The pretreatment of aluminum bondpads for electroless nickel bumping

The Characterization of Al Bond Pad Surface Treatment in Electroless Nickel Immersion Gold (ENIG) Deposition

“…In conventional zinc solutions, first zincation is not uniform and only a small amount of zinc will be deposited. Only after the second zincation process, the zinc layer covers the entire surface and is considered [3][4][5][6][7][8][9] . However in this case, we clearly see that the coverage for each zincations are good regardless of the number of zincation processes, compared to the conventional solution.…”

“…EDX analysis as shown in Table 2 confirms the additional nickel layer instead of a solely zinc layer deposited during the zincation process. This zinc-nickel combination is thought to be an improvement over the pure zinc system because of a more homogenous attack on different aluminum-alloy compositions and providing improved nucleation to subsequent process with good zinc coverage on the bond pad surface [5] . The reduction and oxidation in this process are given as: Reduction: Zn 2+ + 2e Oxidation: Al Al 3+ + 3e -E 0 = -2.31 V Some samples of the first zincation process were then subjected to various dipping times during the zincation process.…”

“…If the three sigma error bars is not considered then the second zincation with 60s exposure is identical to the third zincation result. This might be the reason the second zincation is considered optimum for zincation process that has been practiced by the industry [3][4][5][6][7][8][9] especially with 60s zincation time. This result also proved to be in support of previous study, whereby the rough and coarse surface adversely affects the ball shear force [6] .…”

“…Zincation is one of the most suitable processes to activate the aluminum surface for the electroless technique [3] . Multiple immersion zincation with minimum aluminum attack/etching is necessary for achieving better adhesion strength of the zinc layer [4,5] . However, common practice is a double zincation process [3][4][5][6][7][8][9] .…”

“…Multiple immersion zincation with minimum aluminum attack/etching is necessary for achieving better adhesion strength of the zinc layer [4,5] . However, common practice is a double zincation process [3][4][5][6][7][8][9] . It is essential to etch the deposited zinc with a suitable acid solution between the consecutive zincation steps in order to successfully carry out the multiple zincation operation.…”