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The simulation results show that larger contact opening may not benefit on relieving current crowding effect in the flip-chip solder joints. For the solder joints with a 10-mum thick Cu UBM, the diameter of the contact opening for the lowest current crowding effect is 60 mum. It means that using larger or smaller diameter will increase the current density in the solder bumps. In addition, the optima diameter of the contact opening with a thin-film UBM is larger than that with a thick UBM. In this letter, the optimal design rule of contact opening for relieving current crowding effect in solder joints will be provided.