http://slideplayer.com/1555517/5/images/30/Electroless+Copper+Plating%3A+The+board+is+first+sensitized+by+immersing+it+in+a+solution+of+stannous+chloride.+The+stannous+ions+are+absorbed+on+the+board+surface%2C+particularly+onto+the+exposed+resin+of+the+hole+walls.+This+is+followed+by+immersion+of+the+board+in+an+acidified+solution+of+palladium+chloride.+The+palladium+ions+are+reduced+to+the+colloidal+state+and+form+a+thin+layer+which+catalyses+electroless+copper+deposition.+Electroless+copper+deposition+takes+place+in+a+bath+with+solution+containing+copper+sulphate%2C+sodium+hydroxide%2C+formaldehyde%2C+a+reducing+agent+and+other+special+additives.+Here%2C+the+copper+ions+are+reduced+to+metallic+copper.+This+results+in+deposition+of+copper%2C+whose+thickness+is+determined+by+the+duration+of+the+board+in+the+solution.+Usually%2C+a+thickness+of+about+40+microns+of+copper+is+built-up+on+the+base+copper+and+on+the+hole+walls..jpg