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solder fatigue properties retry

Discussion in 'Electronic Design' started by Glen Walpert, Dec 7, 2013.

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  1. Glen Walpert

    Glen Walpert Guest

    There should now be a couple of pages from a SMT magazine article on ABSE.

    Posted in response to Phil's post in the voids in solder joint thread:

    "
    This reference:
    http://www.fujitsu-ten.com/business/technicaljournal/pdf/33note2.pdf

    suggests that a fractional strain of 1% gives a fatigue life of 1000
    cycles to failure with PbSn eutectic, and that the lifetime goes as
    1/(epsilon**2). Lead-free solder has about a quarter of the fatigue life
    of PbSn.
    "

    A single test showing separation of the solder from the pad is hardly
    justification for the conclusion that "Lead-free solder has about a
    quarter of the fatigue life of PbSn."

    Solder joint reliability is a complex phenomenon involving surface
    finish, processing parameters, solder creep and fatigue properties, etc.
    The only test that matters is a test of your particular processed
    assemblies, usually thermal cycling. The most challenging connection to
    make reliably is usually large BGAs, where the corner balls are most
    highly strained by thermal cycling. While I can't find them now, I have
    seen a few published test results comparing different lead free solders
    with tin-lead solders in large BGA thermal cycle to failure tests, and in
    these tests some lead-free solders underperformed tin-lead by about 20%,
    and some outperformed it by over 50% (not the cheap ones of course).

    High reliability soldering is a complex issue, and if you want to
    understand it you should follow a few of the manufacturing trade rags
    like SMT.

    While searching for the BGA test results I also happened to notice that
    the EMS division spun off by IBM has folded. Too much effort "proving"
    the unsuitability of lead-free and not enough time developing processes
    that work well, perhaps. All of the surviving large EMS companies have
    reported improvements in yield and reliability accompanying the
    conversion to lead-free, and virtually all exempt high-rel manufacturers
    are in the process of qualifying lead-free materials and processes, not
    because of any need to comply with ROHS but because these materials offer
    significantly improved reliability when selected and used in an optimal
    manner.

    Tin-lead is still easiest to use, and I would not suggest that small
    exempt manufacturers convert due to the high cost of qualifying new
    processes. But those who have done the work are achieving excellent
    results.
     
  2. Glen Walpert

    Glen Walpert Guest

    I don't doubt that a lot of great R&D was done by IBM, and that the real
    reasons that they are no longer the leader in electronic manufacturing
    that they once were has nothing to do with lead-free. Bad management
    caused by the replacement of engineers with bean counters is more likely;
    when the failure of a company appears to be due to something other than
    bad management, it was managements job to deal with those other issues.
    I did say "perhaps", and was really attempting to comment on the general
    attitude towards lead-free displayed by you and other regulars on this NG,
    who insist, against all evidence, that tin-lead is inherently more
    reliable than lead free. It was the poor fatigue performance of tin-lead
    solders that initially drove the development of lead free solders, not
    ROHS, and virtually every large EMS reports better performance under
    shock, vibration and thermal cycling with optimized lead-free processes
    than with equally optimized tin-lead manufacturing processes.

    While clay minerals do greatly slow the transport of heavy metals, and
    while the problem may well not have been severe enough to justify the
    money spent, lead has been found in the leachate of some landfills, some
    electronics does go into incinerators, and the claim that heavy metal
    poisoning is not a problem is contradicted by legitimate testing of lead
    in the blood of persons near lead processing facilities. And when has
    any real cost-benefit analysis been a part of any political process?

    Regards,
    Glen
     
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