Reliability challenges in 3D LSI associated with mechanical constraints induced by Cu TSVs, μ-bumps and crystal defects, crystallinity in thinned Si wafer and metal contamination induced by Cu diffusion from TSVs and thinned backside surface are mainly discussed. Mechanical stresses induced by Cu TSVs and μ-bumps are strongly dependent on design rules and process parameters. DRAM retention characteristics were severely degraded by Si thinning, especially below 30 μm thickness. Minority carrier lifetime was seriously degraded by Cu diffusion from Cu TSVs as the blocking property of barrier layer in TSV is not sufficient. A dry polish (DP) treatment produced a superior extrinsic gettering (EG) layer to Cu diffusion at the backside. We suggest the nondestructive failure analysis using X-ray CT-scan to characterize TSVs connection and μ-bumps joining in 3D stacked LSIs.