Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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for each individual part. The goal of the MCC is to monitor the CCC output and make any required adjustments to keep the output current at the required val- ue (which is stored on the computer). It is clear that an individual CCC unit for each part is necessary in order to have maximum flexibility in adjusting the electrical current at each part. Furthermore, this flexibility is at the core of the Smart Rack technology's ability to dramatically narrow plating layer thickness distributions. (A schematic of the control circuit is shown in Figure 7.) APPLICATIONS FOR SMART RACK TECHNOLOGY There are a number of applications where Smart Rack technology can have a strong impact in improving performance and manufacturability. • Noble metal plating to reduce the amount of material required to meet minimum thickness and, thus, lower cost. Key metals would be Au, Pt, Pd and Rh. • Rack plating AuSn for solder or eutectic die attach. For this application, Au and Sn layers are deposited on plated surfaces and then diffused. The deposited layer thicknesses must be very consistent (+/- 3%) in order to achieve eutectic composition melting. • Uniformity of thick plated layers such as Cu conductors. Because the conductors can be very thick (50–500 µm), large variations in Cu thickness can lead to dimensional tolerance issues for the final plated part. • Bumping wafers that are 6 inches or less on a plating rack rather than using a fountain plater. The advantage is higher throughput and lower tool cost. SUMMARY This paper has described a new electroplating technology that is focussed around individual electrical control at each plating site. This electrical control is achieved by a special control circuit that monitors and adjusts the electrical current dur- ing plating at each site to a pre-set value. This approach also requires a plating rack that is made from an electrically insulating material with metal traces to each plating site. This type of rack can be made using PCB fabrication technology to lower cost and weight. If electrical current is set to the same value at each plating site, a significant improvement in the plating thickness distribution can be achieved. However, if small additional adjustments are made that boost the thickness in areas below the mean value, and reduce the thickness in areas above the mean value, even a narrower distribution can be attained. The basic electrochemistry behind this observed phenomena was discussed. Data was presented for Ni plating and Ag plating. For Ni, the distribution using a traditional "shorted" rack, where all of the parts are connected to each other and to the cathode, had a distribution of +/- 20%, which is typical for Ni sulfamate. If all parts were run at an individually controlled and equal value, the distribu- tion width dropped to +/- 8.5%. If each plating location current was optimized, 141

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