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84 PCB007 MAGAZINE I MAY 2020 end ink, as well as the adhesion of conformal coating and other coatings. Plasma creates a wettable surface without discoloring the solder mask. As a result, the surface tension is strong- ly increased (plasma treatment with O 2 at low pressure, one step, 10% inert gas). Desmear of Hole Fill Paste Before Plating [3] Plasma treatment is a good alternative to a wet process with permanganate. This process in one step is as follows: 1. Mixture of CF 4 and O 2 with 10% of an inert gas, nitrogen, argon, or helium (5–30 minutes, depending on the energy of the RF generator). This process in three steps is as follows: 1. Oxygen until 90°C is reached on the board surface. 2. Mixture of CF 4 , O 2 , and 10% of an inert gas (10–30 minutes). 3. Oxygen to burn the residues (5–10 minutes). Cleaning of a Gold Surface Before Soldering and Bonding Plasma cleaning of a gold surface (ENIG, electroless, electroplated, soft gold, hard gold), full body or selective, significantly improves the solderability and wire bonding, as the disper- sion will be smaller, and the pull-off values for the wires and solder joints are much more con- sistent. There are several gas mixtures to per- form this cleaning process; very good results are obtained with argon plasma in one step. Cleaning of Surface: Deoxidation Before Soldering Plasma cleaning of PCBs before soldering is a viable alternative to wet cleaning processes and is done in one step. Hydrogen, or a mix- ture of hydrogen with argon, is used to make up the plasma (10 minutes, give-or-take). Drilling of Polyimide and Flexible Circuits Plasma treatment is an elegant alternative for polyimide machining (drilling) or laser ma- chining. With plasma, holes, via holes, and blind vias up to a diameter of 50 µm can be generated. The plasma drilling process is com- pleted in one step, with a mixture of oxygen and tetrafluorocarbon with 10% argon (nitro- gen). Milling and the creation of irregular con- tours are also possible with this technology. Surface Activation of Polyimide for Fully-additive Copper Plating For flexible circuits, a flexible polyimide cop- per clad is generally used. Surface patterning is done with photolithographic wet subtractive techniques. For the highest pattern resolutions, it is possible to use additive patterning. Then, plasma can be used to achieve the best surface activation for palladium adhesion. Plasma- based surface activation is performed with a mixture of hydrogen with nitrogen in one step at low pressure. With this treatment, the amine groups are created on the polyimide surface, which ensures a good palladium adhesion. Panel/Layer Descumming [3] Descumming is the removal of resist resi- due left over from the circuit image developing process. Developing the resist on panels and layers with fine pitch design features will sometimes leave a developer scum between the circuits. If this residue is left on through plating or etch- ing, the residue will cause an electrical short. Plasma can be used to remove scum between the fine pitch features. The process can be performed in one step with oxygen. The duration of plasma treat- ment depends on the power of RF/microwave generator, and the scum thickness that needs removal (15–30 minutes is usually sufficient to remove the variable thickness of resist sum). The Final Surface Finish of a PCB for Copper Preservation (Semblant) The Semblant plasma finish (SPF™) is de- posited using a plasma polymerization pro- cess in a vacuum chamber. The process cre- ates a dense, highly crosslinked, ultrathin fluoropolymer coating on the surface of the

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