(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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