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We investigated electroless Ni uniformity on Al metal pads connected to different size pads or a pn junction for under bump metallurgy in flip-chip assemblies. In an electrically isolated pad, Ni thickness decreased as the pad size decreased. Because of nonlinear diffusion of Pb2+ stabilizer in the plating solution, fewer electrons were supplied to the smaller pad than to the larger pad by an anodic oxidation reaction on the pad surface. In pads smaller than 50 mum square, the Ni thickness increased when connected to a 100 mum square pad. This increase might be caused by electrons flowing from the 100 mum square pad to the smaller pad to produce an equipotential for the connected pads. In addition, the Ni thickness was increased by electrical connection to an n-type Si in the presence of fluorescent light illumination for a pn junction area larger than 100 mum2. For a pad connected to a p-type Si, however, Ni thickness decreased in comparison to that of an electrically-isolated pad, regardless of the light illumination or pn junction area. The change of Ni height on pads connected to the pn junction is attributable to photoelectrons injected into the n-type Si, or to electron-hole recombination in the p-type Si. These results indicate that the pads should be of the same size within a chip for better Ni uniformity. Moreover, blocking light during Ni electroless plating can eliminate Ni thickness differences due to an n-type Si connection.
Components and Packaging Technologies, IEEE Transactions on (Volume:30 , Issue: 3 )
Date of Publication: Sept. 2007