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That brown glue stuff

Discussion in 'Electronic Repair' started by N_Cook, Jul 13, 2008.

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  1. N_Cook

    N_Cook Guest

    Used for fixing otherwise floppy bits to boards.
    Is it hygroscopic ? whatever the word is, but pulls dampness out of the air.
    Beocenter 4000 from 1985 with the LC7815 (not V reg) electronic 2 pole 4 way
    switch, internally locked into Aux only input, so disabling everything else.
    0.7 inch pitch 28 pin DIP but now removed and replacement available.
    Affected input was on one side of the IC and some of this shitty brown stuff
    on the other. But now removed , the 2 glued pins are almost corroded through
    but not obvious before removing the IC, and have presumably conducted
    moisture, capillary fashion, into the IC.
    So will remove that glue and another dollop near another IC. Will hot-melt
    glue go the same way after a qurter of a centuary.
    Is it bulking filler in these glues that cause the problems ? I have used
    talcum powder to bulk out on occassions but could that be problematic in
    future , so a better choice?
  2. ian field

    ian field Guest

    The brown glue that usually causes the problems you describe normally
    becomes very brittle and chips off easily.

    If something really does need tacking down I'd use a very small spot of
    superglue or a blob of 2-part epoxy, for larger components that need support
    but need to flex with thermal expansion silicone RTV is a good choice,
    instant gasket is sometimes cheaper from auto accessories stores, or if you
    use a lot of it a tube of RTV for a "mastic gun" from a builders yard
    usually works out cheapest.
  3. junebug1701

    junebug1701 Guest

    Nigel, from my experience this brown glue eventually turns dark brown,
    and when it does it becomes conductive. I have found it difficult to
    remove. A plastic serrated knife like you might find in a take-away
    food box works well and is soft enough not to damage the traces on the
    board. I really hate that glue.

  4. N_Cook

    N_Cook Guest

    On first sighting of this latest case, I tested the glue and it was more
    than 30 Megs , just retested a bit and over 500 megs.

    I suspect the plain gummy petroleum glue is ok over time, but it is a
    problem with a filler/bulking agent used. I agree it goes brown with age,
    perhaps it is something like corn flour or vegetative starch as filler. I
    doubt a mineral like talcum or French chalk, as a filler, would go this very
    dark brown or be a problem. The recent failure was due to corrossion
    localised to where this glue had glooped over a few pins. Those pins looked
    ok but after removal I could see corrosion and one of the pins, without the
    pcb support, has almost sheared off due to the underlying corrosion.
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