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Gold-Plated PCB-mount Sockets?

Discussion in 'Electronic Design' started by [email protected], Mar 23, 2007.

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  1. Guest

    I've been reading about how two different types of metal, when in
    contact as an electrical connection, can cause bad things to happen.
    Specifically, for my current interests: gold and tin (or tin/lead)
    connector-contacts should not be mated. So, for example, it would be
    a bad idea to use DIP IC sockets that have gold plating, unless the IC
    pins are also gold. So far so good. I can (not) do that.

    I've also been reading that gold-plated pins should not be soldered
    with tin/lead solder, IIRC. Is that correct? I've think I've read
    that doing so will cause the solder joints to be more brittle and have
    higher resistance, and possibly get worse with time.

    But then what about all of those nice gold-plated Molex PCB headers
    and sockets? What should I use to solder them to a PCB? Or should I
    just switch back to tin?

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html
     
  2. John  Larkin

    John Larkin Guest


    We do solder or tin coated pins in gold sockets all the time. No
    problems in the last 20 years.
    Not a problem either. The gold on most pins is so thin, microinches,
    that it totally dissolves in the solder.

    John
     
  3. Guest

    Thanks, John.

    It's reassuring to hear the results of your experiences. From
    everything else I had read, mating tin and gold contacts sounded like
    a very bad idea. But I couldn't really tell HOW bad. Maybe it's not
    as much of a problem as I was beginning to think it was.

    I have just read some threads about mating tin and gold connector
    contacts, in the Google Groups archive, that imply that the "leaf-
    spring"-contact types of gold-plated IC sockets are much worse to use
    with tin or tin/lead IC pins than are the "turned" or machined types
    of gold-plated sockets, at least as far as galvanic corrosion of the
    tin- or solder-plated IC pins is concerned.

    There's also the thermocouple effect of the two dissimilar metals in
    contact, creating a small series voltage which might be a problem in
    high-precision circuits (but probably only something like a 1 uV
    offset).

    And, FWIW, I just found a short Kester article about soldering gold-
    plated metals, which mentions the sometimes-major effects of different
    thicknesses of the gold plating, which can be downloaded at:

    http://www.kester.com/en-us/technical/kb_detail.aspx?KnowledgeID=1

    I also read somewhere that using silver-bearing 62/36/2 solder helps
    to alleviate some of the problems associated with soldering to gold-
    plated metals, by preventing most of the leaching of the gold during
    soldering.

    All in all, if possible, it seems best to try to mate contacts of the
    same metal. But in the case of ICs and sockets, since almost all
    plastic DIP through-hole ICs have tin or solder-plated pins, is there
    any advantage in using sockets with gold-plated contacts, to make it
    worth facing the possibility of eventual degradation from galvanic
    corrosion (assuming the use of sockets, at all, is a given)?

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  4. Eeyore

    Eeyore Guest

    A classic error is to use the popular 0.1" pitch IDC connectors with gold plated
    pins and tin plated sockets. The trouble is that it's difficult to see there's a
    problem until it fails.

    What would you use instead of tin-lead or even lead-free solder ?

    Graham
     
  5. Eeyore

    Eeyore Guest

    It is
    It's bad !
    Oh it is but that has nothing to do with soldering gold plated leads.

    Graham
     
  6. krw

    krw Guest

    The main issue mixing sockets is the contact pressure. Gold is soft
    and requires (withstands) little pressure. Tin is harder and needs
    some contact pressure to penetrate the surface. A connector designed
    to mate with gold may not have enough pressure to mate[*] with a tin
    contact. A contact designed to mate with tin may scratch through
    gold.

    [*] though the machined gold contacts are quite good.
     
  7. John  Larkin

    John Larkin Guest

    We generally use tin/solder plated cheap, spring-type ic sockets, and
    the gold ones are the round socket machined types.

    I suppose in a very humid environment you could see some corrosion
    effects form mixing metals, but I haven't personally. For bad
    environments, we tend to avoid sockets and conformally cost.

    That can be corrected by proper layout, making sure all such
    connections are isothermal. See the pcb pic I recently posted to
    a.b.s.e, on the subject of slit ground planes. The lower-right region
    is thermocouple stuff.

