The effect of pretreatment process on electroless nickel bumping for different Al pads

We present a study on optimization of pretreatment process of the electroless plating to obtain good aspect and process steadiness of nickel UBM on different aluminum surface morphology. Pitch 250μm and circular Al pads with 85μm opening in the passivation layer were prepared for our study. Six kinds of aluminum pads were produced by the sputtering system: Varian-3180. The compounds of the Al targets were 98.95%Al-1%Si-0.05%Ti in the Varian sputtering system and some set parameters of sputtering system were varied, the heterogeneous surface morphology of aluminum samples was gained in our research. All the samples underwent the electroless zinc/nickel/gold plating with low kinds of different pretreatments. The effect of aluminium surface morphology on the nickel UBM was observed. The results demonstrated that the pretreatment of Al pad surface by heat sodium hydroxide and mixed acid provided denser zinc seeds. That led to better electroless nickel plating. The reasonable and acceptable pretreatment of Al pad surface is critical for the nickel UBM.