Electronic – Which flux type to use for SMD hobbyist (home-made) PCB

fluxpcbsoldering

"Short" question

Which flux type to use for SMD hobbyist (home-made) PCB?

  • How to know if a specific flux (e.g. F-SW-33) is no-clean or require cleaning.
  • How to know if a specific flux conduct electricity?
  • What is the impact of Halides?

Longer question, including context and previous researchs

I just had a hard time searching why my SMD circuits was not working properly, up to the moment I found the flux deposit was conducting enough to switch some transistors ON.

After some research, I found the following info:

  • Wikipedia flux specification differenciate types:

    • Base material: Resin, Organic, Inorganic
    • Corrosivity: not, weak, strongly.
    • With/Without Halides
    • Base component
    • etc.

  • Different types as:

    • no-clean (which unintuitively do not need to be cleaned)
    • water-soluble
    • etc.

On the other side; With my poor understanding of PCB fabrication, the process is simplified to:

  1. The circuit is printed (UV-positive painting, exposure, development, etching)
  2. I protect the circuit with some soldering mask/protection (recommended)
  3. Silk-printing (optional)
  4. Soldering (using flux)
  5. Cleaning of flux ?

But, how to clean below SMD small resistors or IC? That seem to me not an easy task. My conclusion is: Better use a no-clean flux, or at lest a non-conductive flux.

But which type match this specification?

This answer suggest that for electronic I should only use Rosin flux. Is that generally correct? or could an F-SW33 fill the task?

Are all the F-SW26 to F-SW32 equally good?

Related questions:

Best Answer

If a flux is "no clean" it means it doesn't, under normal circumstances, conduct electricity. The exception is when the flux gets burned by too high a temperature and turns brown. At this point it can start to conduct due to carbon deposits.

Water soluble flux conducts and must be cleaned before use.

My personal choice is to never use water soluble flux and to clean flux off with isopropyl alcohol as a matter of course.

I have two fluxes - one in a pen form and one in a syringe. I find I use the syringe one more than the pen one these days.

Both are Rosin based.

As always the exact properties and handling / cleaning instructions can be found in the datasheet.

Fluxes designed for metalwork, such as soldering pipes, is generally an acidic flux and must never be used on PCBs or it will corrode things horribly. Only ever use electronic-grade flux.

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