Electroless Ni/immersion Au interconnects: Investigation of Black Pad in wire bonds and solder joints

Black Pad was observed on Electroless Ni/Immersion Au (ENIG) wire bond pads. Thick immersion Au on highly corroded electroless Ni was detected. It was determined that the pads were electrically connected to the Cu ground plane due to a Ni bridge formed inside normally open photovias. The mechanism of the bridge formation was verified and preventative actions were taken; it was demonstrated that formation of Black Pad could be switched on and off. The mechanism of Black Pad formation is proposed to be defective ENIG plating involving variation of both the electroless Ni and immersion Au plating processes. The intermetallic structures of solder joints on the above pads were studied. The study was conducted on both defective and non-defective pads to show differences in intermetallic structure and composition. Me3Sn4 and Me3Sn2 (Me = Cu, Ni, and Au) intermetallics were formed on non-defective pads, which nucleated on the Ni layer and grew inside the molten solder. However, only the Me3Sn intermetallic was formed on defective pads inside the corroded Ni layer. Both mechanisms of intermetallic formation were found on pads with mildly corroded Ni and intermediate Au thickness (4.5-7 mu in).