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<p>The decline in surface free energy of OFC (oxygen-free copper) during heat treatment at 200–400 °C for 1–5 min was studied by considering the coupled effects of copper oxide layer growth, evolution of surface microstructure, and adsorption of organic functional groups. OFC oxidation forms a CuO-dominated layer with thickness increasing from &lt; 40 nm at 200 °C to ~ 200 nm at 400 °C, and conical nanostructures (80–120 nm in diameter) develop on the surface at 300–400 °C, which increase roughness <i>Ra</i> up to 0.1243 ± 0.005 μm. The conical nanostructures act as a barrier to trap air, markedly increasing the water contact angle from 65° ± 1° to 124° ± 1°. The solid–liquid contact fraction (<i>f</i><sub><i>sl</i></sub>) decreases from 0.89 at 200°C to 0.38 at 400 °C, confirming the transition to a Cassie–Baxter wetting state, further supported by low contact angle hysteresis and the corresponding decrease in surface free energy from 38 mN/m to below 28 mN/m. XPS analysis revealed that the proportion of non-polar groups (C–C/C-H) on the OFC surface increased from 50.63% to 58.84% (200 °C), 56.44% to 78.67% (300 °C) and 59.44% to 78.65% (400 °C), while polar oxygen-containing groups (C–O–C and O-C = O) decreased correspondingly. This indicates that a thicker oxide provides more sites for hydrophobic group attachment, leading to the correlation between oxide thickness, <i>f</i><sub><i>sl</i></sub>, and surface free energy. Critical thresholds (oxide thickness &gt; 80 nm, hydrophobic/hydrophilic ratio &gt; 1.71) are identified where surface free energy falls below the industrial welding requirement (&lt; 38 mN/m). These findings provide essential insights for the application of OFC in the semiconductor and electronic component manufacturing industries.</p>

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Coupled oxidation–nanostructure–adsorption mechanism of surface free energy reduction in oxygen-free copper

  • 期刊论文

  • 发表日期：

    2026年4月24日

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  • Hao Cheng,
  • Xin Gao,
  • Xiaolin Zhang,
  • Heyuan Huang,
  • Bing Li

摘要

The decline in surface free energy of OFC (oxygen-free copper) during heat treatment at 200–400 °C for 1–5 min was studied by considering the coupled effects of copper oxide layer growth, evolution of surface microstructure, and adsorption of organic functional groups. OFC oxidation forms a CuO-dominated layer with thickness increasing from < 40 nm at 200 °C to ~ 200 nm at 400 °C, and conical nanostructures (80–120 nm in diameter) develop on the surface at 300–400 °C, which increase roughness Ra up to 0.1243 ± 0.005 μm. The conical nanostructures act as a barrier to trap air, markedly increasing the water contact angle from 65° ± 1° to 124° ± 1°. The solid–liquid contact fraction (fsl) decreases from 0.89 at 200°C to 0.38 at 400 °C, confirming the transition to a Cassie–Baxter wetting state, further supported by low contact angle hysteresis and the corresponding decrease in surface free energy from 38 mN/m to below 28 mN/m. XPS analysis revealed that the proportion of non-polar groups (C–C/C-H) on the OFC surface increased from 50.63% to 58.84% (200 °C), 56.44% to 78.67% (300 °C) and 59.44% to 78.65% (400 °C), while polar oxygen-containing groups (C–O–C and O-C = O) decreased correspondingly. This indicates that a thicker oxide provides more sites for hydrophobic group attachment, leading to the correlation between oxide thickness, fsl, and surface free energy. Critical thresholds (oxide thickness > 80 nm, hydrophobic/hydrophilic ratio > 1.71) are identified where surface free energy falls below the industrial welding requirement (< 38 mN/m). These findings provide essential insights for the application of OFC in the semiconductor and electronic component manufacturing industries.