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May 2014 • The PCB Magazine 19 GRAPHITE-BASED DIRECT METALLIzATION continues Comparison to Conventional Electroless Copper There are few similarities between the graphite based direct metallization system and conventional electroless copper. Differ- ences in water consumption, chemical usage and environmental concerns have been well documented in the Design for the Environment Project on Making Holes Conductive, conducted by the United States EPA [2] . However, what are not clearly defined are the limitations of the processes in terms of ability of the pro- cess to successfully metalize small diameter, high aspect-ratio blind vias. Plating copper in 6-mil diameter, 3-mil deep blind vias is not a routine task. However, with respect to conventional electroless copper, significant improvements have been instituted in rack designs, filtration and solution movement to ensure that larger blind vias can be metalized. The real challenge is in the 3-mil diameter and smaller blind vias. Depositing a uniform coating of copper is compromised because it is believed that hydrogen gas bubbles (hydro - gen is a by-product of electroless copper re- action) lodges in the via, inhibiting plating. Any discontinuities in the electroless copper deposit lead to high resistance. The high re- sistance will lead to thin plating in the elec- trolytic copper process. Conversely, the graphite coating is con- tinuous and has very low resistance. Since the graphite process functions as a coating technol- ogy and not an electroless process, hydrogen gas evolution is not an issue. The electrode- position of copper proceeds by way of a mov- ing front mechanism. The high conductivity and continuity of the graphite coating permits the rapid deposition of copper, thus minimiz- ing any tapering effect in the hole-wall. This is extremely critical as aspect ratios of blind vias reach 1:1 and greater. Also, the higher aspect ra- tio through-holes present additional challenges as gas bubbles may lodge in the PTH, potential- ly leading to voids. This is not an issue with a graphite-based direct metallization process. Application of Graphite Metallization for Flexible Circuits and HDI While graphite based direct metallization is a production-proven process for rigid PCBs, the process is gaining a much larger share of the new capacity coming online for HDI and flexible circuits. A major reason is driven by the cost of equipment for electroless copper versus direct metallization. In addition, as has been described elsewhere in this paper, the pro - ductivity of a graphite-based direct metalliza- tion process is much greater than conventional electroless copper. Most importantly, however, Figure 8: High t g Fr-4 after permanganate desmear (left) and polyimide flex after plasma desmear (right).