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SMD soldering : Clearing flux residue ?

Discussion in 'Electronic Design' started by Sylvain Munaut, Mar 16, 2005.

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  1. Hi,

    I'm hand soldering my first SMD board. Since it has soldermask, soldering the SMDs is pretty easy, even the 0603. To do so I use additionnal solder flux. I bought it at farnell, a Multifix 425 flux, rosin based. But after soldering there is a lot of flux left, how can I get rid of it properly ?

  2. Robert Baer

    Robert Baer Guest

    One could use solder with the more modern "no-wash" fluxes, and use a
    flux pen for additional purposes, like fluxing the IC leads and pads first.
    If one does that, use isopropyl alcohol ~75% (balance is water) for
    wash as follows:
    1) Soak for 0.5 hr, rinse and soak again, then final rinse. Air or oven dry.
    2) Rinse and use artist short-hair brush to remove flux; repeat at least
    3 times, then two final rinses. Air or oven dry.
    That being said, i have yet to find a relatively safe solvent for
    rosin; isopropyl soak method removes most, but not all of it.
    Using a toothbrush can be of great help, but places that are not too
    accessible tend to have some residue left behind - giving a whitish cast
    around the offending joint.
    Using a small pin point to scratch off that whitish residue helps,
    then repeat the cleaning as above may get all visible rosin off.
    Good luck.
  3. I read in that Robert Baer>) about 'SMD soldering : Clearing flux residue ?',
    Have you tried air agitation (or ultrasonics)? Air agitation performs a
    similar function in PCB etching, after all.
  4. Brian

    Brian Guest

    Alcohol in an ultrasonic cleaner works good, in fact, excellent.

    Acetone and a toothbrish works, too.
  5. dlharmon

    dlharmon Guest

    soldering the SMDs is pretty easy, even the 0603. To do so I use
    additionnal solder flux. I bought it at farnell, a Multifix 425 flux,
    rosin based. But after soldering there is a lot of flux left, how can I
    get rid of it properly ?
    Try water soluble solder and flux. I use kester 331 solder and a
    2331ZX flux pen. It cleans up easily with hot water. I have some
    notes on my webpage at

    Darrell Harmon
  6. Ken Smith

    Ken Smith Guest


    I have learned the hard way: never use a "no-wash" flux and then expose
    the PCB to any kind of solvent. "no-wash" fluxes contain basically a
    plastic that is supposed to trap the ionic stuf that is in the flux. If
    you expose the board to a solvent, the plastic stuff partly loses its grip
    on the ions and they head straight for the most sensitive circuit on the
    PCB. The remaining plasic stuff then serves to hold the ions exactly
    where they cause the most trouble.

    The company that I use to do coating and potting strongly recomends that
    easily washed fluxes be used. Next on their list is the stuff that takes
    fair amounts of non-polar solvent. For the "no-wash" stuff they suggest
    you send it somewhere else to be cleaned.

    There is evil smelling stuff called "ensolve" that works but you can't buy
    it in the hardware store.

    Some people claim to have luck with switching back and forth between
    mineral spirits and water based cleaners. The mineral spirits lifts the
    non-polar stuff to let the water at the ionic stuff. The water gets the
    ionic stuff away so the mineral spirits can get to the next bit of
  7. Ken Smith

    Ken Smith Guest

    With film capacitors, acetone will often change their value slightly.
  8. Ken Smith

    Ken Smith Guest

    Depending on your local water supply, finish up the washing with distilled
    water. After the PCB is clean, leave it in a hot dry environment over
    night before you apply power.
  9. Bob

    Bob Guest

    YESSSSS!!!!! Heed this warning!!!!

    I, too, found out the hard way.

    If you use no-clean flux then either do nothing else (after soldering) or
    clean the entire board. If you wash part of the board, after soldering, then
    ions will form nasty little electrically-conductive things called dendrites.
    Dendrites will short-out your electronics.

    No-clean flux leaves an ugly looking residue, but it is non-conductive.

    If you use other types of flux (other than no-clean) then clean the entire
    board after soldering.

  10. giorgi

    giorgi Guest

    i clean my pcb as follows:

    1) clean with normal fuel car (in Italy we call it "benzina" ) with aid
    of a short-hair brush

    2) after, clean with water and liquid soap (and brush)

    3) be shure of removing all soap. It happens when disappears bubbles
    during brushing.

    let warm for humidity removal. Sunlight is good.
    The aid of compressed air during 1) and 2) is useful.

    bye - S.
  11. Leon Heller

    Leon Heller Guest

    I use an old toothbrush and IPA, a few seconds of brushing removes all the

  12. Boris Mohar

    Boris Mohar Guest

    I switched to all water soluble flux. Flux pen is great chips. Just
    saturate the pins and drag the iron tip over them. With plenty of flux
    solder snaps in place without smearing. You must wash off the water soluble
    flux thoroughly and shortly afterwards. It is very conductive and any
    residue will play havoc wit high impedance circuitry. Does that answer why
    the 32kHz clock does not start? If you have to interrupt the work, store it
    along with some desiccant in a airtight container. Humidity in air reacts
    with the flux residue and forms white crud that is difficult to remove. I
    use electric toothbrush for cleaning the boards.


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things)
  13. dalai lamah

    dalai lamah Guest

    Un bel giorno Sylvain Munaut digitò:
    Someone says that a dish cleaner works pretty well. I haven't tried it
  14. Terry Given

    Terry Given Guest

    The thread 'OT: Check out this idiot' on ABSE shows one technique.


  15. Is that with or without the optional blow torch igniter?
  16. Brian

    Brian Guest

    But probably not as bad as stuff getting stuck in the residue does :)

    I have had RF board completely go nuts when flux residue remained after the
    wash process.
  17. Brian

    Brian Guest

    No clean means that though, no clean. It also means don't coat with any
    conformal coatings that use a solvent. Use a water-based coating instead.
  18. Ken Smith

    Ken Smith Guest

    Yes, the goal *IS* to remove the flux without removing the circuit in the
    Me too + low noise amplifiers weren't + 5V regulator wasn't + high
    impedance input wasn't.
  19. Fred Bartoli

    Fred Bartoli Guest

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