Solder Paste vs Solder Flux

Question

I bought a "Solder Paste". It is in quote because I'm not really sure if it's a Solder Paste itself.

I researched about what Solder Paste is and, by definition, is defined as a mixture of small solder spheres with flux. It's also grey-like in color since it has tiny solder spheres.

So, it can be directly applied to the pads of the PCB and perform reflow soldering afterwards.

But here is the actual product I bought:

As you can see, it's brown-ish in color and I don't think there are tiny solder spheres in there.

And the direction in using this is to put ample amount on the pads then apply solder with iron or torch

So my question is, is this really a solder paste or just flux? How can I use this, then?

Answer 1

This is not the stuff to use to solder electronic boards, this is for plumbing fittings, a flux to be used in addition to stick of solder.

Electronic boards require either 'flux-cored solder' for hand use, or 'solder paste', which is a mixture of small solder balls in a suitable flux, for machine use.

This flux is likely to be too aggressive to leave on a board, and will require cleaning, otherwise it's likely to damage the board. The flux used in flux-cored solder or solder paste is designed to be compatible with the materials on a PCB, and it's usually safe to leave the residues there indefinitely.

Answer 2

This seems to be an Asian-English thing- calling flux "soldering paste" or even "solder paste". Sometimes the word "cream" is in there.

I've seen it in China, Hong Kong and Taiwan, and this "soldering paste" is from the Philippines.

Here is some Asian-origin (real) solder paste:

As you can see, the alloy (Tin 63%/Lead 37%) is specified as well as the ball size 25-45um diameter. Most lead-free alloys are mostly tin, with a bit of copper and possibly other metals mixed in. There is a tin-bismuth alloy for the big spenders. It looks like a grey paste and under the microscope you can easily see that it is composed of tiny balls in a matrix of flux.

What you have is flux of some unknown type (although it's specified as non-corrosive so at least it (probably) isn't the typical highly acidic plumber's flux). You can stick multimeter probes into it and see if it is conductive.

Typical classes of fluxes are RMA (Rosin mildly activated), "no-clean" and water soluble. They have different characteristics, though for non critical circuits it may not matter which is used.

Here is a typical flux (RMA) from the same manufacturer:

As you can see, it's a translucent gel-like material with a yellowish tint, not dissimilar to what you have (however given the reference on the can to 'torch' I would be reluctant to try yours on circuits - it may be similar but have impurities that would be electrically conductive, for example).

Solder paste photo (from Adafruit):

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Edit: From the comment/link by @JRE, the flux you have is claimed to be suitable for electronics:

"Noisy set" indeed. If you use acid flux at mains voltage (as a hapless customer of mine once did) the "set" will be noisy from all the electrical arcing.

Answer 3

It's called soldering paste, not solder paste. Solder paste has a grey color due to the contained solder.

Answer 4

This is flux, not solder paste.

and concerning how to use it, you gave us the instructions, just put some where you will be soldering (or just dip your iron in the stuff) and apply the good ol' solder & iron technique.

I doubt it would be useable in reflow soldering though.

Answer 5

It's most likely flux. I say that because it is brown (as opposed to the grey that solder paste would be due to the tin content) and the suggestion of a torch, which is normally used for things like soldering pipes, not electronics.

Since electronic solder paste contains solder, it should specify whether it is lead-free or 60/40, 63/37, etc. This one doesn't say.

Answer 6

I’m no expert at this, but it appears, if some online sources such as here are accurate, that there are three things to distinguish:

1. Flux, which cleans metal surfaces
2. Soldering paste, which allows the flowing molten solder to adhere to other metal surfaces
3. Solder paste (and presumably solder flux) which contains tiny solder particles or balls

The likely mixes of some or all of these, plus interesting translations, might not help always make it clear which is/are involved.

I was recently looking for #2 for electronic hand soldering work, without which molten solder acts as pretty little mercury drops that don’t stick to anything. (A lot depends on the shape and composition of what’s being soldered, and mine wasn’t a typical one.)