Another question made me curious, does the NEC allow copper wire to be joined to aluminum wire by soldering?
You mean like a rattail or Western Union splice, Al-Cu?
Your first problem is that soldering in general is so alien that an AHJ is likely to reject it out of hand, even if it were all Cu. The ruling would likely be "You can solder if you can show how every other splicing method is impracticable".
But when it comes to Cu-Al splicing...
NEC 110.14 Electrical Connections. Because of different characteristics of dissimilar metals, devices such as pressure terminal or pressure splicing connectors and soldering lugs shall be identified for the material of the conductor and shall be properly installed and used. Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors (such as copper and aluminum, copper and copper-clad aluminum, or aluminum and copper-clad aluminum), unless the device is identified for the purpose and conditions of use.
(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.
The sections I put in bold are in contradiction with each other. 110.14(B) requires direct contact between the Cu and Al wires, yet 110.14 forbids it.
The path to soldering is plainly identified in 110.14 with "soldering lugs".
I'm not an expert in this area but I can't see any way that copper wiring can be safely soldered to aluminum wiring. Aside from the galvanic corrosion that would probably occur, copper and aluminum expand at different rates when heat is present from current passing through the wire. The joint will ultimately weaken and fail and I don't see anything in the NEC implying it's safe. You need to use approved Al/Cu connectors such as Alumiconn.
I would think that the biggest problem would be trying to solder the Aluminum to begin with. Normally it is a major pain to solder for the simple fact that it will start to oxidize as soon as air hits it and just swiping it with flux is not enough.
As soon as you get heat near it, unless it is completely covered in flux, it will form a layer of oxide and prevent a good joint from forming. And in this case it's going to be even worse for the simple fact that the copper will suck heat from the Al making it that much harder.
The one time in the past when I HAD TO solder it, I ended up having to hold the junction in liquid flux while I applied the gun(there was no way any of my irons would have been able to sustain proper heat) to it.
Afterwards, I would clean it with something like contact cleaner and coat it with liquid tape or self vulcanizing tape.
EDIT: BTW, that is how you tell that you have Al wire and not tin coated copper, when you try to solder, you end up with a black mess instead of a good joint.
This is a very good, interesting question.
The question specifically asks about the NEC, but code aside, aluminum is not really suitable for soldering in the field. Typical tin-lead solder used for electrical / electronic applications will not bond well to aluminum or the aluminum alloys used for aluminum wire.
The National Electrical Code has changed a great deal over time. The first edition was published in 1897, before electrical utility power was even available most places. It has gone from a skinny book you could carry in your pocket to a phone book sized volume (for those of us old enough to remember thick phone books).
I think soldering went out as a normal practice in the 1960's before aluminum wire came into regular use in the 1970's and the code never really put much thought into soldering aluminum. At this point soldering remains in the code mainly as a holdover, nobody solders any more. But soldering isn't treated nearly as rigorously in the code as newer methods.
I think if soldering was a current practice, solder would be UL listed, soldering irons would be specified in the manufacturer's instructions, the types of wire suitable for soldering would be spelled out, etc. etc. This is not the case, I think because the code was not so rigorous in the 1940s and 1950s when soldering was still a common practice. The vestige that is held over hasn't been updated to the rigor of the current code.
@Harper 's answer makes a decent argument that there are two requirements for soldering and you can't readily meet both when soldering aluminum. But this line of reasoning is it's a bit convoluted and the code should really be more direct and clear. It is perfectly direct and clear with respect to connectors.
I believe it's a great question because when dealing with the aluminum wiring that was popular in the 1960s, making connections that comply with modern code can be challenging. There are specialty products and connectors that will work, but they can be hard to find and hard to use. Soldering, although labor intensive, might be an option in some cases when working with these old aluminum installations. I suspect that may be the intention of the question.
In fact, code aside, aluminum is not really suitable for soldering. The hard oxide that almost instantly forms on aluminum prevents a good bond to the solder. There are specialty products and methods for industrial applications that must solder aluminum, but t's unlikely that you'll get a good reliable connection in the field with regular tin-lead solder commonly used for electrical / electronic applications. I would not sleep soundly in a house wired that way.
So to me, the answer to the question is: you may or may not be able to slide it by an inspector, but just don't do it.