Phase formation in Cu/Ni/Sn thin film systems

As the void formation in the Cu3Sn-phase in standard flipchip micro-bumps is a concern, several stacks containing Ni in the UBM are evaluated for their capability of forming the preferred intermetallic (Cu, Ni)6Sn5-phase. For this purpose two thin film stacks were processed. The first thin film stack, referred to as F1 consisted of a 1.5 μm Ni, 0.5μm Cu and 3μm Sn, while the second one, F2, consisted of 10 μm Cu, 0.5 μm Ni and 3 μm Sn layer. By following the phase transformations in these two different thin film stacks during ageing at 150, 175 and 200°C for three different ageing times, an adequate stacking sequence and thickness of the Cu and Ni layers, that enables exclusive formation of the preferred (Cu, Ni)6Sn5-phase, was determined. The results obtained showed that only in the F1 thin film stack the preferred (Cu, Ni)6Sn5-phase forms exclusively. For that reason this stack was used for further diffusion study in order to determine the appropriate diffusion parameters, such as, the activation energy and the pre-exponential factor for the growth of the(Cu, Ni)6Sn5-phase. Knowing the values of these diffusion parameters is crucial for the prediction of the amount of the formed intermetallic phase and the UBM consumption for any selected thermal budget.