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Does it have to be gold?

Discussion in 'Electronic Design' started by Frank Bemelman, Oct 2, 2003.

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  1. LCD/PCB with zebra strips always have gold plated pads on the PCB,
    at least that's what I've always seen.

    I was thinking if it were okay to use simple tinned pads. Looking
    for some guidelines, I found this:

    http://www.fujipoly.com/design/genDesignGuidelineArticle.asp?ContentItemID=dga_1007

    They say:
    c.) Contact pad material on the PC board, as well as the LCD, should be
    smooth
    and regular with thickness tolerances established.

    Is that the key issue, a smooth surface? They don't specifically mention
    gold plated.

    Anyone tried tinned pads before?
     
  2. Ban

    Ban Guest

    Frank,
    the usual tinning process cannot guarantee a smooth surface. I have been
    making boards for PGA-casings and these require a gold finish, the tin is
    too uneven.

    ciao Ban
     
  3. Mario Trams

    Mario Trams Guest

    I don't know whether you once disassembled and reassembled things like
    digital watches, calculators, digital thermometers etc.

    My experience when I did this, is that these things are quite
    difficult to get working back again without any problems.
    Obviously (and as Ban said) you need a very good finish for
    proper contect between PCB and this "magic" rubber. And
    even in this case everything has to be perfectly mated.

    Regards,
    Mario
     
  4. Carbon coating or nickel is also used.

    Here's what one manufacturer suggests (but that gold thickness sounds
    pretty generous to me)

    ---
    PCB Coatings
    The best material to use for the contact area is gold.
    Alternatives are carbon, tin-lead or nickel.
    Gold: 50 millionths over 50 millionths nickel
    Nickel: 100 millionths
    Carbon: 0.001 carbon over copper
     
  5. Thanks, luckily they don't advise *against* tinned pads. My favorite
    prototype service doesn't provide gold plated contacts. I figure
    I can get away with tinned pads, perhaps give it a swipe first,
    with that eh, solder-sucking-litze.

    I was also thinking about gold plating them myself, with some
    cheap $69 kit, but I would have to remove the soldermask, which
    isn't very easy to do...

    It's all for an ambitious hobby project, for which I want to
    use low cost graphic lcd as used in Nokia phones. These are
    easy to get and don't cost much. See Ebay #3352101054. There's
    a little zebra connector on the back, 9 pads of reasonable
    size. The control is easy, not much more than +v and serial
    clock and data.
     

  6. I just opened up my $49 cordless phone, the big caller-id LCD display of
    which has lost most of its segments,
    slowly, during the two years we've had it, making it hard to tell who's
    calling.

    I peeled off the carbon zebra flexy pc board, and noticed the pads on the PC
    side were tin plated.
    The glue used was really strong, but apparently not good enough to sustain
    contact for several years.

    Soo if you believe this data point, tin plating will work, for about a year,
    then gradually decay,
    leading to much cussing.


    Regards,


    George
     
  7. They don't use any glue. The stickiness was just from the rubber.

    Best regards,
    Spehro Pefhany
     
  8. Tilmann Reh

    Tilmann Reh Guest

    They do. It's a hot-melt technology, and I also made experiences
    that this is not too reliable...

    --
    Dipl.-Ing. Tilmann Reh
    Autometer GmbH Siegen - Elektronik nach Maß.
    http://www.autometer.de

    ==================================================================
    In a world without walls and fences, who needs Windows and Gates ?
    (Sun Microsystems)
     
  9. Tilmann Reh

    Tilmann Reh Guest

    They do, at least some of them.
    I also checked this detail some time ago, and talked to many
    LCD module makers. Most of them didn't know anything more
    detailed ("we get them completely assembled from taiwan,
    and we don't know why they always use gold").
    One of them, however, told me that the tinned finish is
    subject to creeping surface corrosion from the open sides,
    which produces a long-term reliability problem.

    So, the known-good materials are only gold, carbon, and nickel,
    as mentioned before.

    --
    Dipl.-Ing. Tilmann Reh
    Autometer GmbH Siegen - Elektronik nach Maß.
    http://www.autometer.de

    ==================================================================
    In a world without walls and fences, who needs Windows and Gates ?
    (Sun Microsystems)
     
  10. Phil Hobbs

    Phil Hobbs Guest

    Right. Elastomeric connectors don't produce gas-tight connections, the
    way that self-wiping metal connectors or wire wraps do. That means that
    base-metal contacts will corrode eventually. For prototype use, tin
    ought to be fine.

    It will also depend on which style of Zebra connectors you're using--the
    ones with the metal wires will probably work better with tin than the
    silver- or carbon-loaded ones.

    Cheers,

    Phil Hobbs
     
  11. Okay, you mean the heat-sealed flexible connectors. That's not called
    "zebra".


    Best regards,
    Spehro Pefhany
     
  12. Tilmann Reh

    Tilmann Reh Guest

    Right - maybe I misinterpreted his sentence, maybe you wrongly focused
    on "zebra"...

    From his "carbon zebra flexy pc board", I assumed he was reffering
    to a flexible foil with zebra-like carbon traces, and that fits the
    hot-seal technology.

    --
    Dipl.-Ing. Tilmann Reh
    Autometer GmbH Siegen - Elektronik nach Maß.
    http://www.autometer.de

    ==================================================================
    In a world without walls and fences, who needs Windows and Gates ?
    (Sun Microsystems)
     
  13. SioL

    SioL Guest

    I wonder if carbon would work for PCI bus connector on the PCB?
    Anyone have experience with that? Gold costs too much in small prod. runs.

    Siol
     
  14. What do/did you pay for the gold plating? What quantities?
     
  15. SioL

    SioL Guest

    100 - 200 boards.

    A quote I got about 6 months ago was about a double (total for a PCB)
    for gold plated edge connector.

    Siol
     
  16. Double??? Good grief... I'll think I'll order that DIY gold plating
    kit and start a business ;)
     
  17. Zak

    Zak Guest

    I've seen phone displays with a flex strip connection as well, but I
    have no idea about the control of those. I think the flex can be soldered.


    Thomas
     
  18. SioL

    SioL Guest

    Exactly, ridiculous prices.

    In fact that kit might be a cheap way to go ;) I wonder how effective
    that KIT thing really is?

    Siol
     
  19. Semih Hazar

    Semih Hazar Guest

    It's all for an ambitious hobby project, for which I want to
    there are LCD modules with metal contacts which are easier to deal with.
    (exactly same display but different contact type)

    See www.jelu.se / shop / misc parts / PART-NOKIALCD
    and also http://www.myplace.nu/mp3/nokialcd.htm


    Hope that helps,

    Semih
     
  20. Thanks, I've seen them. These are removed from original
    nokia 3310 replacements, have a contact block of goldplated
    contacts. Originally it all sits in a plastic frame with
    a keypad. The contact block is a loose part, and has to
    be glued on. Then you need to make a new bezel or frame to
    mount it. Doable, but not very practical. The 5110 just
    snaps on, provided you have a PCB with the proper pads
    and slots.
     
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