I’ve found it almost impossible to find decent Solder these days 
Since the removal of the Lead content being replaced by Tin and Alloys for “safety concerns”, modern Soldering Wire is just difficult junk in comparison to the old 1970’/80’s Solder that I had been using for decades.
Trying to join two battery wires together on a new waterproof ESC for the 1800mm HUSKY yesterday, I simply gave up in the end and just crimped the bastards together!
I ended up doing the same for the time being, still need to open the gearbox again but it’s been a disgustingly muggy day today, shitty insulation in my current place, especially now it’s raining a bit, and I’m avoiding using the aircon, also havnt slept lol
Yes I have the same trouble trying to solder solar panel connections on the roofski. Fk this here get a piece of old copper water pipe ya bastard I had spare in the shed. 
@shadow187 make sure your solder is like a 60/ 40 rosin cored about 1mm wire. Lead free is fine just have to use more rosin and heat I use 50/50 alloy fine for battery cables and stuff. 320c. Get on ya bastard.
Like this 60/40 Tin lead Solder Wire Rosin Core Soldering Kit Tool 2% Flux Reel 0.5mm-2mm | eBay Australia
Grab this too to apply to both surfaces before attempting to solder 2X 10g Soldering Flux Paste PCB Rosin Welding DIY High Intensity Solder ZJ-18 | eBay Australia
Modern Solder simply won’t “take” into the joint, no matter how much heat and flux is applied, the shit just constantly puddles off of the joint instead of melting into it.
I gave up on the ESC wiring yesterday simply because there was half a roll of solder running off the joint and puddling up underneath on the bench without actually “melting into” the wires at all!
Even if you do manage to get it to flow into the wiring, it ends up as a dull dry connection, which is shit house for solid connectivity and produces high resistance issues.