Key Takeaways & Executive Findings
- •• High strain cold drawing of single-crystal copper produces 18 μm ultrafine wire with a tensile strength of 729.8 MPa and electrical conductivity of 86.9% IACS. • As drawing strain increases, the average lamella thickness refines dramatically from 1.63 μm to 102 nm. • Texture evolution is inhomogeneous: <111> and <112> dominate the periphery, while <001> and <111> dominate the center; hard <111> orientation grows at the expense of <112> and <001>. • Grain, dislocation, and texture strengthening are the primary strengthening mechanisms, with dislocation density being the main factor reducing electrical conductivity.
Abstract
The ultrafine copper wire with a diameter of 18 μm is prepared via cold drawing process from the single crystal downcast billet (Φ8 mm), taking a drawing strain to 12.19. In this paper, in-depth investigation of the microstructure feature, texture evolution, mechanical properties, and electrical conductivity of ultrafine wires ranging from Φ361 μm to Φ18 μm is performed. Specially, the microstructure feature and texture type covering the whole longitudinal section of ultrafine wires are elaborately characterized. The results show that the average lamella thickness decreases from 1.63 μm to 102 nm during the drawing process. Whereas, inhomogeneous texture evolution across different wire sections was observed. The main texture types of copper wires are comprised of <111>, <001> and <112> orientations. Specifically, the peripheral region is primarily dominated by <111> and <112>, while the central region is dominated by <001> and <111>. As the drawing strain increases, the volume fraction of hard orientation <111> with low Schmid factor increases, where notably higher fraction of <111> is resulted from the consumption of <112> and <001> for the wire of Φ18 μm. For drawn copper wire of 18 μm, superior properties are obtained with a tensile strength of 729.8 MPa and an electrical conductivity of 86.9%IACS. Furthermore, it is found that grain strengthening, dislocation strengthening, and texture strengthening are three primary strengthening mechanisms of drawn copper wire, while the dislocation density is the main factor on the reducing of conductivity.
1. Introduction
In recent years, the emergence of 5G networks has significantly propelled the rapid development of artificial intelligence industries, particularly in the domains of mobile communications and industrial automation [1, 2]. Consequently, there is an escalating demand for integrated circuits that meet performance requirements. Bonding wires, as the lead wires connecting the chip to the frame, play an important role in realizing the interaction between the chip and the external electrical signals.
Historically, gold bonding wires are mainly used due to their remarkable performance. Subsequently, it is found that the copper wires exhibit excellent electrical, mechanical properties and solderability, along with its relatively inexpensive price advantage. Whereas, package failure due to the inherent characteristics of copper wires, such as the susceptibility to oxidation and relatively high hardness, remains an urgent issue. Currently, adding trace alloying elements and coating an antioxidant metal layer on the surface of copper wires effectively addresses these issues. With ongoing improvements in processing techniques, copper wires exhibiting enhanced overall performance and finer diameters are produced to meet the demands of narrow-pitch, low-length arc electronic packaging [3, 4]. Therefore, using copper wires as replacements for gold wires holds broaden application prospects and vast development potential [5−7].
For copper bonding wires, selection of the drawing process plays a pivotal role in determining the mechanical property and electrical conductivity. The high-quality copper rod obtained by the directional solidification continuous casting process, which approximates single crystal structure, exhibits excellent electrical conductivity and plasticity. Moreover, it can prevent the formation of transverse grain boundaries, thereby enabling the production of finer copper wires through cold drawing. It is noticed that the single crystal copper exhibits excellent information transmission performance due to the elimination of signal interference caused by grain boundaries. Up to now, the single crystal copper is widely utilized in communications, civil electronics, networks and other fields [8, 9]. GU et al [10] utilized the cold mold downward continuous casting method to prepare a Φ7.8 mm Cu-4Ag rod billet.
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LIU Jin-song, ZHOU Yan, WANG Song-wei, CHEN Shuai-feng, SONG Hong-wu, ZHANG Shi-hong (2025). Exploring the evolution of texture and properties of ultrafine copper wire during high strain drawing process. Journal of Central South University. https://doi.org/10.1007/s11771-025-5896-4
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Frequently Asked Questions
What is the strongest and most conductive ultrafine copper wire produced in this study?
An 18 μm diameter copper wire prepared by high strain cold drawing from a single crystal downcast billet achieved a tensile strength of 729.8 MPa and an electrical conductivity of 86.9% IACS.
How does the texture evolve during the high strain drawing of copper wires?
As drawing strain increases, the average lamella thickness refines from 1.63 μm to 102 nm. The peripheral region is dominated by <111> and <112> orientations, while the central region is dominated by <001> and <111>. The volume fraction of hard <111> orientation increases through consumption of <112> and <001>.
What are the main strengthening mechanisms of drawn ultrafine copper wires?
The primary strengthening mechanisms are grain strengthening, dislocation strengthening, and texture strengthening. Dislocation density is the main factor that reduces electrical conductivity.
Why is single crystal copper used for producing ultrafine bonding wires?
Single crystal copper obtained by directional solidification continuous casting approximates a single crystal structure, exhibits excellent electrical conductivity and plasticity, and prevents transverse grain boundaries, enabling the production of finer copper wires through cold drawing.
What is the significance of <111> texture in drawn copper wires?
<111> is a hard orientation with low Schmid factor. Its volume fraction increases with drawing strain, and for the 18 μm wire, higher <111> fraction arises from consumption of <112> and <001>, contributing to texture strengthening.
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