Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 136 of 707

electroplating solutions New Technology for Electroplating Metal Layers Aims to Improve Thickness Control BY G. CARRASCO, DR. J. HARRIS, T. BECKETT, AND E. RUBEL, CMC LABORATORIES, TEMPE, ARIZ. CMC Laboratories has developed a new technology for electroplating metal layers that results in significant improvements in thickness control.1 CMC's tech- nology, which is called "Smart Rack," utilizes active control of the plating process to improve thickness versus traditional passive controls such as current thieves. CMC's Smart Rack technology entails a new electroplating circuit as well as a completely different approach to rack design. Though this technology was developed for electronic devices or package applications, it can easily be applied in other areas that utilize precious metal plating. The focus of this new technology is to narrow the distribution of plated lay- er thicknesses across a plating rack. Narrowing this distribution can have a sig- nificant impact on precious metal plated layers, such as Au, Pt, Pd, Rh, and Ir. When a minimum thickness of these expensive metals is specified, as is typical, a narrower distribution results in less metal used for significant cost savings. A tighter plating distribution can also be important for other metal plating processes. For example, currently most wafer bump plating is done using low throughput, high tool cost fountain platers. For small wafer size (< 6 inches), it may be possible to use Smart Rack technology to bump multiple wafers at one time in a rack configuration. Since Smart Rack only improves part to part thick- ness control, not the thickness distribution across a part, the technology would not help for very large wafers. The ability to plate AuSn at the eutectic composition on die attach pads would also be very important in many high power electronic applications. However, since the composition of AuSn varies with current density, it is very dif- ficult to achieve the 80/20 eutectic across a large plating rack. AuSn compositions that are off eutectic will not melt at the 400C process temperature.2 Traditional electrolytic plating uses a plating rack that is made of a highly con- ductive metal such as copper or stainless steel. All of the parts being plated are attached to this rack. During the plating process, the rack is attached to a con- stant current power supply (plating rectifier). Current is pumped through the rack structure, forcing the plating reaction to occur at the interface between the conducting work parts and the plating solution or electrolyte. The plating rate at each individual part depends on a number of factors: • The electrical current density at the individual part • The availability of metal ions in solution at the location of the individual part • The availability of other electrochemical reactants at the plating interface which compete with the plating reaction 135

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