The degradation of the Cu₆Sn₅ intermetallic compound layer caused by pore formation in fine pitch Cu/Sn microbump interconnects is reported in this study. Die-to-die stacking was carried out using the solid-liquid interdiffusion principle. The diameters of the microbumps on the top (Cu/Sn) and bottom die (Cu) were 15 and 25 μm, respectively. The stacking process was carried out in air atmosphere at 240 and 260 C with varying holding time at the peak temperature 10 s, 1, 2, 3, 4, 10 and 20 min. Flux and flux-containing no-flow underfill were used for stacking. Subsequent thermal storage experiments were done in N₂ and in air at 240 and 260 C for 10 min, 20 min, 1, 3, 24 and 96 h. The pores start to form after 1 min bonding at the edges of Cu₆Sn₅ exposed to flux/underfill. These pores propagate to the center of the interconnect with longer bonding time till the complete Cu₆Sn₅ layer is affected (after 4 min). The possible mechanism of the pore formation is the dissolution of Sn atoms from the Cu₆Sn₅ matrix due to the reaction between Cu ₆Sn₅ and flux residues. The remaining pore layer has the composition of Cu₃Sn. The results of a subsequent thermal storage show, that a complete transformation of Cu₆Sn₅ into Cu₃Sn without further degradation is possible after the removal of the flux residues.