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Goo on calculator PCBs

Discussion in 'Electronic Design' started by Ghazan Haider, Nov 20, 2004.

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  1. When I opened up a few calculators, and Commodore64 expansion
    cartridges a long time ago, some PCBs have wires that converge into
    what seems like a solidified drop of black plastic or ceramic. The
    drop looks like it was liquid once upon a time, and is too hard to
    remove, to check if there was an IC underneath or just silicon.
    Apparently the object (IC or just silicon die) was placed on the PCB
    connected to the wires, and the goo dropped on and solidified.

    What is this process? Why dont they do this anymore?

    And can home hobbyists do it?
  2. John Larkin

    John Larkin Guest

    Globtop. A bare IC die is epoxied to a PC board, wire-bonded to
    traces, and a blob of epoxy is dumped on top. Calculators and cheap
    toy-type things still do this a lot.

  3. Guy Macon

    Guy Macon Guest

    That's epoxy over a bare die wirebonded to the board. Same technology
    that's inside an IC, except the little wires go to the PWB instead of
    the IC lead frame.
    Who says we don't dom it anymore?
    I have a prototype-quantity wirebonder in my lab, but it was quite
    expensive, Far better to use ICs until you reach the point where
    you are prototyping something that you will make 100,000/day of.
  4. Tim Wescott

    Tim Wescott Guest

    -- and any home hobbiest with a wire-bonding machine and access to bare
    dice can do it (sorry, couldn't resist).
  5. It's called chip-on-board (COB) in the industry.

    The die is bonded to the gold-plated PCB and individual wires are
    bonded between the chip and the PCB where you would otherwise have
    pins soldered.
    They do; it's very common. Usually in low end consumer goods such as
    calculators, scales, LCD games, watches, exercise equipment computers,
    LCD modules, LED flashers etc. It requires gold plating on the board
    for the die and wire bonding (you also need gold for elastomer LCD
    connectors, so it's a natural fit for products that contain a micro or
    an ASIC and an LCD). It's typically used on a product where there are
    almost no components, so the board is thrown in the garbage if it
    doesn't work right off.

    The main advantage is cost, at the expense of yield and reliability.
    I don't think size is any smaller than QFN and similar packages.
    Some moderately expensive equipment is required. Other than that, it's
    entirely possible to do it in a relatively clean area in a basement,
    if you can get the parts and supplies. If a manual wire bonder is
    used, each wire has to be stitched from the pad on the die to a pad on
    the PCB (using a microscope and a manipulator), so doing 15,000 boards
    with 80 pins each might be a bit of a drag. It's much faster than it
    sounds, and there are (even more expensive) automated machines.

    Best regards,
    Spehro Pefhany
  6. john jardine

    john jardine Guest

    They're still using it.
    I've been looking for a bottle of that special Pitch for a long while. Would
    make life a lot easier :)
  7. Thanks everyone for the answers.

    It does raise another question. How much cheaper is a die compared to
    a package, and who sells dies anyway beside Intel (OK 2 questions.)

    Most MCU manufacturers only sell in standard packaging. Can I get the
    picmicro, the avr, xilinx spartan, AMD Geode and ARM MCUs in die
  8. Guy Macon

    Guy Macon Guest

    A lot cheaper when you are paying less than a dime per uP
    and you are buying 100,000 every day.
    Nobody at the low end buys Intel. Way too expensive. Asian vendors
    such as Winbond, Sunplus. EMC/Elan and Sonix own that market.
    Be prepared for minimum orders of 50,000 and up with masked ROM,
    long lead times, data sheets in Chinese, buggy DOS-based tools,
    chips that don't work as advertised, and no support.
    Really? News to me.
    Put in a standing order for 100,00 per day and any manufacturer
    will sell you bare die parts. Hobby quantities? Go pound sand.

    BTW, if you are in the market for 87 metric tons of
    Jar Jar Binks dolls, I can get you a *great* deal... :)
  9. Pig Bladder

    Pig Bladder Guest

  10. crzndog

    crzndog Guest

    This is called die-bonding (Wire bond-out also). The Goo is epoxy which
    cures after being poured onto the board.

    This is really cost effective when you are into production volumes of 50K+
    per month (ie 0.5 Million units per year). The die Bonding machines weld
    the wires onto the pads of the die and the PCB. The Goo (as you call it) is
    then poured over the die (which also has some silica gel on to remove any
    residual moisture) to stop the wires being deformed/shorted and to keep the
    die dry. Oxygen [air bourne or water bourne] will oxidize parts of the
    silicon and degrade it's perfomance/functioanlity/life.

    Now you know!

    If you are interested in doing this, please ask, I can put you in contact
    with people who do this for a living!
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