.g. 40g of solder instead of 100g).
I always use Multicore/Loctite brand. Most of the time I don't need flux with that.
yes
I apply liquid flux ( it spreads better over the part than thick ), trying to make sure that it moistens the entire joint of the parts as much as possible.
You now know that this happened because of the weak solder you used.
But even after you have fixed that, I would ask why you are connecting Dupont wires to the pins? This will result in a messy, fragile circuit.
The ESP board is designed to be plugged in to a breadboard for prototyping, or soldered to a protoboard/stripboard/veroboard/PCB for the final version of the circuit.
Guys! I have used the above mentioned solder (which has flux in it) and the results are incredible! Now it is a really strong connection! Thank you my friends! I was definitely lacking flux!
