Liquid Flux Pen - Water Soluble

Replacement: None at this time. We are currently looking for a less hazardous flux pen, check back later for more information. This page is for reference only.

This is a water soluble liquid flux pen. A flux pen is a must have when performing any kind of solder rework. The flux is the substance that prevents beading of the solder and helps the solder flow cleanly onto the parts you are soldering. However, flux is mildly corrosive over long lengths of time (months). The water soluble flux will cleanly wipe away after wetting the soldered surface.

Check out our Surface Mount Soldering Tutorials for some great soldering examples!

Note: Due to restrictions with the flux pen, we are currently unable to ship this product. It is still available for local pickup if you're in Colorado.

Be sure to use in a well ventilated area.


Looking for answers to technical questions?

We welcome your comments and suggestions below. However, if you are looking for solutions to technical questions please see our Technical Assistance page.

  • RedHeron / about 10 years ago / 2

    What hazards are with this pen? It's the one I've been told by basically everyone is the best. It's flux, so the purpose of it is chemical in nature.

    Kester also has a no-clean pen which I've also been told about (I'm hanging with some electrical engineers). It's more expensive, but certainly less chemical-y. And Amazon ships it, so you might be able to set up fulfillment by Amazon and avoid other shipping issues (it's a workaround, but it might work). I'm sure that Kester might be willing to work on it with you, as well, since SparkFun does volume.

    Just some ideas.

    • Mayo2017 / about 10 years ago / 1

      Here's another vote for the Kester 951. I can't live without the damn stuff.

  • No! I need more flux, I'm sure it is safe... enough.

  • ulidtko / about 11 years ago / 2

    Not shown on the photos, but the pen has hard porous tip, with very usable push-doser and holder. It's so cool, I like it.

  • I absolutely love this pen. Too bad I can't buy it from SFE, but I just found it at Allied Electronics:

    If you guys buy all of them before I can place my order, may the back-EMF of 1000 motors attack your motor controllers!

  • Shadow6363 / about 10 years ago / 1

    So…what was the danger with this? I've been using it for over a year now and am wondering if I'm going to suddenly pass out and die or something the next time I use it.

    • Mayo2017 / about 10 years ago / 1

      Water-soluble fluxes are highly aggressive and will corrode boards if left uncleaned. This can be a real problem in some cases where it gets under things like BGAs especially.

  • schwal / about 10 years ago / 1

    If you are in the Boulder area and hurting for flux pens, JB Saunders finally has some back in stock.

  • LightManCA / about 11 years ago / 1

    Glad I bought this while it could still be shipped. And thanks Asuraku for the suggestion of kimwipes. I'm gonna get some right now.

  • nmorisod / about 12 years ago / 1

    I've tried this flux and it's working well.

    But effectively a big warning if you don't clean the flux after using it, because it is more conductive that you might think.

    I've used this to transplant a super gameboy cpu to a gameboy and it was not working after the transplant. i've seen like a electrolysis between cpu pins (SMT) when power was applied. anfter cleaning the board everything was working fine ..

    • TD Gonzales / about 12 years ago / 2

      I have had the same problem with this stuff. I'm new to soldering and it feels weird to shove a board under a faucet and scrub it with a toothbrush. Although for finer more detailed stuff stick with rubbing alcohol.

  • mpechner / about 12 years ago / 1

    When ever I want to try a new surface mount technique I use this kit:

    It has practice parts and a red greed led blinker using a 555.

    I used the flux pen to solder this kit. I licked it. It stayed wet long enough to do a few parts. It washed off easily. I used the dedicted toothbrush to scrub the board. Then the heat gun to dry and evaporate the water.

    I do prefer water based flux. No chemicals required to clean. And it does not become a sticky as the no-clean stuff.

  • MoriFi / about 13 years ago / 1

    nice little flux pen very easy to use, to clean just use Rubbing Alchol (I use that stuff for everything but in a well ventaled area but if you are using flux you are allready going to be in one so I guess you are okay)

  • JBeale1 / about 14 years ago / 1

    Kester's 2331-ZX data sheet says cleaning is recommended, as below. So are people actually washing their boards with DI water at 140 F ? Just curious.
    "...It is not recommended to use high mineral content tap water. Otherwise, tap, deionized or softened water may be used for cleaning. The optimum water temperature is 54-66?C (130-150?F), although lower temperatures may be sufficient."

    • Camalaio_ / about 11 years ago / 1

      A bit late on the reply, but common household hot water tanks hold water around 60C (140F). Basically all that is needed hot tap water that isn't super hard, and a brush wouldn't hurt. Deposits from hard water could cause problems, but unlikely for hobby use.

  • This stuff is more conductive that you might think. I was working on a high-gain instrumentation amp circuit prototype on a board with no solder mask and the the resistance between adjacent traces was a megaohm or less!
    That's many microamps of leakage in the circuit and it drove my circuit to the rails and caused all sorts of oddball behavior. A solder mask would have reduced the interaction area between the traces and the flux, but the it still would have caused problems.
    If your circuit will function with megaohm resistors sprinkled all over you may not have to clean before you test, but you should.
    Also, the stuff is corrosive and will cause problems over time.

  • SteveL / about 14 years ago / 1

    Is the solder flux conductive??

    • It's barely conductive. But, it is corrosive. It's best to clean off any flux residue with alcohol when you're done.

      • Electrical Juggernaut / about 12 years ago / 2

        Having used this one, I prefer this one, specifically the no clean version. I have tested it up to 900 MHz and up to 100v, it has no noticeable effects of capacitance or conductivity. It's great because you don't have to worry about corrosion from leftover flux under BGA or other surface mount chips. Any chance you could stock some?

        • Thanks for your feedback and product suggestion. I will pass along to our main product research person!

  • This is the best way to improve your solder joints. This stuff works great, very good flow, easy to clean, and easy to apply. Made more of an improvement in my solder joints than a temperature controlled iron or anything else in my toolkit.

  • I tried several varieties of Kester flux and this one is by far the best. It stays liquid for a long time in the tube, it's easy to dispense, and it is an excellent wetting agent for liquid solder (especially the finicky lead-free stuff I'm using).
    Its only disadvantage is that it does conduct electricity (a little) when dry, so you really should wash your boards afterward. Well, probably not ... except if you're soldering triacs for use on line voltage because otherwise pretty sparks will result.

    • Asuraku / about 15 years ago / 3

      Always clean your boards! I don't know if it's because I'm IPC certified for class III PTH as well SMT, but even with a Low flux, corrosion can occur. Just make sure you clean off a board with rubbing alcohol and some non-fibrous tissues (Kimwipes are really good). And it helps to use a flux brush (commonly sold at hardware stores for applying acid based fluxes for copper pipes) to scrub with the alcohol. And note that the alcohol doesn't act as a solvent, it just removes the flux from the board and mixes with with the alcohol. Then use the Kimwipes to clean it up.
      Happy soldering!

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