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78 PCB007 MAGAZINE I MAY 2020 Article by Nikolaus Schubkegel Plasma, which consists of ionized gas atoms, is the fourth state of matter. On Earth, plasma does not occur naturally, but it is sometimes visible at high altitudes as auroras. But off- planet, elsewhere in the universe, almost all visible matter is plasma. Plasma is a mixture of positively charged atomic hulls, free elec- trons, free radicals, and neutral particles; the total electrical charge is neutral, conductive, and highly reactive. Due to permanent recombination, plasma lights can come in different colors. The colors depend on the nature of the gas [1] : • CF 4 : Blue • SF 6 : Pale blue • SiF 4 : Light blue • SiCl 4 : Light blue • Cl 2 : Light green • CCl 4 : Light green • H 2 : Pink • O 2 : Pale yellow • N 2 : Red to yellow • Br 2 : Reddish • He: Red to purple • Ne: Brick red • Ar: Dark red The plasma particles have great speed, and thus, high energy content. Irving Langmuir was the first who called ionized gas "plasma" [2] . Although there are several methods known to produce plasma, only one method is wide- ly used in manufacturing PCBs: RF discharge at 40 kHz or 13.56 MHz or microwave dis- charge at 2.45 GHz at low pressure 0.1–1.0 mbar. This method produces what is com- monly known as "cold plasma." There is also plasma at atmospheric pressure, with techni- cal applications. This article deals only with low-pressure plasma. Equipment Available to Produce Plasma Most plasma equipment is comprised of dis- continuous (batch) systems, although there are also continuous systems available. The batch systems have some volume of l.0 to 10 m³ and more. The common units in the PCB industry have volumes of approximately 0.2 to 3 m³. Continuous systems are available for flexible circuits, reel to reel, and rigid products as well. All systems have one or more vacuum pumps, a gas supply with a plurality of gases (oxygen, argon/nitrogen, tetrafluoromethane, ammonia, hydrogen and nitrogen, and some- times helium), a control unit with timer, and Plasma Applications in the PCB Industry