Creating PCB Pattern on the Substrate
Printed circuit board patterns can be created by a additive process or a subtractive process. During the additive process, copper is plated or added to the surface of the substrate in the desired pattern so that the remaining surface is not plated. In the subtractive process, the entire surface of the substrate is first electroplated and then the area that is not part of the desired pattern is etched away or subtracted.
The foil surface of the substrate is degreased. The panel passes through the vacuum chamber where a layer of positive photoresist material is firmly pressed against the entire surface of the foil. Positive photoresist materials are polymers that become more soluble when exposed to ultraviolet light. Vacuum ensures that no bubbles are trapped between the foil and the photoresist. A printed circuit pattern mask is placed on the top of the photoresist and the panel is exposed to intense UV light. Since the masks are clear in the area of the printed circuit pattern, the photoresist in these areas is irradiated and becomes very soluble.
The mask is removed and an alkaline developer is sprayed on the surface of the panel which dissolves the irradiated photoresist in the area of the printed circuit pattern, exposing the copper foil to the substrate surface.
The panels are then electroplated with copper. The foil on the substrate surface acts as a cathode in this process and the exposed foil area of copper foil is plated to a thickness of about 0.001-0.002 inch (0.025-0.050 mm). The area still covered by the photoresist can not act as a cathode and is not plated. Tin-lead or other protective coating is plated on top of the copper plating to prevent oxidation of the copper and act as a resist for the next fabrication step.
Photoresists are stripped from the plate using a solvent to expose the copper foil of the substrate between the electroplated printed circuit patterns. A circuit board is sprayed with an acidic solution that eliminates copper foil. The copper plating on the printed circuit pattern is protected by a tin-lead coating and is uninfluenced by the acid.