While using Cu electroplating to metallize silicon cells is of great interest, the long term reliability of such cells have not been well understood. In this paper silicon solar cells metallized with electroplated Cu were thermally stressed. The cell performance was characterized in conjunction with microscopic structure analysis to understand the long-term degradation mechanism and the effect of each differentmetal layer. The Ni silicide layer had little impact on the performance degradation at 200°C. Increasing the thickness of a second Ni layer between the silicide and Cu significantly delayed this degradation, acting as a diffusion barrier. A series of cells annealed at different temperatures were used to extrapolate the degradation kinetics. Two different mechanisms were observed. Further silicidation of the second Ni layer was observed and was believed to cause the degradation at a higher temperature of 250°C. However, this silicidation reaction rate dropped quickly as the temperature decreased and Cu diffusion became the dominant mechanism for cell degradation at temperatures below 200°C. These two different mechanisms cross over at about 200°C in this study, where both silicidation and Cu diffusion were observed. An improvement in the cell lifetime by replacing pure Ni with NiCo alloy was studied.