Plastic deformation processes in Cu/Sn bimetallic films

Abstract

Although the driving force for the growth of Sn whiskers from the surface of Sn coatings on copper is thought to be internally generated stress due to the formation of Cu 6 Sn 5 at the Cu/Sn interface, little is known about the nature of this internal stress and how it cracks the surface Sn oxide (an important precursor to whisker formation). Arguments based on elasticity alone do not appear to be sufficient and suggest an important role for plastic deformation. Direct observations, made by transmission electron microscopy of cross-sectioned bimetallic Cu/Sn thin-film specimens, confirm plastic deformation of the Sn grains due to the formation of Cu 6 Sn 5 . Dislocation motion and pile-up at the surface Sn oxide, rotation associated with subgrain boundary formation, interaction of the subgrain boundaries with the Sn surface, and diffusional processes are various mechanisms that can produce stress at the Sn surface and crack the Sn oxide.

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

“…13,14,16,18 TEM analysis also provides evidence of dislocation activity within the Sn layer, particularly near IMC particles. 13,14 This suggests that IMC growth induces sufficient stress within the surrounding Sn grains to cause dislocation-mediated plastic deformation.…”

“…Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses reveal that IMC particles nucleate within the Sn at the Cu-Sn interface, typically with greater probability at the triple junctions where grain boundaries between Sn grains meet the Cu interface. 13,14,18 After nucleation, growth of the particles proceeds at the IMC/Sn interface, 5,19 with IMC particles primarily growing up into the Sn film. 13,14,16,18 TEM analysis also provides evidence of dislocation activity within the Sn layer, particularly near IMC particles.…”

“…13,14,18 After nucleation, growth of the particles proceeds at the IMC/Sn interface, 5,19 with IMC particles primarily growing up into the Sn film. 13,14,16,18 TEM analysis also provides evidence of dislocation activity within the Sn layer, particularly near IMC particles. 13,14 This suggests that IMC growth induces sufficient stress within the surrounding Sn grains to cause dislocation-mediated plastic deformation.…”

“…13,14 There is no evidence of isolated IMC particles away from the Sn-Cu interface, either within Sn-Sn grain boundaries or in the Sn grains themselves. Thus, a model for stress generation must explain how IMC growth that is restricted to the base of the Sn layer results in the development of stress throughout the thickness of the 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. This conclusion is also consistent with reported stress measurements within the Sn layer which are very near the nominal yield stress for Sn (14.5 MPa 15 ) and remain relatively constant over time.…”