What Is a Multilayer PCB?

A Multilayer PCB is a circuit board consisting of several conductive copper layers separated with insulating materials. This enables higher signal integrity and improved functionality.

However, a multilayer PCB can have some downsides that you need to consider before choosing it for your next project. This includes increased manufacturing time, cost-inefficiencies and potential design issues.

High Layer Count

Multilayer PCBs are characterized by having a high layer count. These circuit boards are made up of many layers, including insulating and copper layers. These are stacked one upon another to form a printed circuit board (PCB).

Multilayer boards have metallized holes that serve as electrical connections between layers. These plated holes are called vias or traces. These conductive pathways may be passersby, blind, or buried. The buried or blind vias are buried inside the layers, whereas the pass-by ones are on the surface of the layers.

Because of the complexity, a multilayer PCB requires a more experienced designer and manufacturer to build it. It also has a higher cost compared to single- and Multilayer PCB double-layer options. But, for many applications, the added functionality, performance, and reliability of a multilayer PCB are worth the extra investment in design and production costs.

Moreover, the high-layer counts of multilayer PCBs allow them to have better impedance control and other functionalities that can boost their overall performance. These characteristics make them ideal for complex electronic devices and advanced technology applications. They can also help reduce the size of a device while maintaining its functionality and power requirements. Lastly, the durability and ruggedness of multilayer PCBs can provide peace of mind for users when using the product in harsh or hazardous conditions.

Controlled Impedance Traces

The traces in a multilayer PCB need to be designed with controlled impedance in order to keep high-speed signals moving smoothly. This is important to prevent problems such as signal distortion, reflections, and electromagnetic interference (EMI). These issues can cause a device’s functionality to fail.

This is especially true when a PCB must operate at high frequencies. To avoid these problems, engineers and PCB fabricators must use controlled impedance techniques that include precise layer stack-ups, optimized trace routing, and accurate impedance calculations.

Trace impedance is influenced by several factors, including the copper thickness and dielectric thickness. When designers need to increase the impedance capability of a circuit, they may reduce the copper thickness and dielectric thickness, or use thinner cores. However, this can cause a reduction in the board’s overall performance.

A more effective solution is to use a stripline configuration, which sandwiches the trace between two reference planes. This offers full shielding of signal traces and helps meet EMI and RF requirements. However, it does require thicker cores to maintain impedance values.

It is essential for both designers and manufacturers to collaborate on the impedance specifications of a multilayer PCB. This is because vague specifications can lead to delays in production as the manufacturer tries to understand the exact parameters that need to be controlled. For example, a designer may specify that the traces need to be controlled, but leave out details such as copper weight and trace width.

Buried or Blind Vias

When it comes to vias on multilayer PCBs, there are different types that can be used. The most common is a through hole via which passes through the entire circuit board connecting all layers at once, the other is a blind or buried via which connects only some of the inner layers, but does not expose itself on either the surface or outer layer.

The process of making a buried or Smart door lock supplier blind via is very different from through-hole vias. First, the blind or buried holes must be drilled before the layers are laminated. Then the holes need to be copper plated and made conductive. Finally, the holes must be filled or plugged with polymer to prevent solder from entering and degrading the connection.

Blind and buried vias are typically used for high-density interconnect PCBs, or HDIs, to increase the layer count and provide space for more components. They are also useful for reducing signal lengths, which can be problematic in high-speed circuit boards.

Because buried or blind vias require multiple lamination steps compared to through-hole plating, they can add significant cost to the board. For this reason, it is recommended to only use them when necessary in a design. This helps to ensure that the board has the proper performance and reliability required for your specific application.

Thermal Resistance

Several aspects of a multilayer PCB affect its thermal resistance. One is the stackup. A good design aims to separate copper planes by their logical function (signal, power, ground). This prevents a signal from crossing the same level twice and disrupting its quality or speed.

The other key factor is the substrate material. PCBs made from materials with optimal thermal properties stand up better to high heat. They also have lower thermal resistance values, meaning they dissipate heat more quickly and efficiently.

To improve a PCB’s thermal resistance, designers can choose to use heavier copper conductors or alternative substrate materials with higher thermal properties, such as ceramics. They can also opt for metal core PCBs to dissipate heat more effectively from the internal components.

Another way to improve thermal resistance is to use buried or blind vias. These are holes or via barrels that do not connect to an inner layer, but penetrate through the board’s insulating layers instead. These help reduce the amount of heat that is transferred to the outer layer, which can disrupt or even damage the integrity of the signals. They are usually placed in areas that need to be insulated. However, if they are used too often, they can cause problems such as ghosting or signal reflections. For this reason, it is important to keep the number of buried or blind vias to a minimum, while ensuring they are properly sized to minimize heat transfer.