The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes.  These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper. COUNT 5
QSN1 What do the hole walls in two-layer PCBs have to be before they can be electroplated? ANS1 conductive QSN2 What's the minimum number of layers a PCB can have to be considered "multi-layer"? ANS2 three QSN3 What process does a multi-layer PCB usually undergo before plating to make sure the layers have good connectivity with each other? ANS3 de-smear QSN4 What type of boards undergo the etch-back process? ANS4 high reliability QSN5 Etch-back removes glass fibers and what other material? ANS5 resin