Mechcables Sky Dolch Custom-Sleeved USB Cable

That's what I suspected. I had been playing with kits I got from Zapcable. It turns out my lead-free solder can work near 470-490F (recommended working temp but it actually melts near 422F and can work with SMD LED). Most people still use about 650F or higher probably because the solder they use. Under that high of a temp, plastic is gone in the blink of eye. Lower temp is actually much safer and won't do that much damage. But that means solder becomes very expensive. The last time I checked, some high quality solder that melt at 360F costs $50USD for ~0.1 lbs. They definitely won't use those for your cable.