Heavy Copper and Drilled Hole Fill
Thick metal layers are often incorporated into a PWB’s as an integrated heat sink or heavy metal ground plane. 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 and etched copper features 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 number of holes and cutouts that may need to be filled.
Arlon offers options depending on the material type, metal thickness, and other design complexities. The 44N is a filled epoxy resin based preprep that may work on up to 0.020 inch metal thickness and meets the requirements of IPC-4101/98. The 84HP which contains additional ceramic fillers to prevent shrinking and cracking, is recommended over 84N for hole filling and heavy metal ground planes. 84HP is a polyimide resin-based prepreg which can be used up to 0.020 inch. Above this thickness, or when the dielectric space must be kept to a minimum, the HF-50 is a polyimide powder that can be applied to provide the extra resin necessary to fill thicker metal layers.
Arlon’s 84HP is a pure polyimide resin with proprietary micro-disperse ceramic fillers engineered for the filling heavy copper ground planes and clearance holes. The 84HP is commonly used in conjunction with Arlon’s 85HP, which is also a filled pure polyimide resin with the same proprietary micro-dispersed ceramic fillers as the 84HP to prevent cracks from forming during mechanical drilling operations and thermal stressed environments. 84HP conforms to IPC4101/40, /41 and is validated to IPC4101/43 . 85HP is validated to IPC4101/40, /41 and /43.