Herein we report on a copper (Cu) sinter die-bonding paste that sinters under pressureless bonding conditions for power devices operating at high temperatures. The shear strengths of pressureless-sintered Cu on four different metallization layers (Ni, Cu, Ag, and Au) were studied by experiment. The sintering behavior of Cu nanoparticles and diffusion coefficients at interfaces between a bulk Cu layer and the metallization layers was also evaluated by molecular dynamics (MD) simulation. After aging (573 K, 8 h), the shear strength of the Cu sintered to the Ni and Cu layers increased, whereas that of Cu sintered to the Ag and Au layers decreased. The interdiffusion at the interfaces between the sintered Cu layer and the Ag or Au layer increased the interfacial porosity of sintered Cu, which decreased the shear strengths in the sintered Cu/Ag and Cu/Au systems. In contrast, the interfacial porosity between the sintered Cu and the Ni or Cu layer hardly changed after aging. MD simulations revealed that Kirkendall voids were promoted by higher interdiffusion coefficients and a higher ratio of intrinsic diffusion coefficients between a bulk Cu layer and an Ag or Au layer. This consequently increased the porosity of Cu sintered near the interfaces.