Microstructure Modification and Enhancement in Mechanical Properties Cu/Sn-Ag/Cu Microbump Joints with 5-Micron Bonding Height Via Alloying of Zn and Ni in Cu Ubm
Authors: Lee Yin-Ku , Chang Shou-Yi , Duh Jenq-Gong 
Published: August 2024

With the rise of artificial intelligence (AI) and the 5G era, advanced microelectronic packaging is gaining attention, shifting towards high-density interconnection and downsizing packaging structures. In response to increasing demands for high power density, low energy consumption, and low latency, 3-dimensional chip stacking technology (3D-IC) has emerged. In 3D-IC structures, Pb-free microbump solder joints, notably with diameters potentially below 10μm, are widely used, exhibiting sizes 10 times smaller than traditional flip chip joints. However, this miniaturization poses challenges, leading to interfacial Intermetallic Compound (IMC) overgrowth and high internal stress due to volume shrinkage and natural brittleness. To address this, Cu-Zn-based substrates are employed, forming a Zinc-rich layer that efficiently reduces IMC thickness. The transition of hexagonal η-Cu6Sn5 to monoclinic η'-Cu6Sn5 and the formation of porous Cu3Sn under certain conditions impact mechanical reliability. The addition of Ni and Zn in solder joints is reported to enhance phase stabilization and mechanical properties. Considering the influence of solder alloy composition on melting points, adding Zn and Ni in substrates is suggested to ensure a stable source of dopants and stabilize molten behavior. Co-doping Zn and Ni in substrates aids in Cu-Sn IMC suppression.

Previous work on double-sided Cu-26Zn-18Ni substrates soldering with Sn-3.5Ag solder optimized IMC thickness, inhibiting the brittle Cu3Sn phase and Kirkendall voids, while inducing the tough γ-Cu5Zn8 phase. This phase exhibits favorable toughness and shear modulus, enhancing mechanical strength.This study investigates the microstructure evolution, elemental distribution, and mechanical reinforcement of Cu-26Zn-18Ni/Sn-3.5Ag/Cu-26Zn-18Ni microbumps with a bond height below 10μm. A field emission electron probe micro-analyzer (FE-EPMA) was utilized to examine microstructures and elemental distribution. Additionally, transmission electron microscopy with energy dispersive spectroscopy (TEM/EDS) was employed for detailed voiding formation and phase identification. Electron backscatter diffraction (EBSD) confirmed the presence of highly randomly oriented (Cu,Ni)6(Sn,Zn)5 with a submicron grain size. Fracture modes of the microbumps were characterized through shear tests and correlated with microstructures. In comparison to Cu/Sn-3.5Ag/Cu microbumps, the alternative Cu-Zn-Ni substrates significantly improved shear strength (19.6% to 97.8%) and failure toughness (10.6% to 132.1%) under different reflow times. The microstructure of Cu-26Zn-18Ni/Sn-3.5Ag/Cu-26Zn-18Ni microbumps, featuring solid solution strengthening and refined (Cu,Ni)6(Sn,Zn)5 along with tough bulk (Cu,Ag)5(Sn,Zn)8, demonstrated remarkable toughness.These findings offer valuable insights into the design of microbumps with a bonding height in the sub-10-micron scale.

Keywords: Microbump, Shear strength, Microstruture, Intermetallic compounds

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