Same here, rarely ever seen the solder separate from the Cu.
The junction where the 2 metals meet (1 metal and 1 "compound") forms a sort of new valence bonding.
ie at that point, you have a new alloy, after all, the metals chosen in the solder are selected EXACTLY for that
purpose. Alloy junctions can have weird properties, DIFFERENT to the base metals. It's kinda a voodoo science.
SO to answer a previous question, IF you separated the Cu and Solder and calculated resistance separately,
it would differ slightly to them combined because that junction has different properties.
Mixing different metals together to find new and often strange properties is an awesome field. Some of my
colleges have spent 20-30 yrs playing with that shit.