I want to know how flux help to solder in detailed step of soldering.

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 3, 2016.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    4
    Hello and happy new year!

    I would like to know what actually happens during soldering with flux.

    I've learn that flux has two major roles; 1. it becomes corrosive at high temperature thus any dirt and oxide film on the metal pad of PCB will be gone when it is touched with heated solder iron before it evaporates. 2. It reduces surface tension of the melted solder thus solder can easily flow with the flux.

    Based on these knowledge, I imaged what happens during soldering as described below. (It is the situation that the pad of the PCB and surrounding area like solder mask is applied by flux and the solder is melted and stayed on the solder iron tip then the tip is touching the pad and flux right now)

    1. As temperature of flux rapidly rises, the flux becomes corrosive so any dirt things on the pad including oxidized surface is gone. At the same time, liquefied solder starts to flow along the flux due to the fact that flux reduces surface tension of the liquid solder.

    2. As temperature of flux rises more and more evaporation begins. The evaporation help solder to touch pad as flux between solder and pad is gone.

    3. Once some portion of solder stays on the pad, its surface tension pulls rest of the solder thus good blob of solder is made on the pad.


    Is my imagination right or there are more process I have to take into account?


    And I have one more question about liquid type flux as I'm using now. Does it also have same function to other flux like rosin? What make it different from the other?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I agree with the sequence you describe, except step 2. Solder is much denser (heavier) than flux, so it will flow under the flux on the pad due to surface tension to make a joint even if the flux did not evaporate.

    John
     
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  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Thanks.

    Then I have to modify middle sequence like this; Flux gets hot so becomes corrosive before solder flows underneath of it. Surface tension of the pad attracts solder so the solder stays at right position.

    Right now, I'm curious whether the pad is really attracting solder due to surface tension between metal?
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The surface tension of the liquid metal has to be roughly equal, ideally, lower than the surface tension of the metal pad. Otherwise, if the liquid solder had higher surface tension than the copper pad, the solder would bead up like water on a freshly waxed car.

    Adhesion is not the same as surface tension but having the right surface tension during the bonding certainly helps get good "wetting" of the substrate so bonding can occur.
     
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  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Try this: Flux gets hot so becomes corrosive "and removes oxidation from solder, which allows solder to flow" underneath it. Surface...

    Yes, solder flows on the pad because of surface tension.

    John

    PS: I have tried to keep close to your style.
     
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  6. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Hello.

    So, the mechanism of bonding of solder to the metal pad is actually not surface tension but adhesion. Is it what you mean?
     
  7. SLK001

    Well-Known Member

    Nov 29, 2011
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    The solder joins to the metal pad via an intermetallic bond.
     
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