using plumber's copper solder as a "solder paste" for SMD components

The "no-clean" rosin flux used in most electronics solder has two interesting properties:

• The junk it leaves on the board is a pretty good insulator (not perfect, but unless you do very high impedance sensitive analog stuff, it's fine to leave it on).

• It doesn't corrode the copper, the pins, etc...

However, other types of flux, like some water-soluble ones that are easy to clean are actually conductive (ions... in water...) and corrosive (will eat your copper!)

So you must clean them (preferably with an ultrasonic cleaner), which is kind of annoying. Good luck getting it out from under a QFN.

Now, you got... Zinc Chloride!

It is very hygroscopic, which means it will suck up any moisture in the air, and then it will be conductive (water+ions) and your board will get electrolyzed and destroyed pretty quick.

It's also very good at corroding metals...

Wikipedia says:

Because of its corrosive nature, this flux is not suitable for situations where any residue cannot be cleaned away, such as electronic work.

So... nope. Get a solder specified for electronics, preferably with lead, it melts at a lower temp.

Zinc chloride flux is very corrosive, and can eat through the traces on a PCB if the residue is not completely removed. If this is something that needs to be working 2 months from now, don't use it.