The Electrochemical Effects of Immersion Gold on Electroless Nickel with Conclusions Oct 10th 2009

We have analyzed the effects of immersion gold (i-Au) and electroless nickel (e-Ni) thickness, on the reliability of a semiconductor device which has special constraints on the absolute Ni and Au layer thickness. The electrochemical reactions involved in the deposition of immersion Au over Ni involves a substitution process by which Ni atoms are replaced by Au atoms. We demonstrate that in some cases this reaction changes the microstructure of the nickel near the perimeter of the pad where the nickel layer overlaps the passivation layer of a semiconductor die, forming a mechanical seal to the passivation. This seal is extremely important for hermetic applications where one wants to keep moisture and contaminants away from the active pad area of the semiconductor die. We have found that extended exposure of this nickel-to-passivation interface, to the Au plating chemicals, damages this interface, leading to reliability issues. In this work, we analyze the phenomena by changing the Ni and Au layer thicknesses and observing its effects on MOSFET devices which are subjected to accelerated reliability stress testing in an autoclave. We conclude with a suggested solution to minimize the effect.