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.
PCB Substrate Manufacturing
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
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.
Vias Electroless Plating
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”
PCB Pattern Plating
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., <0.001″) to be reliable. To address this, second electro-chemical plating is performed. Pattern plating deposits a material over the copper circuitry that will protect it during a subsequent etch stage. A material such as tin can be used to cover the copper traces to protect them. The copper is first thickened using an additional Electro-chemical plating process. Once applied, the tin is deposited on the pattern. After etching, the tin can be stripped off or left on depending on the manufacturer.
Solder Mask – Hot Air Surface Leveling (HASL) or Organic Coating
Solder mask is a protective insulating layer that goes over the outer sides of the PCB. This is typically the green material you see on boards. Copper is very susceptible to oxidation so if the pattern is going to be exposed to ambient air, they need to receive additional plating to protect against oxidation. Oxidation is the reaction of oxygen with the copper. During this process, the copper is actually consumed. So a thin layer of copper can actually be completely oxidized into an insulator. Solder mask is a layer of polymer that can be applied using either silk screening or a spray. The solder mask covers all conducting circuits on the board with the exception of any pads that components will connect to.
The solder mask can be dry film or liquid. Both are photo-imageable and require a very clean environment. After the mask is applied over the entire board surface, UV-blocking artwork is placed over the board. This artwork blocks UV radiation from solder mask so that the pads can be exposed for component placement during assembly.
UV radiation is then applied to the board. The mask that is protected by the UV blocking artwork remains soft. The mask not protected by the artwork is exposed to UV, which begins polymerization of the mask. Polymerization is the linking of the polymer molecules so the mask becomes hard enough to protect the board surface from scratches and contaminants. The mask must be hard enough to protect the board surface from scratches and contaminants. This is done by the polymerization (i.e. linking of the polymer molecules).
The mask that was protected by UV radiation is still soft and is removed by high-pressure water spray in a machine called a developer. Then the board is heat cured to harden the mask to get its final form.
This process is also referred to as Solder Coat or Exposed Conductor Plating. The pads of a board must receive a special surface finish to resist oxidation from long periods of storage while waiting for loading and to prepare them for the application of solder.
To accomplish this, a layer of conducting material is applied to the pads after solder masking.
Tin-Lead Solder (industry is trying to move to “lead-free” plating)
Gold Silver (Lead-Free compliant, ROHS)
This step is what gives the pads on the board the shiny look that we see.
Silk screening is the process of adding documentation to the board. The term silk screen refers to the process of transferring a pattern using a special stencil. A stencil is a sheet of material that has physical openings in it that represent the pattern to be transferred.
In a silk screen stencil, the openings are typically a set of small dots (i.e., a screen). The stencil is laid on top of the board and then a documentation material is applied to the entire board using a roller &squeegee or spray. After removal of the stencil, the documentation material remains in the pattern of the openings on the stencil. The board is hardened the documentation material by baking it.