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soldering small SMT resistors, capacitors and transistors, I can't get them flat

Discussion in 'General Electronics Discussion' started by danny davis, May 10, 2012.

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  1. danny davis

    danny davis

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    May 9, 2012
    When soldering small SMT resistors, capacitors and transistors, I can't get them FLAT on the PCB

    I have to use the Hot Air gun to melt the solder to make the component FLAT on the PCB

    If I use the soldering iron to solder SMT components, The SMT components have a Slant or they are lifted on one side

    How do you place and push down or keep the component in place while soldering?

    Using tweezers the small SMT part will fly away or it keeps slipping out of the tweezers arms and falling out and it's really had to place it and solder , tack in the SMT component

    When I use the Hot Air gun it will heat up both sides of the SMT resistor or capacitor and it will melt the pads flat and I can remove and place it better

    But most of my SMT solders using a soldering iron on Slanted or they look like the are lifted on one side? How can I get it flat?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Are you using solder paste?

    The problem with components standing up on end as they are soldered (tombstoning) is generally caused by uneven heating. If the solder melts simultaneously on both ends of the component, this rarely happens.
     
  3. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    Hold the part down with the tips of your tweezers or a pick while tacking.

    Dry tack.


    Wet tack.


    Bump tack.
     
    Last edited: May 10, 2012
  4. danny davis

    danny davis

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    May 9, 2012
    I usually place the part on the pads with a tweezers, then use the tips of the tweezers to gently press down on the middle of the package while I solder one contact. Once one side is down, I solder the other side (no need to hold the part down, because the first solder joint holds it in place). I've never had an issue with flatness using that method.

    I have tried this method and it doesn't work for me
    1.) When I press down on the package it moves side to side, it's very hard to center the SMT resistor in the middle so both pads line up

    What can I use besides tweezers to hold a SMT resistor or capacitor down?
     
  5. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Only apply solder to one pad, place and solder the chip down, than solder the other side... It shouldn't move once the first side is soldered down flat, but if you have solder on both pads at the same time it can be tricky without hot air or an oven...
     
  6. danny davis

    danny davis

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    May 9, 2012
    you have solder on both pads at the same time it can be tricky without hot air or an oven

    Yes that is my problem because I'm doing rework at this job I'm at and you have to desolder the SMT resistors and capacitors

    So both pads have solder already on them. When I'm placing the new SMT resistor it's very have to get it FLAT, because it look slanted and not flat to the PCB

    Another problem is when I'm desoldering, trying to lift one side up is hard too, What is the best tool to use because I can't heat up both of the pads at the same time so trying to lift up one side is a pain with only one soldering iron
     
  7. KJ6EAD

    KJ6EAD

    1,114
    157
    Aug 13, 2011
    I've had success reworking chip capacitors using hot tweezers and holding the component tight to the board with a pick.
     
  8. danny davis

    danny davis

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    May 9, 2012
    My last job used a Hot Air gun to solder and desolder SMT components

    Using a soldering iron is much harder

    The problem also using a Hot air gun is the AIR will blow the SMT resistor away

    So it's the way problem I have of finding the best tool to use to keep a SMT resistor from moving?
     
  9. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    If you have solder on both pads, remove the solder on one pad with wick before reworking...

    With a rework hot air gun, and tweezers this shouldn't be a problem... I can dial down the airflow on my rework air to the point that it hardly blows even the smallest chips around, and won't move a chip at all once the liquid solder gets a hold of it... If you are using a paint stripping heat gun or whatever get a better hot air tool that is made for the job...

    You can also use a large flat tip soldering tip that will heat both sides of the chip at once if you come at it from the side...

    But honestly I think you are over complicating the issue... I reworked SMD components 12 hours a day 60 hours a week for several years with just an ordinary iron and small heat gun for larger ICs... I think more practice and less desire to find the miracle tool will be your biggest advantage...
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Use solder wick to remove any existing solder before you replace the device.

    even with a soldering iron, solder paste can help you.

    place the paste on the pads, sit the component on the top of it, hold down with tweezers and touch with a soldering iron.

    Even if you have to come back later and apply a little more solder, the small amount that remains will hold the device firmly in place.
     
  11. danny davis

    danny davis

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    May 9, 2012
    Isn't the solder paste just the same as adding solder to the pad before you place component its the same thing right? no much difference

    When I use the tweezers I have no control at all of the SMT resistor to keep it straight and flat to the PCB, It just keeps moving
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, solder paste is a paste made from very small balls of solder mixed with flux.

    if the pads are clean, the component shouldn't move a lot.

    If you use stainless steel tweezers you can rest your hand somewhere while you hold the component and even if you have shaking hands, this should allow you to keep the component still.

    If you're REALLY having problems, get some adhesive and apply a dot to the back of the component. You would have to be sure that it's not corrosive, but there are adhesives made especially to stick components to boards.

    What size are we talking 0805, 0603, 0402? I can understand if you have trouble with 0402, they're hard enough to see let alone to hold.
     
  13. danny davis

    danny davis

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    May 9, 2012
    0805, 0603, 0402

    Mostly 0805 and 0603

    Do you know what type of adhesive

    I'm mostly doing rework , which is taking SMT resistors or caps, and replacing them with the correct values. so both the pads have solder already

    It's really hard to keep the SMT resistors from moving and making it sit flat
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Use solder wick to clean up the pads. That will make your life much easier.

    edit: It's normally only a cosmetic issue if the components are not flat, as long as all the leads are connected.
     
    Last edited: May 10, 2012
  15. CocaCola

    CocaCola

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    Apr 7, 2012
    Have you ready any replies? Clean the pads of the existing solder... It's not hard to get the parts flat, it just requires you learn a technique that works for you... Tweezers will hold the part just fine, I can't understand why you say they won't as they most certainly will...
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    With 0402, and probably 0603, you should be able to melt the solder on both ends of the component simultaneously.
     
  17. danny davis

    danny davis

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    May 9, 2012
    how do u melt the solder on both ends of the component simultaneously with only one soldering iron?

    the tweezers don't work for me
     
  18. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    May I asked how much experience you have in reworking and soldering?

    You keep saying won't, doesn't and can't but I know for a fact that the suggestions above WILL work, I have done them all and can do them all successfully... You just need to apply yourself and learn the skills necessary...
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    you hold the component with the tweezers and use a tip large enough to touch both pads at the same time.

    I typically just put the pointy end of a pair of tweezers with bent ends on top of the component and apply a small amount of pressure. They rarely move. With clean pads and solder paste you can tack the component down without even touching it in most cases.
     
  20. danny davis

    danny davis

    306
    0
    May 9, 2012
    What do u use to clean the pads? if i use a solder sucker it lifts the pad and sucks the pad off of the trace

    I guess solder paste makes the pads sticky to tack the SMT resistor so it doesn't move?
     
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