HOW TO MANUFACTURE PCB? – Embedded Flakes
There is a variety of methods to manufacture PCB. We will discuss the most common processes in PCB fabrication. The process of removing copper and leaving only desired interconnect (tracks) patterns on a sheet of copper attached to an insulator is called a subtractive method.
Following are the major process steps used in creating a PCB.
A substrate is manufactured from thin sheets of a dielectric material bonded to a sheet of electrically conductive material. FR4 is the most common substrate used in printed circuit boards. Epoxy resin is used to bond fiberglass to copper foil in the creation of FR4. A fire retardant is added to the substrate can be safely soldered in later processes.
Some other substrates are:
- polyimide/fiberglass, which can sustain higher temperatures and is much harder than FR4
- FR2-fire retardant coated phenolic/paper, which is cheap and used mainly in low-cost consumer electronics
- Flex circuits, usually polyimide, in some cases polyester, used in automotive and other applications where space and weight are at a premium. New technology allows adhesive less material for even thinner flex circuits.
After the substrate has been made, drilling machines drill holes of different diameters in the exact locations on the board. These holes are called vias. Via is a structure that electrically connects two different layers of copper in a PCB. For that, a hole of suitable diameter must be drilled to the core. Drilling can be done using mechanical drills or laser drilling. Mechanical drilling is mostly used approach. Laser drilling is used mainly for small holes.
There are two common techniques to remove material from the core to leave only the desired conduction paths.
- Photoengraving or photolithography
An image of the circuit pattern is transferred to the copper foil on the surface of the board with either a UV photo-resist film or an ink screening process. Another step is required to remove the resist material in the UV process. Incomplete removal of this resist material can cause solderability problems later. Resist residue on the copper will not allow solder to bond to the pad.
PCB milling is another subtractive technique that uses a milling bit (similar to a router or drill bit) to mechanically remove copper from the CORE leaving only the desired pattern.
Next step in creating via is to deposit copper into the holes in order to plate the inner diameter of the via with a conducting material. PCB via plating is accomplished by an electroless plating process in which a series of chemical reactions are performed to transfer copper atoms from a sacrificial source to the barrels of the via holes. In this process 0.0000150 – 0.000020 inches of copper is chemically deposited in the drilled holes. This plating provides a base on which more copper can be electrically plated. This process ends with a structure that connects different layers of a PCB. The “via” is also called a “Plated Through Hole”
The copper deposited from the Electro-less Plating step applies a thin layer of copper on the entire surface of the CORE in addition to inside the drilled via holes. The plating in via barrels is typically not thick enough (i.e.,