PrePreg orPreimpregnated Bonding Sheet
It’s the “glue”that holds the cores together. There are many types of materials, we use FR4 –a woven fiberglass cloth preimpregnated with epoxy resin – known in the industry as Bstage.
The resin is activated and “melts”during the lamination process from pressure and heat. It flows across copper features and exposed laminate on the core and as it cools bonds the layers
of foil and core together.
Inner layer core, copper foil and prepreg are bonded together under heat and pressure, sometimes in a vacuum, during the lamination process. The result is a panel with several layers of copper inside as well as the foil on the outside
The production process for both multi-layer and double sided panels is generally the same after lamination of the multilayer panels
Primary or First Drill
Holes of various sizes are drilled through a stack of panels (usually 2 to 3 high). The locations are determined by the board’s designer to fit specific components. Drilled
hole sizes are usually 5 mils larger than finished plated through hole sizes to allow for the copper plating process.
Deburr is an abrasive mechanical process that removes the raised edges of the metal or burrs surrounding the holes that occur during the drilling process. Any debris that may be left in the holes is also removed at this time.
Desmear – Multi-layer Boards Only
Desmear generally applies only to multilayer boards. It is a chemical process that removes the thin coating of resin from the inner layer connections that is produced by the heat and motion of the drill bits as
they create the holes. Removing the resin smear improves the electrical connectivity
Electroless Copper Deposition
Once the smear is removed, a thin coating of copper is chemically deposited on all of the exposed surfaces of the panel, including the hole walls. This creates a metallic base for electroplating copper into the holes and onto the surface. The thickness of the electroless deposit is between 45 & 60 millionths of an inch
Dry-film Resist Coating Outer Layer Panels
The same resist or light sensitive film used on the inner layers is used for the outer layers. The film covers the entire surface including the drilled holes
Outer Layer Expose & Develop
After dry film lamination the panel is exposed and developed using the same procedure used for the inner layer cores. Clear areas in the film allow light to pass through and harden the resist creating an image of the circuit pattern All of the drilled holes that are exposed will be plated through