Laser-Structured Copper Surfaces for Paste-Free Direct Cu/Cu Joining

Abstract The increasing thermal and power-density requirements of modern electronic systems create a growing demand for joining technologies that provide high electrical and thermal conductivity, sufficient mechanical stability, and reduced process complexity. This study investigates a paste-free approach for direct Cu/Cu joining using laser-structured copper surfaces. A kinematic surface simulation was used to identify laser parameter combinations that generate pronounced microstructured topographies. Two structure variants with different lateral feature sizes and surface-enlargement factors were subsequently produced and joined under heat and pressure without a filler metal, sintering paste, or permanent intermediate layer. The resulting surfaces and joints were characterized by optical microscopy, scanning electron microscopy, laser scanning microscopy, cross-sectional analysis, fracture-surface examination, and preliminary adapted die-shear testing. Apparent shear strengths of approximately 16 and 35 MPa were obtained for structure variants I and II, respectively. Variant II exhibited finer and more densely distributed features, a larger surface-enlargement factor, and a more continuous interfacial contact morphology. The results indicate that laser-generated microstructures can support paste-free direct Cu/Cu joining and that the lateral distribution of the surface features is an important factor in joint formation.