Hi,
It´s shouldn´t be hard to find the copper´s tempco ... in times of internet.
The tempco of copper is huge, thus you rather may use it as temperature sensor than as precise resistor.
For solder: you have to contact the manufacturer, if it´s not listed in the datasheet. Different solder types will have different tempco.
If you see a lot of tempco whil you use kelvin wiring, then maybe your wiring technique is not optimal.
--> Post the part of the PCB layout.
Klaus