The strong acid in plumbing welding is very corrosive and is used to remove the oxidation layer from the surface of pipes as the solder melts, allowing it to adhere and form a waterproof joint. However, this corrosive acid flux will quickly degrade wiring if plumbing solder is mistakenly used for electronics. The key difference between electric and plumbing welding lies in the type of flux used in each application. Flux is used to clean the metals to be joined, eliminating any oxidation and preventing it from forming.
Plumbing flux is much more corrosive than electronic flux and is used to remove oxides on a much larger scale. While it may be more effective at removing oxides, it is too efficient in terms of electronics and could easily destroy a printed circuit board (PCB). Electronic soldering is a more delicate process than pipe welding and requires a flux that is much less aggressive. Do not use flux for pipes instead of flux for electronics.
Electrical solders have a lower melting point (around 360° F), while solders used in plumbing have higher melting points, generally more than 400° F. Therefore, it is not a good idea to use plumbing solder for electrical connections, since acid in the flux can damage the wiring and cause the connection to fail. The user in the question that led me to ask here thought their flux was corrosive because it had warning labels, but here is a U grade. Uncleaned fluxes leave much less residue than conventional R type flux, and this lower amount of residue will generally not interfere with the operation of the board or cause long-term corrosion-related failures. Fluxes designed for metal work, such as solder tubes, are generally an acidic flux and should never be used on PCBs or they will corrode things horribly.
Although tinning flux may be less corrosive than plumbing flux, it is not recommended to use it in electronic appliances. When used in this manner, without the addition of acid activators, it is referred to as type R rosin flux. Flux is not only used in electronic soldering, but also in plumbing, which uses different types of flux, such as water soluble flux. Non-clean rosin-based flux solutions are essentially the same as rosin fluxes (type R), but generally contain natural gum rosin at a much lower concentration than that used in R (R, RMA and RA) type flux solutions. That's why electric welding can be done with a soldering iron, whereas most plumbing welds use a gas torch to heat metals.
However, most local codes require the use of lead-free solder for all drinking water plumbing connections. Electric welds also tend to be smaller gauge, meaning they melt faster, unlike plumbing solder, which is generally thicker gauge and requires more heat to melt.
