Effect of introducing high temperature gradients on microstructure evolution and mechanical properties in Cu/Sn-3.5Ag/Ni microbump joints during thermocompression bonding
摘要
In this study, a fast thermocompression bonding method incorporating a high-temperature gradient (≥ 12,667 °C/cm) was developed for preparing remelting-resistant and anti-collapse full-intermetallic compound (IMC) microbump joints for three-dimensional packages. In response to the slow formation of full-IMC-microbump joints (several hours) in conventional transient liquid-phase diffusion (TLP), the introduced temperature gradient significantly accelerates the diffusion of copper from the hot side (Cu-side, 320–325 °C) to the cold side (Ni-side, 160 °C) in the Cu/Sn-3.5Ag/Ni microbump joints, shortening the processing time to 1 min. Microstructural analyses and die shear tests revealed that temperature gradients facilitated the diffusive growth of columnar Cu6Sn5 IMC to span the microbump joint, eliminating the weak Sn/IMC interface. The findings indicate a notable enhancement in shear strength as the IMC ratio rises. The shear strength of the 40 μm-diameter full-IMC-microbump joint reaches 86.7 MPa (2.1 times that of the 2 s bonded joint), while the shear strength of the 100 μm-diameter full-IMC-microbump joint reaches 101.8 MPa. Most importantly, the 40 μm-diameter near-full-IMC-microbump joint has a higher shear strength (85.0 MPa) than that of the 100 μm-diameter non-full-IMC-microbump joint (76.1 MPa) at 30 s of bonding, which counteracts the reduction in strength due to the reduced size. When the IMC ratio exceeds 75%, the fracture mode transitions from tough-brittle mixed fracture to brittle fracture, which correlates with interfacial evolution. This approach provides an efficient stacked-chip solution for three-dimensional packaging.