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solder fatigue properties retry

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:

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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.

Reply to
Glen Walpert
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I used to work in the packaging research department of IBM Watson, and I know the inventors of both the plated and injection moulded solder processes. (We filed a few patents together on the creative use of solder for optical alignment--see

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.)

Your characterization of IBM's effort couldn't be further off base--a lot of very smart people spent over a decade getting lead-free right, well ahead of the rest of the industry. A huge waste of time, effort, and talent, due to idiotic bureaucrats solving a non-problem. (Heavy metals don't go anywhere in landfills, due to extremely strong ion exchange with clay minerals.)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

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

Reply to
Glen Walpert

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