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

Can I ? :D

It is what I currently have plenty of lying around and, well... it says S-Sn97-Cu3 at composition

If not, could I use regular solder wire, coat the SMT pads with it, apply flux afterwards, place the SMD part and hold it into place, then gradually apply hot air to it ?

edit The package I am looking to solder in this way is a no leads type. Sorry, forgot to mention.

edit2 Found this paste over here. It has the same binder and the same flux/binder ratio as the one that I have. So it would appear that all that remains is the flux. I do not know what it is, but on my flask it says it contains zinc chloride, it thoroughly warns against inhaling, ingesting, eye and skin contact. Also there are some writings: R36/37 S 1-2/S 20/S 26/S 36-39

Charge: FD

DVGW: DV-0101 AT 2247

I wonder if the regular electronic parts solder paste has any warnings against skin/eye contact, inhaling and ingesting.

• The point of the flux is to deoxidize and clean the pads, so you'd apply it before soldering. If you want to go that way, add flux first, then add a bit of solder, add more flux, then hot air, and when the solder is molten, drop the part on top. Lots of extra effort IMO. – Simon Richter Nov 10 '17 at 14:05
• You probably can use the plumber's solder paste, although watch out for melting point issues. Would you be using it in reflow or with an iron? If you're doing it manually you can actually just place the part first, flux, then solder with solder wire. – pjc50 Nov 10 '17 at 15:21
• Regular solder doesn't work well in place of solder paste. NEVER use anything marketed for plumbing use to solder electronics. Plumbing stuff uses acid as flux. This will destroy your traces and pads. – JRE Nov 10 '17 at 15:23
• Just use the iron and regular electronics solder and solder the parts by hand. By the time you manage to do it using hot air or oven at home AND get it all correct, you could have done it by hand many times over. – JRE Nov 10 '17 at 15:25
• @JRE without disagreeing with your conclusion, a lot of plumbing solder has no flux in it, and is intended for use with separate flux – Chris H Nov 10 '17 at 21:10

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.

• Yikes! Ok, ok, I give up. And surely enough, the label says 100% water soluble :). But I wonder why it's being used on copper plumbing if it is so corrosive... – kellogs Nov 10 '17 at 16:21
• @kellogs Because plumbers wipe their joints clean as they cool and also most good ones will flush a new system before it is put into service so removing most if not all of the remains of the flux... – Solar Mike Nov 10 '17 at 16:28
• Copper pipe has 1mm thickness vs copper trace on PCB is 35µm, so the pipe doesn't mind a little residue, plus it's easy to wipe... Also there should be no current in the copper pipe (normally...) whereas on the PCB you can get electrolysis between nearby pins... – peufeu Nov 10 '17 at 17:30
• Also, raw copper pipe that has, in the worst case, been stored outside or had dirty fingers on it will need flux with more oomph than flux for electronics work. – rackandboneman Nov 10 '17 at 22:17

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.