The effect of lead free soldering on formation of black pad failure
By this time usable solder alloys for lead-free soldering had already been developed. NEMI (National Electronics Manufacturing Initiative) recommends Sn/3.9Ag/0.6Cu (±0.2%) solder alloy for reflow soldering instead of leaded solder alloys, due to its excellent mechanical properties and other good parameters. The next step is the choosing of suitable pad finish for lead-free soldering. In the age of fine pitch assemblies (QFPs (Quad Flat Pack), BGAs (Ball Grid Array) and CSP (Chip Size Package)) flat and planar plated surfaces are needed. Electroless Nickel Immersion Gold (ENIG) pad finishes are used, when there is a need for aluminum wire bonding besides soldering. At the interface of NiAu plating and the solder a failure can occur, called 'black pad'. Black pad is not only a cosmetic problem, but also a source of weak wire bonding and soldering joints. The question is, whether the lead free soldering affects the formation of black pad failure or not. Experiments were made to answer this question, and this paper will show the results.
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. Me2Sn4 and Me2Sn2 (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 in).
Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in low cost flip chip technology
Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in the Si/SiO2/Al/Ni–P/63Sn–37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni–P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni–P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni–P to Ni3P was induced by the depletion of Ni from electroless Ni–P to form Ni3Sn4. The interface between electroless Ni–P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni–P was determined. Based upon microstructural and diffusion r...