Thick metal layers are often incorporated into the center of a PWB as an integrated heat sink. These metal layers often require clearance holes in them to provide for through hole interconnections to the circuitry on each side of the metal plate. These holes must be filled with a material that insulates the holes from the metal, can be drilled using common processing without fracturing or damage, and can be plated along with the layers on each side. Filling these holes can sometimes be accomplished in the same lamination cycle as the rest of the board, but sometimes requires a separate cycle for the hole filling process. The specific process will depend on the thickness of the metal core and the amount of holes and cutouts that may need to be filled.
Arlon has several options depending on the material type, metal thickness, and other design complexities. The 44N is an epoxy material that may work on up to 0.020 inch metal thickness. The 84N is a polyimide resin based material also used up to 0.020 inch. Above this thickness, or when the dielectric space must be kept to a minimum, the HF50 is a polyimide powder that can be applied to provide the extra resin necessary to fill thicker metal layers.