Electroplating of solder was developed as a less costly and more flexible method than evaporation. The UBM is typically an adhesion layer of titanium tungsten (TiW), a copper wetting layer, and a gold protective layer. The UBM is sputtered or evaporated over the entire surface of the wafer, providing a good conduction path for the electroplating currents.
Bumping begins with photopatterning and plating a copper minibump on the bump sites. This thick copper allows the use of high-tin eutectic solders without consuming the thin copper UBM layer. A second photopatterning and plating of the solder alloy over the minibump forms the solder bump. The photoresist is then removed from the wafer and the bump is reflowed to form a sphere.
Electroplated bumping processes generally are less costly than evaporated bumping. Electroplating in general has a long history and processes are well characterized. The UBM adheres well to the bond pads and passivation, protecting the aluminum pads. Plating can allow closer bump spacing (35 to 50 microns) than other methods of bump formation. Electroplating has become more popular for high bump count (>3,000) chips becasue of its small feature size and precision.
Plating bath solutions and current densities must be carefully controlled to avoid variations in alloy composition and bump height across the wafer. Plating generally is limited to binary alloys.
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