Yes, strain relief is key. Even more so for soldered connections where vibration is significant. But really, no joint is immune to problems without some kind of

Yes, strain relief is key. Even more so for soldered connections where vibration is significant. But really, no joint is immune to problems without some kind of strain relief to prevent stress concentrations at the rigidity transitions. I agree that soldering probably won’t produce any issues if the wire is not subjected to significant flexure or vibration, or if it properly strain relieved and secured to a bulkhead so there is no movement at the limit of solder wicking. It’s just that I’m usually too lazy to do that perfectly, so I tend towards more forgiving solutions in vehicles and equipment. But sometimes, solder is the one true way, and I always feel better about soldered joints even if I know I’m statistically wrong. As for wire being 100 percent copper? I’m not sure, but I have definitely noticed different rigidity of copper fibers. Might be impurities, annealing, or work hardening from the drawing process… but there is definitely variation. Some wire can be really susceptible to fatigue cracking, while others seem relatively immune. Not sure of the why. Also, don’t even get me started on copper plated aluminum fiber wires.