    John
     
  8. Joerg

    Joerg Guest

    As had been evidenced by the old HP4191 here in the lab. One sunny day I
    couldn't calibrate it anymore. Turned out they had pushed fancy
    gold-plated mil-type PROMs into el-cheapo spring type sockets. Like
    Porsche Design leather seats in a Yugo. It did work for 20 years but
    then quit.

    Still, if it isn't absolutely necessary to use sockets, don't.
     
  9. Eeyore

    Eeyore Guest

    No, it's fretting corrosion.

    Graham
     
  10. Rich Grise

    Rich Grise Guest

    Not necessarily. With the cheapie leaf-spring contacts, that make contact
    surface-to-surface, as it were, for some reason the tin and gold gall -
    I think maybe some gold dissolves into the tin and they make some kind of
    alloy that separates into nonconductive flakes; the gold-plated machine
    screw sockets don't have this problem, because the inside of the pins is
    shaped kind of like a funnel - like inside-out of a barbed hook, which
    actually dig into the pin, scraping the gold of the socket and the tin off
    the lead, so you get at least four gas-tight contact points that are base
    metal to base metal, and the rest of the assembly is corrosion-resistant.
    He was worried about soldering gold-plated pins of gold-plated sockets,
    and that's never been a problem in most people's experience.

    Cheers!
    Rich
     
  11. Eeyore

    Eeyore Guest

    For an IC socket it may be fine but for connectors generally it most certainly
    isn't.

    Graham
     
  12. krw

    krw Guest

    Wrong. You can easily see it happening with gold/tin SIMMs.
     
  13. Eeyore

    Eeyore Guest

    The problem that happens with classic ribbon cable type conectors is entirely a
    fretting corrosion issue. It's caught loads of ppl out.

    Graham
     
  14. Guest

    Right. I was asking about two separate problems. One was about
    putting DIP ICs with tin- or solder-plated pins into sockets that had
    gold plating. The other was about soldering the gold-plated pins, of
    Molex gold-plated wire-to-board sockets, to PCBs.

    Thanks.

    - Tom Gootee
     
  15. Guest

    Thanks for the response, Graham.
    "I also read somewhere that using silver-bearing 62/36/2 solder helps
    to alleviate some of the problems associated with soldering to gold-
    plated metals, by preventing most of the leaching of the gold during
    soldering."

    - Tom Gootee
     
  16. Guest

    That is yet-another problem with it.

    Thanks.

    - Tom Gootee
     
  17. Guest

    Twenty years? Not too bad! Better than what I'd been led to believe
    from what I've read about the problem. (But probably not good-enough
    as an actual lifespan design goal, in my case.)
    That is, no doubt, the best solution.
     
  18. Guest

    Thanks, Rich.

    That agrees with almost everything else I've read about that problem.

    - Tom Gootee
    That's good to hear, again. The usenet archive posts I've read seemed
    to imply that it was a problem, because the gold that was dissolved
    into the solder would form intermetallics that were relatively
    brittle, and (IIRC) less conductive.
     
  19. Guest

    1. For almost all receptacle-to-pin-mating situations, both socket and
    pin surfaces should be made of the same metal. One exception seems to
    be that acceptable performance can be gotten when putting a tin or tin/
    lead DIP IC pin into a (round) machined or turned gold-plated socket
    (but NOT a gold-plated "flat contact" or "leaf-spring" type of DIP
    socket), although, if practical, no IC socket at all would be best.

    2. Soldering a gold-plated pin into a PCB, using tin/lead solder,
    "should" be OK. But using 62/36/2 tin/lead/silver solder should be
    better.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  20. Joerg

    Joerg Guest

    When I found out what the problem had been I was also surprised that it
    hung on for 20 years. But from such professional equipment I'd expect
    the same life span as people expect from aircraft. Something along the
    lines of 30+ years. IMHO this socket/PROM combination was a design flaw.
    Oh well, we all make mistakes, nobody is perfect.

    Doing that right now. But we have something HP didn't have back in those
    days: EEPROMs. It's amazing, I just spec'd in a 256k EEPROM for under
    $2. Nicely SPI bussed. Heck, it even has pretty good over-write
    protection where you can lock up a part of the address range and leave
    the rest open for writes.
     
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