A microstructure-based electromigration model of Cu interconnects is proposed. Mechanisms of scaling and critical length effects of Cu electromigration are studied by transmission electron microscopy and statistical failure analysis. The results show that the lifetime of electromigration is reduced with Cu grain size decreasing when the width of interconnect is scaled down. Electromigration failure is not observed when the interconnect length is smaller than the critical length due to insufficient vacancies for voiding the whole Cu grains. Some small grains are vacated at the cathode end when the interconnect length is larger than the critical length during the testing. The proportion of failures increases and the lifetime decreases with interconnect length increasing. The failure time is dependent mainly on Cu grain size, and the failure lifetime and failure proportion fluctuate with grain size varying when the interconnect length is beyond the diffusion length.