Acid copper plating is one electrochemical process for manufacturing printed circuit boards. The action of the ionic current in an electrolytic copper plating tank is such that, during plating, any and all suspended particulates will be forced toward the cathode (the panel that you are plating) and deposited. If the particles are large enough (5 to 20 microns), their presence can disrupt the local electrical field enough to reduce the effectiveness of the levelers present in the electrolyte. The net result of this "electrophoretic" migration is rough plating with the contaminating particles trapped in the electrolytic layer right on top of a critical circuit element. In a production setup, the plating electrolyte is usually filtered continuously using one or more 1 micron filters specially made for "polishing" electro-chemicals.