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In plastic power devices, the interfaces of Cu/EMC are most likely to delaminate under thermal loading, especially when moisture diffuses into the interface through EMC. In this work, the bare die samples were fabricated in standard commercial process. Shear test of Cu/EMC interface was designed to measure the strength of the interface with different lead-frame oxidation and moisture absorption time. The samples with pre-cracks on Cu/EMC interfaces were studied by the same shear test method and the critical value of J-integral was investigated by FEA (Finite Element Analysis). The results indicated that the shear strength of Cu/EMC interfaces and the value of J-integral declined dramatically with moisture absorption time. However, the effect of lead-frame oxidation is complex. The 2-D analysis of power device showed that the small initial crack, as small as 0.13 mm, will propagate along Cu/EMC interface when temperature is decreased to -55degC.