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BGA fix needed

Discussion in 'Electronic Design' started by Boris Mohar, Dec 31, 2003.

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  1. Boris Mohar

    Boris Mohar Guest

    One of my customers is in a bit of a pickle. They need to access a ball on
    a BGA chip that is already mounted on a four layer board. The ball is in
    one of the inner rows. The two inner layers are power and ground. The ball
    pitch is 0.8mm and the pad diameter is 0.42mm. This is a prototype fix.
    Does anybody have a 0.3mm ceramic coated metal core self taping screw?


    Boris Mohar
  2. John Larkin

    John Larkin Guest

    Why not just go in there with a tiny end mill and bore through from
    the bottom to the top of the board (and pray you don't hit any signal
    traces)? Then solder a wire to the ball or go in with a tiny pogo.

    On all of our BGA layouts (total of *one* so far) we routed all unused
    balls to a via.

  3. Joe Legris

    Joe Legris Guest

    Drill a large hole (maybe several mm diameter) in a trace-free and
    ball-free area just inside of your target ball. Connect to it more or
    less horizontally using a ball of conductive epoxy on the end of a
    strand of wire. Use a bit of tape to hold it until the epoxy hardens.

    Practice first on an old PCB.
  4. I read in that Boris Mohar <>
    Probably not, but you can fairly easily make a 0.3 mm dia glass tube by
    primitive glass-working techniques and epoxy a stiff wire into it. It
    MIGHT work.
  5. Mac

    Mac Guest

    "Primitive glass-working techniques." Heh. That reminds me. I once
    experimented with heating up disposable glass pipettes with a bunsen
    burner. If you heat up a small part in the middle, you can then stretch
    the pipette and the inner diameter will shrink as the hot section
    stretches, until eventually the tube will close out. I think you are right
    that it wouldn't be too hard, given a glass tube and a torch or bunsen

  6. Bob Stephens

    Bob Stephens Guest

    I met a guy in a bar once who worked for a BioMedical outfit that made
    capillary tubes for blood filters. They took plastic straws about the size
    of a "swizzle stick" - remember, we were in a bar - and stretched them out
    until the inside diameter was the size of a blood molecule, and placed
    bundles of these things inside a patient's blood vessels to filter out
    impurities. They blew air throught the tube while heating and stretching it
    so it wouldn't close out.

  7. It gets mighty small before it closes out, if you don't heat the tube
    too much and stretch quickly.
    A fishtail burner is the preferred type, but I wonder if you can still
    get them. They give a flat flame which allows you to heat either a small
    section or a larger one, and the flame is not as hot as a Bunsen. AIUI,
    there is a limited air bleed into the base of the flame, but nowhere
    near as much air as in a Bunsen on 'kill'. Before the gas mantle was
    invented, they were used for illumination.

    Glass working is an interesting pastime for some kids, provided they
    understand what 'hot' means! I suppose these days you need safety
    goggles and Kevlar gloves, but it can still be fun. It's possible to
    progress either to usable chemical apparatus (a 'tulip' reflux condenser
    is not too difficult when some manual dexterity has been acquired) or to
    art forms, such as glass animals and jewellery.
  8. You might find it easier to make a fix if you remove the BGA first. Then
    have it re-balled and re-installed after the board is reworked.

  9. Ya, that sounds more like it! Or.... decap the part and maybe probe it?
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