Abstract
The microstructure and mechanical property of gold cladding silver composite bonding wire under different true strains were characterized by metallographic microscope, double beam electron microscope, high-low temperature tensile tester and nano-indentor. The results show that the silver alloy core of gold cladding silver composite bonding wire evolves from cellular dendrite to fiber structure along the drawing direction. The gold cladding layer is uniform and continuous and the transition layer near the interface always keeps fine equiaxed or spherical grains during the deformation process. Inconsistent size change among each component is observed during deformation, and the fitted size change constant is not proportional to the change of wire diameter. Microhardness, tensile strength and elongation increase with the increase of deformation amount. During the uniaxial drawing process, the unidirectional tensile strain becomes a complex two-dimensional stress state due to the interaction between each component of the composite wire. Thus the plasticity and toughness of the material can be improved because the alternating stress inhibits the nucleation of crack.
Science Press
Micro-nano composite bonding wire composed of two or more components in the form of surface cladding presents advantages of high strength, excellent chemical stability and low pric
In this study, microstructure evolution, and mechanical performance parameters (such as strength and ductility) during the preparation process of gold cladding silver composite bonding wires were studied, and the mechanical properties of layered composite bonding wires were dis-cussed. The size effect and its intrinsic physical mechanism, the deformation coordination mechanism of the component, and the deformation mechanism during large plastic drawing of the layered composite materials were explored.
A gold cladding silver composite bonding wire with silver alloy as core material produced by Sino-platinum Metals Co., Ltd was studied in this work. Composition of the silver alloy was 0.1wt%~5wt% Cu, 0.001wt%~0.01wt% Sn, and the balance silver. The gold cladding silver ingot (Ф15 mm) was prepared using a composite continuous casting furnace, preheated at 300~500 ℃ for 1~2 h under charcoal protection, hot extruded into a rod of Ф6 mm, and then cold drawn and processed into a Ф0.020 mm sample. Samples were taken when the true strain η=2.25, 4.76, 7.22, 10.41, 11.12, 11.94, 12.56, 13.24 (η=ln(A0/A), where A0 and A represent cross-sectional area of the composite at initial stage and after deformation, respectively. The cross-section and longitudinal section of the as-cast, η=4.76 and η=13.24 samples were taken by electric discharge wire cutting. Metallographic structure was observed using XWJ-02 normal material microscope after corrosion using mixed solution of ferric chloride and hydro-chloric acid for 3~5 s. The cross and vertical sections of the gold wire sample (η=13.24) were cut using the ion beam (FIB) in FEI Versa 3D, a double-beam electron microscope, and the microstructure of the gold wire sample was characterized by electron back scattering diffraction (EBSD). INSTRON LEGEND 2344 high-low temperature tensile tester was used to test the breaking load and elongation under different true strain conditions. The gauge length was 100 mm, and the testing speed was 50 mm/min. Hysitron nanometer indenter was used to test the hardness of the gold cladding and the silver alloy core material with a load of 4 mN.
Microstructure of the gold cladding silver composite bonding wire was observed at different true strains.
Fig.1 Metallographic images of as-cast gold cladding silver com-posite bonding wire: (a) cross section; (b) longitudinal section
Fig.2 Metallographic images of as drawn (a, b) and annealed (c, d) gold cladding silver composite bonding wire when η=4.76: (a, c) cross section and (b, d) longitudinal section
Fig.3 Morphologies of gold cladding layer in processed (a) and annealed (b) conditions
Fig.4 Microstructures of gold cladding silver composite bonding wire at η=13.24: (a) cross section in the lower part and (b) longitudinal section
The size of each phase of gold cladding silver composite bonding wire was measured under different deformation amounts, and the relationship between average size and deformation amount is shown in Fig.5. As can be seen, the average thickness (tAu) of gold cladding layer and average diameter (tAg) of silver alloy core decrease with the increase of true strain. The curves are fitted by power function, and the relationships between the gold cladding thickness, silver alloy
core diameter and true strain are obtained.
| (1) |
| (2) |
According to the definition of true strain η, η=ln(A0/A)= ln(d
| (3) |
Therefore, when the deformation of each phase of the wire is uniform, the relationship between the size of each phase and the true strain should be:
| (4) |
However, the deformation of each phase inside the wire is not uniform, and change of the size of each phase is not propor-tional to the change of the diameter of the sample. Therefore, the coefficient of η obtained by fitting
Fig.6 shows the microhardness of each phase of the gold cladding silver composite bonding wire under different true strains. It can be seen that microhardness of the gold cladding layer dynamically changes and slightly increases with the increase of true strain, and the microhardness of the core material shows an increasing trend. Due to the thin thickness of the gold cladding layer, the tensile structure is cut off by the interface and the transition layer in the deformation process, which reduces the work hardening increment and results in a small increase of microhardness. The center of the core material is hardly affected by the gold cladding layer and the interface. As the true strain increases, the fiber structure increases, the texture is formed, and the hardness increases.
Fig.7 shows the variation of breaking strength and elonga-tion of gold cladding silver composite bonding wire with the true strain. As can be seen, both the breaking strength and elongation of the gold cladding silver composite bonding wire increase with the increase of true strain. The traditional homogeneous materials are subjected to uniaxial tensile stress during the tensile process, but the stress state of layered composites is changed by the mutual binding of the components. When the contraction of the surface layer is restricted by the central part, the surface layer is subjected to a lateral tensile stress. Since there is no external lateral force, the tensile stress of the surface layer is balanced by the compressive stress of the inner part. In the process of tension, the unidirectional tensile stress gradually changes into a complex two-dimensional stress state. The surface layer is subjected to the tensile stress, while the inner part is subjected to the compressive stress. This alternating stress state passivates the crack tip, and the existence of compressive stress can also effectively inhibit the nucleation of cracks, thus improving the plasticity and toughness of material
In conclusion, due to the structural difference of each component and the presence of interface, the structure of the core material evolves from cellular dendrites to fibrous struc-ture, and the transition layer presents cellular grains in the process of tensile deformation. Compared with homogeneous materials, the deformation behavior of gold cladding silver composite bonding wires is complex. The deformation stress is diverse and mutually restricted, and the interface plays the role of stress transfer and redistribution. This stress state will inhibit the nucleation of the crack, so the gold cladding silver composite bonding wire has good plasticity and toughness.
1) The microstructure of the core material of gold cladding silver composite bonding wire along the drawing direction transforms from cellular dendrite to fiber structure. The transition layer near the interface maintains fine cellular grains and the gold layer is uniform and continuous during the deformation.
2) In the process of tensile deformation, the phases of gold cladding silver composite bonding wire are not uniform and mutually restricted, and the change of size is not proportional to the change of sample diameter.
3) With the increase of true strain, microhardness of the gold cladding layer changes dynamically and increases sli-ghtly, and the microhardness of the core material shows an in-creasing trend. The increase trend of cladding layer microhard-ness is small due to the influence of interface and transition layer.
4) The breaking load and elongation of gold cladding silver composite bonding wire increase with the increase of true strain. In the process of uniaxial tension, the mutual constraint of each component of the wire changes stress state of the material, and the unidirectional tensile stress changes to a complex two-dimensional stress state. This alternating stress state inhibits the nucleation of cracks, improving the plasticity and toughness of gold cladding silver composite bonding wire.
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