Transient liquid phase (TLP) bonding of Cu substrates was conducted with interlayer systems with the stacking sequences Ag–Sn–Ag (samples A), Ni–Sn–Ni (samples B), and combined Ag–Sn–Ni (samples C). Because of the low mismatch of the coefficients of thermal expansion, characteristics of the TLP process and mechanical and thermal behavior of TLP-bonded samples could be investigated without interference from thermally induced residual stresses. An ideal process temperature of 300°C, at which the number of pores was lowest, was identified for all three layer systems. It was verified experimentally that formation of pores resulted from volume contraction during isothermal solidification of liquid Sn into intermetallic compounds (IMC). Temperature and interlayer-dependent growth characteristics of IMC accounted for the increasing size and number of defects with increasing process temperature and for different defect positions. The shear strength was measured to be 60.4 MPa, 27.4 MPa, and 40.7 MPa for samples A, B, and C, respectively, and ductile fracture features were observed for Ag3Sn IMC compared with the purely brittle behavior of Ni3Sn4 IMC. Excellent thermal stability for all three layer systems was confirmed during long-term annealing at 200°C for up to 1200 h, whereas at 300°C the microstructure was driven toward Ag–Sn solid solution, accompanied by Cu diffusion from the substrate along grain boundaries and Cu3Sn IMC formation (A), and toward Ni-rich IMC phases (B). Combined IMC interlayers (C) tended to be transformed into Ni-based IMC when held at 300°C; intermixing into (Ni,Cu)3Sn was accompanied by pore formation.