Electroless nickel bumping of aluminum (Al) bond-pads followed by solder paste printing is seen as one of the lowest cost routes for the bumping of wafers prior to flip-chip assembly. However, the electroless nickel bumping of Al bondpads is not straightforward and a number of activation steps are necessary to enable the nickel deposit to form a strong, electrically conductive bond with the Al. For the electroless nickel coating of mechanical components made of aluminum, a zincate activation process has been used for many years; however, extension of these techniques to semiconductor wafers requires careful control over these pretreatments to avoid damage to the very thin bondpads. This paper reports a number of experiments designed to characterize the activation of Al bondpads to electroless nickel plating, focusing on the effects of solution exposure time and bondpad composition. In addition, the results are discussed in the context of other studies presented in the literature to provide an understanding of the mechanism of the zincate activation process applied to Al bondpads.

Electroless nickel bumping of aluminum bondpads. I. Surface pretreatment and activation

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The Effects of Multiple Zincation Process on Aluminum Bond Pad Surface for Electroless Nickel Immersion Gold Deposition

Electroless nickel bumping of aluminum (Al) bondpads
followed by solder paste printing is seen as one of the lowest
cost routes for the bumping of wafers prior to flip-chip assembly.
However, the electroless nickel bumping of Al bondpads is not
straightforward and a number of activation steps are necessary to
enable the nickel deposit to form a strong, electrically conductive
bond with the Al. For the electroless nickel coating of mechanical
components made of aluminum, a zincate activation process has
been used for many years, however extension of these techniques
to semiconductor wafers requires careful control over these
pretreatments to avoid damage to the very thin bondpads. This
paper reports a number of experiments designed to characterize
the activation of Al bondpads to electroless nickel plating, focusing
on the effects of solution exposure time and bondpad composition.
In addition, the results are discussed in the context of other
studies presented in the literature to provide an understanding
of the mechanism of the zincate activation process applied to Al
bondpads.

Electroless nickel bumping of aluminium bondpads. Part 1 - surface pre-treatment and activation

This paper reports the number of experiments that were designed to study the effects of aluminum bond pad surfaces prior to electroless nickel immersion gold (ENIG). During this process, called as zincation, an oxide layer of aluminum will be replaced with a thin layer of zinc metal through the reduction and oxidation process. Zincation process is very important process to provide trustworthy interconnection of under bump metallurgy in advanced packaging. The study was carried out to determine the best conditions, through the various zincation exposure times and multiple zincation process to the surface topographic and morphology. In addition, for further understanding the effect of zincation process, the study includes of investigating aluminum dissolution rate and ball shear strength between eutectic Pb/Sn solder ball and under bump metallurgy (UBM) interfaces. Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM), Focus Ion Beam (FIB) and Intellectest STORM series FA1500 were used as analytical tools in this study. The result suggests, the triple zincation process consistently produces a smooth surface of ENIG UBM and consequently a better shear strength with only minimum aluminum dissolved.

The effects of zincation process on aluminum bond pad surfaces for electroless nickel immersion gold (ENIG) deposition

Al surface morphology effect on flip-chip solder bump shear strength

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 /spl mu/m/spl times/80 /spl mu/m 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.

G.-J. Qi

Papers

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