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Re: [Electronics_101] surface mount

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  • BrianJHoskins
    ... components.  Does anyone know of a website that has a good tutorial for a beginner hobbyist? ... Well, I can give you some advice of my own, if you
    Message 1 of 3 , Aug 3, 2002
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      > So, I was thinking of trying my hand at soldering some surface mount
      components.  Does anyone know of a website that has a good tutorial
      for a beginner hobbyist?
      >

      Well, I can give you some advice of my own, if you like? I'm not sure
      exactly what kind of surface mount components you're looking to solder,
      wether it be small discrete components or ICs. But here goes...

      DISCRETE COMPONENTS:

      First of all, get yourself a quality pair of precision tweezers, it'll
      make your job ALOT easier. The ones that are sprung so that they pull
      together are the best, you wont need to grip the component, just hold the
      tweezers. Also, things will be alot clearer when looked at beneath a good
      magnifier/light tool. The big circular ones which mount on a bench and
      can be twisted to any position you desire are excellent, you wont have to
      hold the magnifier in position which means you've two hands free for
      soldering the component in. They are a little expensive, but keep an eye
      out on Ebay because I got mine off there quite cheap :)

      Right, assuming you've already removed the old component (or you're
      fitting one for the first time) the first thing you'll need to do is clean
      the connection pads. For this I usually des-solder them with braid, give
      them a clean with Flux-Off, and then add a small amount of solder
      afterwards - this way the solder will have bonded well with the pads, and
      will be nice and fresh so it'll flow nicely onto the end caps of your
      component. Now, carefully pick your component up with the tweezers making
      sure that they've gripped properly. If you drop it, don't bother looking
      - it's gone. Get a new one (the ones we deal with in work are the size of
      a spec of dust). Move the magnifier into the correct viewing spot and
      position the component nicely over the pads. Using your iron, tag just
      the one end of the component, and when you're sure that it's bonded there,
      release to tweezers. If you have to mess about re-heating the solder more
      than twice, then start again from the beginning, re-cleaning the pads.
      Once the solder has been heated up twice it never flows properly. Now that
      the component is soldered at the one end you can put the tweezers down and
      pick up your solder instead. Use it in combination with the iron to make
      a good connection on the other end, but don't add very much solder - you
      only want a little bit to flow on the pad and the end cap of the
      component. Any more than that, you'll just have a blob of solder sitting
      on the end of the component. Having done this, take a good look again
      through the magnifier and check that the solder really has made a good
      connection with the end caps of the component - sometimes it doesn't quite
      bond properly. By the way, if you apply to much heat to the surface mount
      component you can damage the end caps completely, and then the solder wont
      take to it.

      And that's it for the discrete component! I've assumed you meant very
      small surface mount components, it's actually quite a bit easier for the
      larger ones - you can normally do away with the magnifier and get away
      with a good pair of long nose pliers if they're the larger type.

      SURFACE MOUNT ICs:

      Right, these are MUCH more difficult. I had to change a tiny surface
      mount microprocessor on a Phillips TV small signals board the other day,
      and it was a right hassle. We've got a proper hot air/suction machine for
      removing them, but you can get away with cutting them out and then
      removing the pins with braid - as long as you're VERY careful not to
      damage the print. Once the old component is out, you'll need to get all
      the old solder off the pads. Be VERY careful when doing this, because
      it's sooooo easy to get a little careless trying to rush the job, and
      before you know it you've damaged the print. Usually the whole board is a
      write off after that, it certainly would be with the Phillips board I
      worked on the other day. After you've got the old solder off, clean it
      with flux-off (again, be careful when using the brush-end because you
      don't want to damage the print) and then re-apply a very small amount of
      solder to each pad with your iron. Next up you'll need a good tube of
      flux - apply it across all the pads. Now, this is the tricky bit...
      Positioning the IC on the pads properly can do your head in quicker than
      your other-half in a shoe shop believe me. You have to make absolutely
      certain that the pins are lined up with the pins properly, and for this
      you'll DEFINATELY need a magnifier. It's a case of gently pressing the IC
      one way or the other until all the pins are lined up, and it'll be alot
      harder if your pads aren't nice and clean I can assure you. The solder on
      the pads needs to be ultra smooth, because if it's lumpy in any way it'll
      hamper yor attempts to move the IC on the pads. Ok, with the IC in
      position you'll need to tag the four corners with your iron. Be extremely
      careful and apply as little pressure as possible - you don't want to move
      the IC. If you tag the IC diagonally, once you've managed the first two
      opposite ends you can normally relax a little bit, it's not going to move
      far after that! After you've tagged the IC in at the four corners, take
      another look with the magnifier to check it hasn't moved out of position,
      otherwise you'll be wasting your time soldering the other pins. Assuming
      you've got the IC all lined up, you can now proceed to solder the rest of
      the pins. Depending on the size of the IC, you can sometimes just run
      your iron down the length of each side and the solder will take to each
      pin perfectly (requires a bit of practise) - the flux allows the solder to
      flow onto each pin. if it's ultra small though it's best to do each pin
      independantly. if you've applied fresh solder smoothly to the pads as I
      suggested earlier, you can normally just tap your iron on the pad and
      there'll be enough solder already on the pad to do the job, but don't be
      afraid to add more solder. Just be careful! By the way, the pointed tips
      are much better for this than the clumsy standard ones.

      After you've soldered all the pins, take a look under the magnifier and
      examine them to be sure there are no bridges between pins, or that the
      solder hasn't taken properly on any of the pins. A good test later is to
      run a precision sharp hook down each side of the IC - if any of the pins
      aren't quite soldered they'll bend out of position and you'll see them.

      After that you've done it!!! Obviously, it's much easier than that with
      the larger surface mount ICs. But a bit of practise and you'll find that
      soldering small surface mounts in becomes alot easier. It's always a
      potch of a job if you don't have the purpose built machines, but as long
      as you're careful and you have pateince, you can normally manage ok.

      I hope that was helpful? Any questions give me a shout :)



      ----
      Brian Hoskins
      South Wales, UK

      Email: BrianJHoskins@...
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