Microstructure evolution and surface defect formation in tin films

Ying Wang, Purdue University

Abstract

Tin (Sn) Whiskers have been identified as one of the biggest concerns in the electronic industry as they can grow spontaneously out of Lead (Pb)-free Sn electrodeposited films as a mechanism to relieve the compressive stress in Sn films. Whiskers can cause short circuiting by connecting adjacent electronic components and, therefore, bring a serious reliability issue. In order to develop more effective strategies to mitigate whisker formation, it is necessary to evaluate and understand whisker growth and film microstructure evolution in different application environments. Room temperature aging condition is a common application environment and whisker growth in such a condition was studied first. The effects of thin film crystallographic texture and substrate roughness on Sn whisker formation were determined during Cu6Sn5 intermetallic (IMC) growth in Sn films on Copper (Cu). Other properties of Sn films were controlled in order to isolate the influences of film texture and substrate roughness. This work supports the relationship between whisker formation propensities, elastic strain energy density (ESED) as determined by Sn film crystallographic texture, and morphological changes to the IMC induced by substrate roughness, providing microstructural guidelines for creating films with a reduced propensity to form whiskers. Sn whisker growth in another common application environment, thermal cycling condition, is also investigated. The evolution of surface morphology, including whisker formation, grain boundary cracking, and subsiding grains, was studied in Sn thin films on silicon (Si) or Cu substrates with physical vapor deposition (PVD) Cu/Ti inter-layers during thermal cycling from -40°C to 85°C in air for up to 250 cycles. Multiple areas were tracked, and the areal density of whiskers and the grain morphologies within these areas were monitored with increasing number of thermal cycles by following a newly designed inspection procedure, which is different from the most commonly used Joint Electron Device Engineering Council (JEDEC) standard test procedure. The Sn films on both Si and Cu substrates were observed to show initial increasing and then decreasing whisker densities. The observed microstructure evolution identified whisker pinch-off and whisker shrinkage processes, suggesting that current JEDEC standard test could lead to an underestimation of whisker risk during thermal cycling. A whisker pinch-off model and a whisker shrinkage model were proposed to explain the observed morphological changes and the resulting decrease in whisker density during thermal cycling. Additional insights into the whisker growth behavior were developed by comparing whisker growth on thermally cycled Sn thin films and on room temperature aged films. It is proposed that there is a critical value of thermal stress. When the thermal stress is higher than the critical stress, the whisker formation will be the dominant stress relaxation process and thermal cycling will lead to a higher tendency for whiskers to form as compared to room temperature aging, and versa vice. Based on the experiments and models in this thesis, whisker growth behavior in room temperature aging condition and thermal cycling condition is better understood and can be predicted, providing a guideline for whisker mitigation strategy development.

Degree

Ph.D.

Advisors

Blendell, Purdue University.

Subject Area

Materials science

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