Hang about, the original piece described an idea, saying it would need
developed and that a product is years off. So whereas it would be legitimate
to draw attention to some needs and practical requirements, I think it’s a
bit premature to have a poke at the guy for not having a developed product.
The piece specifically cited that bond strength is an area which needs
attention. Your speculations on bonding mechanisms are clearly just that. So
far as process temperatures and conditions are concerned, these are yet to
be determined, 200C may require very small particles but consider it is
entirely possible that the material is not pure copper.
In addition the product may well be a process which reacts to dump out
copper particles rather than a suspension of them. That sort of approach can
give low process temps and relatively weak bonds, and would resolve the
oxidation process.
In a few years time we may hear of a super new bonding product. I am not
putting a reminder in my diary though, if it works we will hear all about
it, and if it doesn't there will be plenty of other stuff to think about.
Meanwhile I don't think I have anything more to say.
Best Wishes
Mike-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Thursday, April 11, 2013 7:39 PM
To: [log in to unmask]
Subject: [TN] FW: [TN] copper nanosolder
(posted for Gordon Davy)
Dr. Zinn has responded promptly to one of my comments on the abstract of
the talk he plans to present. While acknowledging that a small particle's
melting point is less than that of the bulk, I said that the nearly
900-degree difference between bulk copper and his copper nanoparticles is
"a stretch." He chided me for not checking my facts before making "such
'bold' statements." Had I known of the work that has been going on in his
field, I would have alluded to it, but the facts as I now understand them
do not alter the substance of what I posted, much of which he ignored.
For interested TechNet subscribers, here is my (readily accessible)
allusion: the Wikipedia article on melting point depression. It expresses
the ratio of melting temperatures (nanoparticle to bulk) by an equation of
the form TMn/TMB = 1 - C/d, where C is a constant and d is the
nanoparticle diameter. In fact, this equation predicts a zero K melting
temperature – for a non-zero diameter. While that might seem startling at
first, it correlates with the common-sense notion that a metal atom bonded
to no other metal atoms is in effect melted, regardless of its temperature.
The graph in the article also indicates that if metal nanoparticles could
be made small enough, they would melt at, or even well below, room
temperature.
Wikipedia also reveals that colloidal gold and silver were known in
antiquity. Gold doesn't form an oxide, and silver oxide is not strong.
Copper of course does oxidize, and coalesce, which makes Dr. Zinn's work
significant.
As a digression, as the number of atoms in a nanoparticle diminishes, the
distinction between a solid and a liquid (and therefore the concept of
melting) gets pretty murky. Above 0 K, a metal's crystal lattice at
equilibrium has a certain number of vacancies. From time to time, a vacancy
and a metal atom trade places – they jump. That's the mechanism for
solid-state bulk diffusion. Grain-boundary and surface atoms jump more
readily – the mechanisms for solid-state grain-boundary and surface
diffusion. Metal atoms in a liquid do not maintain a fixed arrangement. How
frequently do the atoms in a cluster (a teeny-tiny nanoparticle) have to
jump for it to be counted as a liquid? For example, bulk copper has a
face-centered cubic crystal structure. But one cube, with 14 atoms (8
corner and 6 face), has not even one bulk atom. How likely are the atoms in
a 14-atom cluster of copper to be arranged in a face-centered cube, or,
above 0 K, to stay that way?
Dr. Zinn, you discovered how to make nanoparticles of copper small enough
to melt at 200°C (a 65% MP depression). Based on the Wikipedia graph, their
diameter must be ~ 5 nm. Since an atomic diameter is ~ 0.3 nm, I estimate
they contain only ~ 5000 atoms. I, and I think many others, would be
interested to have you explain briefly on TechNet how you determined the
diameter distribution and corresponding melting temperature range.
However, even given that they melt at 200°C, your mechanism seems to
require most of these nanoparticles to:
a. Melt simultaneously (a challenge, even assuming they all had
identical diameters),
b. Wet (react with) the atoms on adjacent surfaces (i.e., board land and
component termination), and
c. Form a bridge (of millimeter dimensions) between them,
before most of the coalescing (and growing) melted copper globules freeze.
I propose it be dubbed the
attachment-by-copper-nanoparticle-melting-and-wetting-before-freezing
(ABCNMAWBF) mechanism, which is how soldering works, with S instead of CN.
I find the ABCNMAWBF mechanism to be a much bigger stretch than
non-coalescing copper nanoparticles small enough to melt at 200°C. For this
reason, until I have more facts relating specifically to your R&D, I still
favor the notion that the connections your paste makes are due to copper
nanoparticles reacting with tin (ABCNRWT) in the termination finish.
What facts am I looking for from you? Nothing proprietary. You can resolve
this question by addressing at least one of my prior requests for:
1. Differential thermal analysis of your paste (plots), or
2. Connection microstructure (photomicrographs), or
3. Whether you have evaluated the connection to non-tin finishes such as
immersion silver, and more importantly, NiPdAu.
Assuming you can get specimens, number 3 seems simplest, and the most
convincing. If, as I suspect, your paste does not make connections to
non-tin finishes, then this would support my ABCNRWT hypothesis.
Dr. Zinn, as a person trained as a scientist, I can overlook your disdain
for my not getting all the facts related to your field that you apparently
expected me to get before I publicly commented on your abstract. (I also
admit – in advance – to not checking to see if colloidal Cu reacts with Ag
or Pd at 200°C, or even if Cu reacts with Ag or Pd at its bulk MP.)
But as a person who had a 34-year career in engineering, I can assure you
that the issue you took issue with is not the most important one. Even if
your paste works by melting (and does make connections to non-tin
finishes), that is still a long way from getting the engineers whom you
hope to interest in your process to regard it as a "drop-in replacement"
for conventional solder paste.
Most of the subscribers to TechNet are not scientists. They don't get paid
to go looking for facts in the kind of publications you cited, and frankly,
many don't care whether or not the copper in your paste melts at 200°C.
Instead, they expect you to provide the facts – at least rudimentary
answers, with evidence, to all of the engineering-related issues I raised:
1. Process The width of the process window – times and temperature range,
for surfaces with marginal solderability (or wettability), including Ag and
Pd.
2. Process and reliability The benefits of reflowing your paste at 200°C
instead of reflowing, at a higher temperature, the paste they are now
using.
3. Quality assurance How well the connection's strength can be judged by
appearance.
4. Reliability How well the (rigid) connections made with this copper
survive thermal cycling (plots of cycles to failure).
Getting satisfactory answers for SAC solder took over a decade – you missed
a lot of excitement during your nine years in Alcatraz.
Finally, here's a fact for you, and two questions. Rework is an unavoidable
part of manufacturing the kinds of electronic products for which most
engineers who subscribe to TechNet are responsible. Regardless of
mechanism, the connections your paste makes have a very high melting
temperature, so:
1. Can they be reworked?
2. Can they even be repaired?
Gordon Davy
Peoria, AZ
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