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SMT question

Discussion in 'Electronic Design' started by o, Feb 1, 2004.

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  1. o

    o Guest

    hi, a smt assembly question ..

    assuming we are talking about a double sided boards , each side active
    chips with .5mm spacing and 0805 resistors etc.

    a local assembly house told me that they would make a solder stencil
    for each side of the board ( $250 each is that ok? ) ... so they print
    solder on the first side , p and p the components and run it on the IR
    reflow belt , then repeat for the other side

    my question #1 ... in the older days with bigger components i know
    they used glue dots on the first side to hold the components so they
    didn't fall off when they reflowed the second side ? do they still do
    this today or won't the back side of the board get hot enough for the
    solder to flow and the components to fall off ?

    the reason i wonder .. is i thought the fine pitch parts need to self
    center during reflow on side one and that wouldn't happen if they were
    glued down ..

    my question #2 ... i guess you can't batch oven relow 2 sided boards ?
    ( unless i guess everything is glued down .. is would seem tough to
    have glue dots small enough for 0805 )

    thanks for any insight ... o
  2. Mac

    Mac Guest

    I'm pretty sure they don't use glue. I think most small parts will stay on
    due to surface tension, even if the solder does reflow.
    That's my understanding, too.
    Don't know. If you can keep one side, at least, to small parts, I don't
    think they will fall off when they are upside down.

    I'm not that involved in fabrication, so I could be wrong.

  3. Leon Heller

    Leon Heller Guest

    Perhaps they cool the underside of the PCB with cold air. Fibreglass
    isn't a very good conductor, anyway.

  4. It is probably not a problem with IR reflow because there is a major
    difference in temperature between the top and bottom sides. With a
    toaster oven it takes more planning. In designing the board put all the
    large components (ICs, inductors etc) on one side and small descreets on
    the other. Small discreets will stay on by surface tension so you bake
    that side first.
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  5. Glenn Ashmore wrote...
    "pictures on this site are current as of February 2000."

    Glen, it's February 2004, how's your boat project coming?
    Also, what's Papa's hangout? :>)

    - Win

  6. Hopefully we set the engine next month and MAY be in the water in September.

    "Poppa's Hangout" is Key West

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  7. R.Legg

    R.Legg Guest

    You still have to ensure that any heavy components presented inverted
    in a reflow process are secured. Normally this means the heavy parts
    all have to be on the last side reflowed, to avoid the requirement.

    Special considerations are required if the finished product is
    intended to be installed using subsequent reflow processes.

    SMD parts are also used on the wave-solder side of an assy. These will
    require adhesive regardless of weight. Obviously this restricts the
    parts that can be used, as not all parts can reliably survive the
    wave, can wave without bridging or can support an adhesive spot within
    their pattern.

    One of the aims of a board designer is to minimize the steps required
    for completion, as each process carries a fixed cost. The intended
    assy process and methods are usually fleshed out before a
    manufacturable board design begins.
    If it doesn't work, you can't do it. The trick is to get everything
    located so that the process steps intended are physically practical.
    Fine pitch devices, or devices with leads on more than two package
    edges cannot be wave-soldered reliably, for a start. Whether thay can
    be reflowed after an adhesive application will depend on your
    materials, equipment and methods.

    Some adhesives can be applied to a part after reflow, if they are
    absolutely required. Final assemblies can be protected from end-use
    re-application of reflow environments, by coating, or by using higher
    temperature processes and materials in their original construction
    Reflowed parts can be inverted in a succeeding reflow process if their
    mass does not cause the parts to separate from the surface tension
    holding them in place, and if they are individually specified for
    repeated reflow cycles.

    You can get answers to some fab questions in the IPC technet forums;

  8. Roger Gt

    Roger Gt Guest

    X-No-Archive: yes

    designing the board put all the

    Excuse me, but in a "toaster Oven" don't you still
    solder one side at a time, by placing a plate
    (tray) under the work? So far I have only
    soldered top side SMT parts with the toaster oven,
    and have always placed the board on rails mounted
    on a tray to keep it from laying against the
    grill. (shelf) Wouldn't this also keep the lower
    side from heating as much, perhaps just warming it

  9. That is what I thought I said. I bake the back side with the small
    stuff first and then place and bake the top side. I use a piece of
    single side PCB board as a tray thinking it might slow the heat from the
    lower element. About the biggest package I have gotten to stick
    reliably upside down is a 1206 but I think I may be getting some over
    run on the temperature just watching that wax pellet melt.

    I am thinking about rigging up the Fuji PRX3 process controller from the
    epoxy post cure oven to do double duty as a toaster oven controler.

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  10. Glenn Ashmore wrote...
    Wow, that's good news! But you've been holding out on us
    with progress photos on your build-a-boat-project website.
    I wanna go too!

    - Win

  11. John Larkin

    John Larkin Guest

    Correct. Small parts (0805's, SOT-23's, stuff like that) won't fall
    off if the board is reflowed with them on the bottom.

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