Well, theoretically!!!! (based on the equation) it might be possible to bring down the melting temperature of Cu to 200 C, but it wouldn't be nanoparticles, but rather kind of clusters of few atoms.
I used to work with 40-60nm Ni particles and the melting temperatures wasn't that drastically lowered.
Regards,
Vladimir
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (647) 495-8727
Cell: (416) 899-1882
www.sentec.ca
-----Original Message-----
From: Bob Landman <[log in to unmask]>
Sender: TechNet <[log in to unmask]>
Date: Wed, 10 Apr 2013 12:19:55
To: <[log in to unmask]>
Reply-To: Bob Landman <[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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