The combined effects of grain and sample sizes on the mechanical properties and fracture modes of gold microwires

doi:10.1016/j.jmst.2018.09.012

Abstract

Abstract:

Hall-Petch relation was widely applied to evaluate the grain size effect on mechanical properties of metallic material. However, the sample size effect on the Hall-Petch relation was always ignored. In the present study, the mechanical test and microstructure observation were performed to investigate the combined effects of grain and sample sizes on the deformation behaviors of gold microwires. The polycrystalline gold microwires with diameter of 16 μm were annealed at temperatures from 100?°C to 600?°C, leading to different ratios (t/d) of wire diameter (t) to grain size (d) from 0.9 to 16.7. When the t/d was lower than 10, the yield stress dropped fast and deviated from the Hall-Petch relation. The free-surface grains played key role in the yield stress softening, and the volume fraction of free-surface grains increased with the t/d decreasing. Furthermore, the effects of t/d on work-hardening behaviors and fracture modes were also studied. With t/d value decreasing from 17 to 3.4, the samples exhibited necking fracture and the dislocation pile-ups induced work-hardening stage was gradually activated. With the t/d value further decreasing (t/d3.4), the fracture mode turned into shear failure, and the work-hardening capability lost. As the gold microwire for wire bonding is commonly applied in the packaging of integrated circuit chips, and the fabrication of microwire suffers multi-pass cold-drawing and annealing treatments to control the grain size. The present study could provide instructive suggestion for gold microwire fabrication and bonding processes.

Key words: Gold microwire, Wire bonding, Tensile testing, Grain size, Hall-Petch relationship

H.K. Yang, K. Cao, Y. Han, M. Wen, J.M. Guo, Z.L. Tan, J. Lu, Y. Lu. The combined effects of grain and sample sizes on the mechanical properties and fracture modes of gold microwires[J]. J. Mater. Sci. Technol., 2019, 35(1): 76-83.

Figures/Tables 12

Fig. 1. The electron channeling contrast (ECC) images of the wires cross-section along the longitudinal direction: (a) as-received, (b) 100?°C, (c) 300?°C, (d) 400?°C and (e) 600?°C samples.

Table 1 The average grain size of gold microwires in the longitudinal and transverse direction at different annealing temperatures.

Materials	Longitudinal direction (μm)	transverse direction (μm)	Average (μm)	t/d
AR (20?°C)	1.35	0.57	0.96	16.7
100?°C	1.24	1.04	1.14	14.0
300?°C	1.62	1.05	1.33	12.0
400?°C	4.91	4.47	4.69	3.4
600?°C	18.59	-	18.59	0.9

Fig. 2. The EBSD maps of the wires cross-section along the longitudinal direction: (a) as-received sample, (b) 300?°C, and (c) 600?°C. The color represents the grain orientation along the longitudinal direction as inserted in (a). The inverse pole figure (IPF) for: (d) as-received, (e) 300?°C, and (f) 600?°C samples. The color represents the intensity of texture.

Fig. 3. The TEM observation of fractured samples: (a) 100?°C (the low magnification observation is inserted), (b) 400?°C and (c) 600?°C. The extractions of dislocations in each annealed samples are represent in (d, e & f), respectively.

Fig. 4. The illustration of gold microwire tensile test: (a) before and (b) after tensile test under optical microscopic; The comparisons of (c) engineering stress-strain and (d) true stress-strain curves of as-received and annealed samples.

Fig. 5. The side and top views of fractures: (a1 & a2) as-received, (b1 & b2) 100?°C, (c1 & c2) 300?°C, (d1 & d2) 400?°C and (e1 & e2) 600?°C annealed samples.

Fig. 6. The in situ observation of fracture: (a & b) 300?°C, and (c & d) 600?°C annealed samples.

Fig. 7. (a) The relation between work-hardening rate (solid lines) and true stress-strain (dash lines), (b) the relation between σθ?and σ of gold wires with different annealing temperatures and values of t/d, and (c) the illustration of σθ-σ?curves.

Fig. 8. The illustration of the fracture behaviors of (a) fine grains and (b) coarse grains.

Fig. 9. Hall-Petch relation applied to the 1% yield strain.

Fig. 10. (a) Illustration of interior and free-surface regions of grain within cross-section of wire along the transverse direction; (b) the volume fraction of free-surface grains with various t/d values.

Fig. 11. The comparison of t/d and normalized flow stress.

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