Enhancing the shear strength of the Au–Ge solder joint via forming a ductile face-centered cubic solid solution layer at the interface

Abstract

The Au-28Ge (at%) eutectic alloy is a high-performance solder with a melting point of 361 °C. However, the brittle interfacial reaction products layer formed between the Au–Ge solder and substrate (Cu or Ni) is always the weak region. Hence, the joint fails at a relatively low shear strength (∼53 MPa). Here, the Cu–Ni alloys (Ni content is 20, 50, or 70 at%) are designed as the substrates to connect with the Au–Ge eutectic at 400 °C. A classic example is represented by the Au–Ge/Cu20Ni joint, which achieves a shear strength of above 100 MPa after soldering at 400 °C for 10–20 min. Microstructure characterization reveals that a face-centered cubic structured solid solution (FCC) layer forms at the Au–Ge/Cu20Ni joint. In contrast, it is mainly the NiGe or Ni2Ge layer in the Au–Ge/Cu50Ni and Au–Ge/Cu70Ni joints. On the one hand, compared to the brittle intermetallic compound products layer, the FCC layer shows adequate ductility, releasing the local strain incompatibility via pronounced local plastic deformation. On the other hand, the fine NiGe and Ni2Ge precipitation and fine grains make the FCC layer stronger than the solder matrix. Therefore, this products layer is no longer the weak region in the solder joint. As a result, the Au–Ge/Cu20Ni joint achieves outstanding shear strength. This study enriches our fundamental understanding of producing ductile phases to enhance the shear strength of the solder joints by designing alloy substrates.