AbstractsEarth & Environmental Science

Stress relaxation mechanisms in thin Sn films and its alloys : Sn whisker formation and its mitigation

by Jendrik Stein




Institution: University of Stuttgart
Department: Fakult├Ąt Chemie
Degree: PhD
Year: 2015
Record ID: 1117720
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2015/9910/


Abstract

The present thesis project deals with the build-up of stresses and stress relaxation mechanisms in thin Sn(-based) films electro-deposited on pure Cu or Cu-alloy substrates. In particular, the spontaneous formation of Sn whiskers, i.e. the growth of single crystalline, needle-like filaments on the Sn film surface, and its mitigation, is taken into focal point of this study. Thereby, new-developed and well-known methods and analysis techniques have been applied in order to investigate i) the specimen composition, ii) the film- and substrate microstructure and iii) the state of the film stress in the initial state as well as during storage at room temperature. Further, the crystallographic growth directions of Sn whiskers have been investigated and a model, on basis of the concept of periodic bond chains as proposed by Hartmann and Perdok, has been derived for explanation. In chapter 2, an in-situ two-dimensional (2D) detector X-ray diffraction technique has been applied on aging pure Sn films electro-deposited on Cu substrates in order to trace local microstructural changes in residually stressed Sn films. Changes of diffraction spots in the 2D-diffraction patterns were observed, as e.g. emergences, disappearances or migrations as well as increases and/or decreases of intensities of diffraction spots. These diffraction spots on the 2D diffraction patterns originate from (usually) only single Sn grains located within the Sn film microstructure. All observed changes of diffraction spots have been assigned to local microstructure changes in the Sn film: Grain rotation, grain growth and grain dissolution processes could be observed during and even before the onset Sn whisker growth. In chapter 3, the impact of alloying Ag to pure Sn films on the formation of Sn whiskers has been explained. In the as-deposited Sn,Ag films (containing silver of about 6 wt.%), a film microstructure with partially columnar and partially equiaxed Sn grains as well as isolated Ag3Sn precipitates along the Sn/Sn grain boundaries have been observed During room temperature storage of these specimens, Sn whisker growth did not occur even though the formation of the IMC Cu6Sn5 took place at the film/substrate interface in an irregular manner with a similar growth rate as observed in whiskering pure Sn/Cu specimens. The Sn,Ag film microstructure allows global stress relaxation since inclined, with respect to the specimen surface, grain boundaries of equiaxed grains are found at many locations within the film. In chapter 4, pure Sn films electro-deposited on Cu substrates containing Zn as alloying element have been investigated. In particular, the impact of the Zn (content) in the substrate on the interfacial reaction between Cu and Sn and its consequences for the formation of Sn whiskers has been in focal point in this chapter. The Cu6Sn5 formation rate was greatly slowed down in Sn/Cu(Zn) specimens as compared to Sn/pure Cu specimens since the driving force for the Cu6Sn5 formation at the film/substrate interface was lower in the Sn/Cu-alloy…