An integrated theory of whisker formation: the physical metallurgy of whisker formation and the role of internal stresses

Oxidation Behavior of Rare-Earth-Containing Pb-Free Solders

“…It is well recognized that compressive stresses are a necessary condition for Sn whisker formation on thin Sn platings. [38][39][40][41] It is believed that Sn atoms diffuse from regions of high compressive stresses (generally caused by the growth of a Cu 6 Sn 5 intermetallic layer) to the stress-free film surface by diffusion along columnar Sn grain boundaries. Observations seen in the current study agree well with those reported in the literature.…”

“…These findings are in good agreement with reported observations of Sn whisker growth on thin Sn films. [38][39][40][41] In these studies it has been shown that whisker grains resulting from the recrystallization of thin Sn platings have boundaries neither parallel nor perpendicular to the columnar Sn grains or the plating surface. These recrystallized grains also have a different orientation than the surrounding as-plated microstructure.…”

Finite Element Modeling of Stress Evolution in Sn Films due to Growth of the Cu6Sn5 Intermetallic Compound

“…However, there is no clear consensus regarding the mechanisms by which localized growth of IMCs produces stress within the Sn film. [5][6][7][8][9][10][11][12][13][14] In order to understand how stress develops in response to IMC growth, it is necessary to identify the mechanical deformation processes operating in the surrounding Sn film. Recent experimental work provides evidence of extensive dislocation activity within the Sn film, especially in the neighborhood of IMC grains, 13,14 indicating that the Sn grains yield plastically.…”

Plastic deformation processes in Cu/Sn bimetallic films

“…11,14,15,21 Various driving forces have been suggested for whisker formation, including recrystallization, 1,[22][23][24][25] oxidation, 7,8,26 and stress in the layers. 5,11,12,21,[27][28][29][30][31][32][33][34][35][36][37][38][39][40] Fisher et al 33 in 1954 first demonstrated that compressive stress in the Sn layer can serve as a driving force for whisker growth by applying ring clamps to Sn-plated steel specimens and measuring whisker growth rates as a function of clamping pressure and time. The current consensus is that stress (externally applied or internally generated) is the primary driving force for whisker formation, but there is still significant disagreement about how and where the stress is generated and how it leads to whisker formation.…”

“…This work included careful characterization of the different microstructures associated with the different layer compositions. To explain the measured stresses, Galyon and Palmer 40 recently proposed a zonal model that includes the effects of vacancy formation in the Cu layer and volumetric expansion in the IMC layer to create regions of different stresses in the Sn-Cu structure.…”