Hi,
How does "Welding" fit into these "Sintering / Melting" processes? I recall
that hot plastic iron can be welded by hitting it together with a hammer.
And there are friction welds and cold welds.
I guess it is all in the resulting grain structures? When I took a short
welding class the motto was "A Weld is Stronger than the Base Items". Not
sure that is always try but that was how my "Welding" was tested! I recall
I at least passed the class. That was stick welding with a Lincoln welder in
"Farm Shop".
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Wednesday, April 10, 2013 12:20 PM
To: [log in to unmask]
Subject: Re: [TN] copper nanosolder
(posting on behalf of Gordon Davy)
Bob, Denny, or Mike, since you're subscribed to TechNet, feel free to post
this reaction to Zinn's abstract (and bio). Perhaps if he sees it, he'll be
able to respond before his June 12 presentation.
Gordon Davy
Peoria, AZ
I don't want to deprecate Zinn's work. Learning how to make copper
nanoparticles and keep them from oxidizing or agglomerating is difficult.
But I am concerned with his claim that reducing the copper particle size
reduces the melting temperature to 200°C. It is true that surface atoms are
not as tightly bonded to their neighbors as bulk atoms, so reducing a
substance's particle size, by increasing the surface-to-volume ratio, does
reduce the melting temperature somewhat below the bulk value. That is the
basis of sintering, which occurs below the bulk melting temperature.
Presumably, if the particles were small enough, no atom would have the full
number of nearest neighbors – it would be all "surface." But the melting
temperature for bulk copper is just short of 1100°C. That's a stretch!
The particles of copper in powders sold for sintering are mostly smaller
than 44 µm (-325 mesh). The process requires compacting the powder (plus
lubricant) at 4,000 to 8,000 atmospheres pressure, then and heating at
750-900°C for 5-7 minutes (see copper-powders.com).
Dr. Zinn's copper is in the form of "nano-particles," so they are
presumably smaller than 1 µm. One can contemplate whether that roughly 1½
to 2 orders of magnitude size reduction is sufficient to account for the
differences between the above conditions necessary for sintering and for
conventional reflow soldering. A differential thermal analysis curve would
support the claim that, with or without compaction, his nano-copper
"solder" melts at 200°C.
Consider this analysis taken from everyday observation. When snowflakes,
which may have nano-scale features, fall and land, they sometimes sinter,
and sometimes (when temperatures remain below about -15°C) they do not.
People refer to that latter kind, once it has landed, as "powder" snow.
Similarly, one cannot skate on very cold ice because the surface lacks the
"liquid-like" layer to lubricate the blade. (Pressure melting is a minor
factor.)
So if dropping the temperature of ice by fifteen degrees below its melting
temperature prevents sintering and skating, is it likely that copper
sintering will occur at a temperature nearly nine hundred degrees below its
melting temperature? As for melting, even if individual particles were to
melt at such a low temperature and join to form a liquid, what would
prevent the liquid, now with dimensions measuring from micrometers to
millimeters, from instantly freezing?
More likely, the copper particles dissolve into the tin or tin-lead plating
on the board lands and component terminations (to form bronze, if the
reaction goes to completion). Even if the nanoparticles are not melting or
sintering, it's a clever idea, but we need to know:
* How well these bronze connections, presumably far stiffer than those of
a tin-based solder, survive temperature cycling. * The microstructure,
so we can understand the attachment mechanism. * The width of the process
window. * Whether the adequacy of the attachment can be judged by its
appearance, and if so, by what criteria. * Whether using Zinn's solder
at 200°C gives benefits large enough to warrant replacing Pb-free reflow
soldering (peak local temperature up to 260°C by convection, lower by
condensation). (For those still using SnPb solder, the conditions don't
seem different enough to warrant consideration.) * Whether Zinn has tried
his "solder" with non-tin land finishes such as immersion silver, ENIG,
and ENEPIG, and NiPdAu termination finish. Since I suspect that the
colloidal copper does not melt or sinter at 200°C, I suspect that
it is not going to perform with Ag or Pd nearly as nicely as with
near-molten Sn, and its reaction with Ni would be even worse.
* If it doesn't make reliable bonds with all the finishes likely to
be present on an assembly, then it is not a "drop-in replacement"
for solder. (Yes, the designer can specify a compatible finish for the lands
of the board he designs, but not the termination finish of the
components he chooses. He can specify, say, immersion tin instead of
ENIG, but must accept NiPdAu on some components.) * Maybe Zinn can add
enough nanoparticle Sn to the formula to provide the necessary wetting
to Pd without increasing the reflow temperature and without introducing
a risk of whiskers from the solder itself. * The risk of short
circuits due to whiskers growing SAC solder appears to be low, from
SnPb solder even lower, and with Zinn's solder it may be zero. But
regardless of the solder used, the overall risk for an assembly remains
high due to portions of Pb-free Sn termination finish that don't get
solder-coated.
Here are two other ways of dealing with the risk of tin whiskers:
1. Mitigation – speculative For assemblies for which Pb is permitted, if
components with a Pb-free Sn tin finish were dipped in SnPb solder paste
(or perhaps a paste of colloidal Sn and Pb) and heated (before or during
assembly soldering), all of the original finish might be covered with a
layer of SnPb, and the assembly would then have a low risk of short
circuits due to tin whiskers. The paste also might bridge.
2. Prevention – reduced to practice After attaching components by
conventional SnPb or SAC soldering, a thin layer of a whisker-impenetrable
metal such as nickel can be applied to the solder and the remaining
(uncoated) Pb-free Sn finish by electroless deposition. The process
requires immersing the entire assembly in the bath for a minute or two. But
because electroless deposition occurs only on conducting surfaces,
insulating surfaces remain uncoated, and the performance of the assembly is
unaffected. (Full disclosure: Bob, Denny, and I are the named inventors on
a patent application for this process – see www.ldfcoatings.com.)
A few additional observations:
* Note the irony, given the meaning of Zinn in his mother tongue, of
him developing a tin-free solder. * Note the nine-year gap (between
1995 and 2004) in his bio. * The claim of "10-15x electrical and
thermal conductivity improvements" is irrelevant: what counts is overall
conductance, and the reduction in a connection's conductance due to use
of conventional solder (compared to copper) is insignificant. * A
substance's melting temperature is a thermodynamic property. Some might
regard the reduction in the melting temperature of colloidal copper as
"significant" or even "dramatic." But it is not "rapid" (a term that
implies a kinetic effect).
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