Re: Soldering SMT Components

Glen Walpert wrote:
On Wed, 23 Apr 2008 21:05:04 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxx> wrote:

<clip>
Lead is in fact not a very good means of preventing tin whiskers, due
to well know problem of poor fatigue strength of tin-lead solders. All high-rel applications which need to withstand shock and vibration
will be converting to lead-free; not the lowest cost SAC alloy but
high performance lead-free alloys which are now being qualified (a
lengthy process) but which clearly blow the socks off tin-lead in
strength, ductility, shock resistance and fatigue life; all of the
properties important for reliability except processing temperature are
greatly improved and the processing temp for these alloys is only
slightly higher than tin-lead. See for instance:
<http://smt.pennnet.com/display_article/323445/35/ARTCL/none/none/1/STEP-3:-Holistic-Lead-free-Reliability/>

The down sides to lead-free are slightly higher cost and far higher
complexity of specifying the optimum alloy, flux, and processing
parameters, plus a need for tighter process controls.

This is not a reasonable forum for learning about soldering or PCB
assembly in general, the issue is too complex and almost no one here
is interested in it enough to have read up on the subject. Read the
trade rags like SMT <http://smt.pennnet.com/home.cfm> and Printed
Circuit Design and Fab <http://www.pcdandf.com/cms/> for a decent
introduction to the subject.
There is way too much serious studies and reports that contradict you.
Name one. Post a link, and I will explain what is wrong with it or
why it does not apply to current materials and processes.

No one who is serious about high reliability is failing to investigate
the advantages of lead-free. It is now clear that tin-lead has no
future in high-rel PCB assembly; its fatigue and shock resistance
properties are not even in the ball park of the latest high
performance lead-free alloys. Read the literature.
I currently work in the packaging research department at IBM Watson Lab. I'm an electro-optics guy surrounded by people who worry about solder and thermal interfaces and 3000g shock tests and other such stuff that you have to get right if your systems are really going to meet a five-nines availability specification. (That's 30 seconds downtime per year, maximum, with a real guarantee that costs lots of real money if it isn't met.)

The sheer volume of labour, of people as smart as any you're likely to meet, that has been poured down the lead-free rathole due to that *meshugginah* European directive would boggle your mind. After years of make-work, my colleagues managed to come up with a solder system that would be that reliable, in the environments our stuff has to work in--_gold/tin_. Talk about reverse alchemy...having to use gold even when lead is better. Turning silk purses into sows' ears makes much more sense.

Lead doesn't go anywhere in landfills, and heavy metals move incredibly slowly in ground water--witness the Oklo natural reactor plume, where the heavy metal fission products moved less than a mile in well over a billion years. Getting lead out of paint and gasoline made abundant sense. Getting it out of plumbing solder made marginal sense, if your tap water is acidic enough and has little or no lime in it. Getting it out of electronics and CRT tubes is just plain ideology.

In a hundred years, we'll be mining landfills--they're high grade deposits of just about everything needed for a technological civilization.

No one else seems to need gold in their lead-free solders to obtain
high reliability, there must be some very unusual situation to require
that, or someone not well informed about other alloys on the market.
Being very smart is not a subsitute for being very well informed. Look
at figure 1 in the following reference, considering that the energy to
fracture is the area under the curve. Then compare the fatigue
properties. It is clear that tin-lead would eventually be replaced by
lead-free alloys for high reliability applications even without ROHS,
because the properties important for reliability are significantly
better.

<http://smt.pennnet.com/display_article/323445/35/ARTCL/none/none/1/STEP-3:-Holistic-Lead-free-Reliability/>
The notion that lead stays put in landfills is simply false, lead is
measurable in the ground water leachate from many landfills. The Oklo
natural reactor plume is not a good model of a landfill. Whether the
amount of lead leached from landfills is worth the cost of removing it
is another matter, I have never seen an economic analysis supporting
either side of that argument, but the cost of converting to lead-free
has been small compared to other ways we squander our money uselessly.

Most who post on the subject here complain about non-issues like tin
whiskers, which are associated only with tin electroplating and have
absolutely nothing to do with lead-free solder (no tin whisker has
ever been observed growing from lead-free solder), or nonsense about
brittle bond layers with ENIG (the gold and nickel are completely
dissolved in the solder and a solid copper to solder bond is formed
with any current lead-free process), or complaining about the slightly
inferior shock resistance of the lowest cost SAC alloy when SAC is
simply not a viable option for anything but high volume consumer grade
electronics. These complaints are pure nonsense, the reliability of
lead free electronics today exceeds the reliability of the old
tin-lead process by a significant margin according to every published
report I have seen from those who measure the reliability of their
products.

You talk a fair bit about all the references you've read, but only cite a trade journal. You also airily dismiss as 'nonsense' the work of a lot of very smart people, with lots and lots of failure analyses of cracked intermetallic layers. This is entirely unpersuasive. The claim that 'no tin whisker has ever been observed growing from lead-free solder' takes you right to the edge of the troll category, and maybe a bit over.

The main issues with lead-free reliability are:

(1) brittle intermetallic compounds: lead-free solders eat most pad metallurgies.

(2) Higher reflow temperatures, leading to much more stress due to thermal mismatch, especially with the simultaneous move to organic chip carriers. (Can you say, "field failures"?)

(3) Higher modulus and lower plasticity in the solder joints, leading to
less stress relaxation, aggravating (1) and (2).

(4) Need for different solidus temperatures to allow multilevel packaging, e.g. microbumps from Si to Si carrier, then BGA from Si carrier to module.

(5) Both plating and injection moulding solder are much harder with Pb-free. If you're only using paste, it isn't such an issue.

Tin whiskers are common in softer lead-free solders, which are otherwise helpful in reducing (1)-(5).

Our applications require, e.g. 5000-pin backplane connectors carrying 5 Gb/s differential signals (Ventura J1), and similar numbers of microjoins.

Most of these problems become quadratically worse with pin count, so some folks haven't run into them yet, but they will.

Cheers,

Phil Hobbs
.

Relevant Pages

  • Re: Soldering SMT Components
    ... to well know problem of poor fatigue strength of tin-lead solders. ... high performance lead-free alloys which are now being qualified (a ... No one who is serious about high reliability is failing to investigate ... solder and thermal interfaces and 3000g shock tests and other such stuff ...
    (sci.electronics.design)
  • Re: Soldering SMT Components
    ... Tin whiskers grow from tin plating and only tin ... And it's OK to use rosin-flux leaded solder, ... lead-free solders. ... is simply not a valid reliability issue. ...
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  • Re: lead free solder with voc free water base
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