Abstract
This study reports a number of experiments that were designed to characterize aluminum bond pad surfaces prior to electroless nickel immersion gold (ENIG). In the ENIG process, aluminum bond pads need special treatment to achieve successful nickel deposition and provide reliable interconnection of under bump metallurgy in advanced packaging. During this treatment process, the aluminum pad was cleaned, activated and then coated with a layer of zinc. Systematic study was carried out to determine the best parameters, through multiple and various exposure times of the zincation process and zincation solution concentration effect on the Ni/Au surface roughness and aluminum dissolution rate on the bond pad during multiple zincation process. The ball shear strength was evaluated between eutectic 37Pb/63Sn solder ball and under bump metallurgy (UBM) interfaces across multiple zincation process. Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM), Focused Ion Beam (FIB) and ball shear tester were used as analytical tools. The results suggest that the multiple zincation process consistently produces a smoother surface of ENIG UBM and consequently provides a better shear strength.
Highlights
Electroless nickel Immersion Gold (ENIG) as under bump metallurgy (UBM) of the bond pads on a silicon wafer is gaining interest in flip chip packaging because of its relatively low cost and simple process
In electroless nickel immersion gold (ENIG) UBM process, the surface treatments starts with surface cleaning to remove any contaminant that are presents on the bond pad surface, followed by an activation process to activate the bond pads for better nucleation for the subsequent process through the micro-etching of aluminum oxide
The information presented may provide the best condition for surface treatment processes to produce a better surface appearance and lead to a more reliable under bump metallurgy (UBM) using electroless nickel immersion gold (ENIG) technique
Summary
Electroless nickel Immersion Gold (ENIG) as under bump metallurgy (UBM) of the bond pads on a silicon wafer is gaining interest in flip chip packaging because of its relatively low cost and simple process. Pure aluminum and high aluminum alloys quickly form a tenacious, refractory ceramic oxide on the surface that requires special treatment to remove before achieving successful soldering This step applied to most of the methods in making interconnections from the bond pad on the Integrated Circuit (IC) die to the leadframe. A series of systematic experiments are carried out to have better understanding of these factors to the surface morphology, surface roughness and dissolution rate to the aluminum bond pad The correlations between these surfaces appearance to the solder ball shear strength is studied. The information presented may provide the best condition for surface treatment processes to produce a better surface appearance and lead to a more reliable under bump metallurgy (UBM) using electroless nickel immersion gold (ENIG) technique
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