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JANUARY 2020 I PCB007 MAGAZINE 81 process is also gaining a much larger share of the new capacity coming on line for HDI and flexible circuits. A major driver is the higher cost of equipment for electroless copper versus direct metallization. In addition, as has been described elsewhere in this column, the pro- ductivity of a graphite-based direct metalliza- tion process is much greater than conventional electroless copper. Most important, though, is the inherent ability of the graphite system to readily adhere to a wide variety of laminate substrates and materials, including polyimide, PTFE, ceramic filled materials, PPO, PPE, halogen-free, and many other materials. As laminate suppliers continue to reinvent them- selves to stake out a position in higher technol- ogy, a versatile metallization process is critical to success. So, why be concerned with the different laminate materials? Aside from the obvious, successful metallization requires that excel- lent adhesion between the conductive coating (either electroless copper or conductive graph- ite) be achieved. It is well-documented that adhesion is favored when there is sufficient surface topography to promote the bond. This is especially true for electroless copper as the seeding of the palladium is required to initiate the copper deposition. However, as one views the SEM in Figure 2, the topography visible after permanganate desmear is rather smooth and unremarkable, further impacting the ad- hesion factor. A critical advantage for the graphite-based system is that the colloidal graphite binds to resin and glass without the need for a high surface area. It is important to recognize that electroless copper is a plating process, where- by a series of reactions take place in order to effect a deposit. In the case of the graphite- based metallization system presented herein, the mechanism is primarily a coating process whereby the binder technology acts to pro- mote adhesion of the graphite particles to even the smoothest of surfaces. Electroless copper lacks in this area. Fabricators must embrace changes in the way printed wiring boards are processed. Di- rect metallization based on colloidal graphite is an enabling technology for HDI designs and flexible circuits. Stay tuned for future columns on direct metallization. PCB007 References 1. IPC, "IPC 2018 Technology Trends Survey," and "IPC APEX EXPO 2019 Buzz Session." 2. Holden, H., "How to Get Started in HDI With Micro- vias," CircuiTree (online article), November 2003. Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 2: High-Tg FR-4 after permanganate desmear (L) and polyimide flex after plasma desmear.

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