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I was at Fry's yesterday, looking for some thin solder to use on an Adafruit kit with through-hole components.

Two of the solders I found were labeled RA Flux and No Clean, respectively. An engineer who was present said that with my use-case I should choose No Clean, as I wouldn't need to clean the board afterwards.

Can anyone clarify how one should select solder based on flux type? I had always been under the impression that you just use some sort of (one size fits all?) rosin-core solder with for basic board work, so I was confused at the different flux labels. I didn't think you had to clean your board (although I do know that after soldering yellow gunk can be left behind, and I suspect this is a flux by-product). I also know that you should avoid solder with an acidic core for electronics work, as that is for pipes + household plumbing.

The wikipedia page for soldering classifies many of the different types of fluxes, most of which require cleaning:

  • R (non-activated)
  • RMA (mildly-activated)
  • RA (activated)
  • No Clean

Can someone explain why/how you should choose a flux type for a given application? Most of the answers below answer "what." I'd like to learn a bit of the theory.

SamGibson
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J. Polfer
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2 Answers2

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For thru-hole work (which it sounds like you are doing) I would go with an active flux that is water soluble. You need to wash the residue off since it is corrosive and can cause shorts. I have had a number of boards mysteriously come back to life after a good cleaning.

For thru-hole parts that may get damaged by cleaning (LED lenses) or SMD parts with heatsink pads I would use the no clean flux.

I have been using the flux pens from Kester. 2331-ZX for water soluble flux and the 951 for no-clean. To clean the water soluble flux I use an "acid" brush with the bristles clipped to around 0.5". I run the board under the hottest tap water I get (probably around 140-160degF) and scrub with the brush. If I do a small repair job I will clean with the isopropyl alcohol I have on my bench.

Here some of the whys --

Why you need flux -- to get a quality joint at a lower soldering temperature. Using a liquid flux lets you spread the flux better over the two surfaces to be soldered.

Why choose an aggressive flux -- an aggresive flux is more tolerant of soldering temperatures. As Leon mentioned this would be a good candidate for a hobbyist. I like the 2331-ZX for this. Since these leave ionic compounds you need to clean.

Why choose no-clean -- eliminates a process step and the issues associated with the extra process step. For example for soldering XLamp LEDs I use a no-clean flux rather than risk scratching the lens.

For more details take a look at http://www.finishing.com/Library/flux.html and the Kester website.

Daniel Grillo
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jluciani
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    At my job some rosin flux was the problem behind a TX antenna mismatching, we now use water based flux. to clean it we give the board a good rinse and then dry it with compressed air. – Kortuk May 03 '10 at 18:17
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RA flux contains activated rosin, and is the most active flux. It's probably more suitable for hobbyists as it'll be easier to get good joints. Use 99% isopropyl alcohol for removing it.

Leon Heller
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  • do not have isopropyl alcohol around so using paint-removal-spray for car-industry or acetone, to remove extra rosin. Spay rosin-blackened spot and then just rinse with water. Probably cheaper than electronics spays as bottles are bigger. – hhh Jun 25 '10 at 11:29
  • Where do you get IPA? – hhh Jun 25 '10 at 11:30
  • IPA is commonly available (at least in the US) in drugstores. Look for the 99% type. Electronics stores often also carry anhydrous (99.9%), but it is a bit more expensive. – Peter Loron Jan 04 '11 at 23:48
  • In the US it's also referred to as "rubbing alcohol". – bitsmack Jan 11 '16 at 18:46
  • Be cautious if soldering optocouplers. Avago recommends [against](https://docs.broadcom.com/docs/AV02-4463EN) the use of halide-containing fluxes (e.g. RA flux) with plastic optocouplers, because the halides can migrate into the optocoupler packages and damage the components. –  Feb 22 '19 at 04:30
  • @hhh do not recommend acetone. It is too aggressive. It will damage plastic parts on the PCB.. – Chupacabras Feb 23 '19 at 06:38