In the case of two- layer and multilayer printed circuit boards, vias must be made between the upper and lower copper-clad printed circuit boards. These vias or through holes are Plated Through Holes (PTH).
They are small holes with diameters of 0.3 mm to 1.0 mm, which have copper or metal layers on the inside walls. The upper and lower contact points of the Plated Through Holes are surrounded and form an electrical contact with the conductive paths.
Plated through holes can be coated with a thin layer of copper through a chemical deposition process. During the deposition process, the copper is also deposited in the hole walls of the vias. The copper layer thickness is about 3 mm after chemical deposition, and about 5 µm after electroplating. Another process for vias is metallization. In this process, the walls of the drill holes are covered with a thin graphite layer. Another alternative is the Any Layer Internal Via Hole ( ALIVH) process, in which the electrical connections between the individual layers are made with a conductive paste.