Soldering is an approved method of splicing.
110.14 (B) Splices. Conductors shall be spliced or joined with splicing devices identified for the use or by brazing, welding, or soldering with a fusible metal or alloy. Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered. All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors or with an insulating device identified for the purpose.
As a matter of fact, soldering is one of the only approved methods for some applications.
The exceptions to this are for service conductors (230.81), grounding and bonding conductors where the connection relies
solely on solder [250.8, 250.148(E)] and connections to grounding and bonding electrodes [250.70].
