Microscopic Structure of Metal Whiskers

Abstract

We present TEM images of the interior of metal whiskers (MW) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as why MWs' diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what is the nature of MW diameter stochasticity, and what is the origin of the well-known striation structure of MW side surfaces. In an attempt to address such questions we performed an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.PACS numbers:

The statistics of tin whisker diameters versus the underlying film grains

“…It explains as well the imperfect MWs cross sections, and the presence of striations on their side surfaces. 7 Earlier, it was observed indeed that the morphology of the whiskers was a result of whether they had nucleated on a single grain or on multiple grains. In the latter case, the whisker surface was fluted or striated.…”

“…Remarkably, that same scenario was recently put forward in order to explain the observed evidence of multifilament structure of MWs. 7 It is based on the electrostatic concept, according to which, MWs grow on local spots exhibiting significant enough surface charge density and its corresponding normal component of the electric field. The charging is related to various surface imperfections, such as a 'wrong' grain orientations, contaminations, deformations, etc.…”

“…[1][2][3][4] Their underlying physics remains poorly understood. [5][6][7][8][9] MWs exhibit significant statistical variations of their lengths 5,6,10,11 and characteristic diameters, 5,12 mutually uncorrelated, both fit well by the log-normal distributions. While the nature of lengths variations was addressed, 8,13,14 the origin of variations in whisker diameters remains uncharted territory.…”

Effect of Cultivation Conditions on Tin Whisker Growth on Ti2SnC

“…The influencing factors include but are not limited to substrate, preparation method, and cultivation condition 5,6 Meanwhile, Sn whiskers have been observed to grow within days in some cases, while there are also some whiskers with an incubation period up to years and even decades. [7][8][9][10] Although many growth mechanisms have been proposed to explain the whiskering phenomenon, including a dislocation-based mechanism, 11,12 recrystallization mechanism, 13 compressive stress mechanism, 14,15 electrostatic mechanism, 16 and interface energy-driven mechanism, 17,18 the conclusions are still inconsistent, even contradictory [19][20][21] Among these mechanisms, the compressive stress mechanism is the most recognized one. However, although the acceleration of compressive stress on Sn whisker growth has been reported in many studies, [22][23][24][25] there are still many results inconsistent with the compressive stress mechanism, such as the whiskering phenomenon under tensile stress, 19 and the great growth rate difference under the same compressive stress level.…”

Kinetics of Sn whisker growth from Sn thin-films on Cu substrate

“…They found that electric charges generated in the insulating glass substrate under the Sn thin film were perpendicular to the Sn film, thus resulted in electrostatically driven whisker growth. Bora and Georgive [28] investigated only the morphology of tin whiskers grown out from Sn thin-film on Cu substrate by FIB-facilitated TEM images. They found that the whiskers consisted of multiple filaments whose individual radii are in the range of tens of nanometers.…”