NEC 110.14 Electrical Connections
Soldering lugs shall be identified for the material of the conductor and shall be properly installed and used.
This is clarified in a sub-paragraph:
Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder.
The simplest interpretation is that you must use soldering lugs, and since the material wasn't specified in the question, the lugs would have to be identified to work with the conductors.
