A multilayer PCB (Printed Circuit Board) is a PCB board with four or more conductive layers stacked on top of each other between insulating layers. They are commonly used in electronic devices that require a high level of complexity and compactness, such as computers, smartphones, and other consumer electronics.

In PCBs, the conductive layers are made of copper, which is etched to create the traces and pads that connect the components on the board. The insulating layers are made of materials such as fiberglass or epoxy resin, which provide electrical insulation between the conductive layers.

The process of manufacturing multilayer PCBs involves stacking and laminating together the layers of copper and insulating material. Holes are drilled and plated with copper to produce vias. Vias are small copper-plated holes that allow electrical signals to pass through the insulating layers and connect to other copper layers of the PCB. In more complex designs, blind and buried vias that do not go through the entire board may be utilized to increase space for routing traces.

Multi-layer PCBs offer several advantages over single or double-layer PCBs, including higher circuit density, better signal integrity, and improved thermal management. They also allow for greater flexibility in routing the traces and can reduce the overall size of the PCB. However, multi-layer PCBs are more complex and expensive to manufacture than single or double-layer PCBs, and they require specialized equipment and expertise.

PCBsupplierChina talk：How are multilayer PCB  produced?

The main difference between single, double and multilayer PCB production is the additional lamination step required to attach extra copper layers together. The process below describes the production of 4-layer boards which is similar to boards with a greater number of copper layers. 

Inner Layer copper etching

The inner layers are etched to produce the internal circuitry. A photosensitive chemical film called photoresist is applied, exposed to UV light through a photomask, and then etched away. The copper exposed by the mask is etched away, leaving the circuit pattern. For four-layer boards, this is similar to two-layer boards, where copper is etched on a copper-clad core, however, regular vias are not drilled at this stage.

Lamination

The copper core is laminated with prepreg insulating layers and copper foil according to the prescribed stack-up under intense heat and pressure. The epoxy in the prepreg glue the layers together.

Outer layer plating and etching

The outer layers are plated and etched to produce the outer layer circuitry.

Drilling

Holes are drilled through the boards to create the plated through holes, including vias. This is done using high-precision CNC drilling machines.

Hole plating

The inside walls of the drilled holes are plated with copper to provide connections between layers. This is done using an electroplating process.

Solder mask and silkscreen application

Solder mask and silkscreen is applied as with single and two-layer boards.

Drilling and Routing

Drilling of non-plated holes and board routing is performed as with single and two-layer boards.

Blind/Buried vias

Buried vias drilled after inner layer copper etching. Blind vias are typically laser-drilled and processed separately after lamination.

How do the vias in a multi-layer board affect the overall performance of the circuit?

Vias are an essential component of a multi-layer board, and they can have a significant impact on the overall performance of the circuit. 

Signal Integrity

Vias can affect the signal integrity of the circuit by introducing parasitic capacitance and inductance, which can lead to signal degradation. The size and placement of the vias can have a significant impact on the signal integrity of the circuit.

Power Integrity

Vias can also affect the power integrity of the circuit by introducing resistance and impedance in the power and ground planes. This can lead to voltage drops and signal noise, which can cause malfunctions or failure of the circuit.

Thermal Management 

Vias can also play a critical role in thermal management by providing a path for heat dissipation. The size and placement of the vias can affect the thermal performance of the circuit by influencing the flow of heat.

Manufacturing

Vias can affect the manufacturing process of the board by increasing the complexity and cost of the manufacturing process, especially blind and buried vias. Advanced vias may require laser drilling, plating, filling and capping, which can add additional time and cost to the manufacturing process.

The reliability of vias is crucial to the performance of multilayer boards and since defects in the internal structure of multilayer boards are harder to detect, designers should choose manufacturers who have a proven track record in multilayer PCB manufacture. 

PCBmanufacturer learned reliable multilayer PCBs are NextPCB's specialty and they pride themselves on being able to produce up to HDI III blind/buried via boards and up to 32-layer multilayer PCBs.