Tin whiskers prefer to grow from the [001] grains in a tin coating on aluminum substrate

doi:10.1016/j.jmst.2021.01.002

Abstract

Abstract:

This work aims at understanding the features of the Sn grains from which whiskers preferentially grow. The growth behavior of Sn whiskers on a 50 μm thick hypereutectic Sn-Al alloy coating was observed in situ by mapping the grain orientations before and after aging using the electron backscatter diffraction (EBSD) technique. Sn whiskers were found to grow preferentially from the (001) or near-(001) grains surrounded by the grains having perpendicular orientations, such as (100), (110) and (210). The compressive stress in the coating was heterogeneous, and the (001) grains exhibited the higher compressive stress close to the grain boundaries. The orientation relationship between α-Al phase and β-Sn phase was confirmed as ${{(\text{2 0 0})}_{\text{ }\!\!\alpha\!\!\text{ -Al}}}$‖${{(\text{2 0 0})}_{\text{ }\!\!\beta\!\!\text{ -Sn}}}$, ${{[\text{0 1}\bar{1}]}_{\text{ }\!\!\alpha\!\!\text{ -Al}}}$‖${{[\text{0 0 1}]}_{\text{ }\!\!\beta\!\!\text{ -Sn}}}$. The plane matching resulted in approximately 0.7 % misfit strain in β-Sn, which had little impact on the growth of whiskers. Dislocations pile-ups were found in the (001) grains and repulsed by the Sn oxide layer, giving the probability of cracking the oxide. Grain boundaries were found between the whisker and underneath grain. The dominant diffusion mode for early whisker growth was grain boundary diffusion aided by pipe diffusion.

Key words: Tin whisker, Preferred orientation, Electron backscatter diffraction, Compressive stress, Diffusion path

Shuang Tian, Yushuang Liu, Peigen Zhang, Jian Zhou, Feng Xue, ZhengMing Sun. Tin whiskers prefer to grow from the [001] grains in a tin coating on aluminum substrate[J]. J. Mater. Sci. Technol., 2021, 80: 191-202.

Figures/Tables 17

Fig. 1. Fabrication schematic of the 50 μm Sn coating on Al substrate.

Fig. 2. Microstructures of the as-prepared coating: (a) cross-section, (b) top-view.

Table 1 The chemical composition of the as-prepared coating.

Composition	Sn (at.%)	Al (at.%)
Coating	90.43	9.57
Primary α-Al phase	2.94	97.06
β-Sn phase	94.79	5.21

Fig. 3. Morphology and density of whiskers on the coating: (a) after aging for 720 h, (b) the filament type whisker, (c) the blocky hillock, (d) whiskers density as a function of aging time.

Fig. 4. Evolution of the residual stress and speed of stress relaxation in the coating.

Fig. 5. Band contrast (BC) figures with the grain boundaries, Euler figures and kernel average misorientation (KAM) figures of (a-c) as-prepared Sn-Al coating, (d-f) the same region after aging for 20 h.

Fig. 6. (a) Proportion of the LAGBs and HAGBs with the same region before and after aging, and (b) the relative frequency of misorientation before and after aging.

Fig. 7. EBSD data of the rectangle region in Fig. 5(e): (a) band contrast figure, (b) Euler figure, (c) inverse pole figure along the Z direction (IPFZ), (d) the whiskers, (e) KAM figure and (f) the direction of the coordinate system.

Fig. 8. (a) BF-image of Sn-Al coating after aging for 20 h, (b) α-Al precipitates in the Sn grains and (c) HRTEM image of α-Al precipitates.

Fig. 9. (a) BF-image of primary α-Al in the Sn-Al coating and (b) orientations relationship between α-Al and β-Sn.

Fig. 10. The distribution of compressive stress in the as-prepared Sn-Al coating measured by nanoindentation method. (a) A matrix of indentation containing the (001) grains and surrounding grains with perpendicular orientations, (b) stress mapping of the Sn-Al coating covered by indentations.

Fig. 11. Representative grain orientation around the whiskering grain: (a) EBSD IPFZ map with β-Sn unit cell, (b) schematic of crystal indices, (c) IPF map of Sn. The whiskering grain with black circle shows (001) orientation and the orientations of neighbouring grain with broken circle are perpendicular to (001).

Table 2 Anisotropic physical properties of β-Sn [ 10,54,55].

Direction	CTE (K^-1)	Stiffness (GPa)	Young’s Modulus (GPa)	Self-diffusivity (cm²/s)
a-axis[100]	14.2 × 10 ^-6+25.5 × 10 ^-9×T	75.3	23.6	8.7 × 10 ^-15
b-axis[010]	14.2 × 10 ^-6+25.5 × 10 ^-9×T	75.3	23.6	8.7 × 10 ^-15
c-axis[001]	28.7 × 10 ^-6+56.8 × 10 ^-9×T	95.5	67.2	4.7 × 10 ^-15

Fig. 12. (a) TEM thin sample with (001) Sn grain, (b) IPFZ image confirming the Sn grain is (001) orientation, (c) KAM image of the (001) grain, (d) Sn sub-grains containing many sub-grain boundaries demarcated by the arrays of dislocations, (e) bright-field image of the sub-grain boundaries and (f) sub-grain boundaries serving as dislocation sources for the motion of dislocations.

Fig. 13. (a) BC image of the cross-sectional Sn/Al bimetal, (b) IPFY image of Sn/Al sample with Sn unit cells, (c) rainbow colored image reflects the BC value of each grains and (d) KAM image of the Sn coating shows the dislocations accumulate in near-(001) grain.

Fig. 14. TEM images of the whisker and the underneath grain: (a) SEM micrograph of the FIB thinned cross-section, (b) TEM micrograph of the position in (a) illustrated by the red arrow, (c) HRTEM image of the interface between the whisker and coting, (d) HRTEM image of the underneath grain with IFFT crystal lattice, (e) HRTEM image of the Sn whisker with IFFT crystal lattice.

Fig. 15. (a) Front view of the cross-sectional whisker, (b) top view of the cross sectional whisker, (c) IPFY image of the whisker cross-section with unit cells and (d) KAM image of the whisker cross-section.

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