Fan out wafer level packaging is a potential solution for combining high electrical performance and low cost. However, with the shrinkage of the package, the mechanical reliability of the copper redistribution layer becomes an important issue. However, based on the processing of fan out packaging, the copper lines were annealed several times during the process. The annealing process is harmful to traditional strengthen mechanisms but not to columnar grains with high-density twinning structure. In this study, the technology of electroplating highly <111>-oriented copper lines was introduced to solve the reliability issue in copper RDLs. By the nature of nano-twinned Cu, the performance of thermal cycling test (TCT) lifetime is much better than normal copper, and the failure mechanisms were discussed in this study.