Not specifically  but from my former life with a solder paste manufacturer, what the abstract states makes perfect sense to me. 

Unwanted Fines &  fine type powders have more oxide of course,  hence the flux spends most of its strength  cleaning the oxide on the powder rather than the surface of the substrate to soldered.  I suspect this shell is a combination of semi-neutered flux (~almost type R) and /or a "flux -to - oxide"  byproduct (spent flux).  If you get enough of this "goo" perhaps it has different rheological and/or flow properties an develops this shell. 



Rich  Kraszewski 

Senior Staff Process Engineer



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-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Nigel Burtt

Sent: Monday, January 7, 2019 9:37 AM

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Subject: [TN] "Flux Shell" phemomenon on solder joint



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.

Came across this terminology in a 2017 paper.



https://ieeexplore.ieee.org/document/8046760



" The constant residue level at small sample size was attributed to the surface adsorption phenomenon, or “Flux Shell” phenomenon, where a layer of flux was strongly adsorbed on the solder powder surface"



This isn't a phenomenon I have seen referred to elsewhere, does anyone have a description of the formation of this shell and its impact?