The effects of immersion zincation to the electroless nickel under-bump materials in microelectronics packaging

One of the methods used for flip chip mounting is by solder bumping that utilizes Ni/Au under-bump metallurgy. Electroless nickel plating has been preferred over conventional electroplating due to its simplicity, good control, and reproducibility. Experiments are carried out to determine the optimum aluminum surface conditions for nickel adhesion, through studies of surface morphology and transformation during pretreatment. Zincation baths are used to condition the aluminum surfaces for nickel plating. The effects of the period and the number of times of the zincation process on the mechanical strength of the electroless nickel deposits are investigated. From SEM and AFM characterization, transitions of zinc grain size and surface roughness are observed. Grains are large with distinct grain boundaries for immersion time of 5 s, but decrease in size and lose their characteristic shapes as the zincation time increases. A double zincation produces a more compact deposit with smaller size grains compared to single zincation. Length of immersion time during the second zincation also affects the physical properties such as shear strength after 1 hr of electroless nickel plating on the 80 μm×80 μm Al bond-pads of a commercial bare microchip. The elemental composition transitions of the zinc deposits formed by different zincation times and bath compositions are also investigated using SEM-EDX and XPS