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July 2014 • The PCB Magazine 89 OXIDE ALTERNATIvE PROCESSES: PERFORMANCE CHARACTERISTICS continues One should note how quickly and efficient- ly the process flows. The organo-metallic coat- ing (formed in the coating step above) is formed very quickly. The organo-metallic bonding pro- cess provides much greater through put than reduced oxide processes. Process Control The organo-metallic bonding process offers advantages over standard oxide processes in terms of consistency and ease of control. Replenishment of the organo-metallic bonding solution is usually done with a "feed and bleed" system. Typically components are pre-mixed in a side tank and then replenished based upon a panel counter, a copper sensor or a specific gravity controller. Each of the control methods has its advan- tages and disadvantages. A panel counter is easy to use and involves little maintenance but it does not accurately track copper build up in the bath when panel size and percentage of copper area vary widely. A copper sensor measures cop- per concentration by determining the amount of light that can pass through the solution. Due to the organo-metallic complex that is formed the etching solution may foul the light sensor resulting in the need for more frequent mainte- nance of the probe. The specific gravity control- ler measures the specific gravity of the solution, which is influenced by all of the components as well as copper concentration. If the bath components are controlled within a consistent range then specific gravity is a good indicator of copper concentration. If the bath components, especially sulfuric acid, are not controlled with- in the proper limits, then the specific gravity controller will not accurately control copper concentration. Despite the drawbacks of the control methods all three of the methods have been used successfully in field applications. As an added benefit, the concentration of all major components of the alternative bond- ing bath can be analyzed using simple analyti- cal procedures. This minimizes performance problems by ensuring the bath components are properly balanced. Proper balance of the com- ponents is necessary so that appearance, bond strength and resistance to thermal excursions are all acceptable. This is especially critical when using high-performance resin systems such as high T g FR-4, polyimide and PPO. Summary Achieving optimum performance with oxide alternatives depends on many factors includ- ing material selection, lamination parameters and process control of the key additives that make-up the system. At the very least process engineers must consider specific gravity as one method to maintain the copper concentration in the working oxide alternative solution. PCB Michael carano is with oMg electronic chemicals, a devel- oper and provider of processes and materials for the electron- ics industry supply chain. to read past columns, or to con- tact the author, click here. researchers from the uSc Viterbi School of engineering describe how they have overcome a major issue in carbon nanotube technology by de- veloping a flexible, energy-efficient hybrid circuit combining carbon nanotube thin film transistors with other thin film transistors. this hybrid could take the place of silicon as the traditional transis - tor material used in electronic chips, since carbon nanotubes are more transparent, flexible, and can be processed at a lower cost. electrical engineering professor dr. chongwu Zhou and his team devel- oped this energy-efficient circuit by integrating car- bon nanotube (cnt) thin film transistors (tFt) with thin film transistors comprised of indium, gallium and zinc oxide (igZo). "i came up with this concept in january 2013," said dr. chongwu Zhou. "Before then, we were work- ing hard to try to turn carbon nanotubes into n-type transistors. instead of forcing nanotubes to do some- thing that they are not good for, we found another ideal material for n-type transistors—igZo." New Hybrid Circuit Could Take the Place of Silicon