How to Design High Multilayer PCBs
Modern-day consumer electronic devices such as smartphones and smart watches use multilayer PCBs. These boards provide excellent EMI performance and functionality.
Compared to double-sided and single-layer PCBs, these boards have more space for placing components. Their complexity, however, increases the design process and requires domain expertise.
The layer stack-up should be carefully strategized to ensure optimal performance. This includes signal integrity and power integrity.
Controlled Impedance Traces
Impedance control involves routing the PCB to feature signal transmission lines that have a defined impedance value. This helps prevent EMI issues, especially at high frequencies.
The impedance of a signal trace is determined by its dimensions and the materials used to make it. Copper thickness and layer build-up have a significant impact on impedance. As copper thickness increases, impedance decreases; however, the opposite is true for dielectric thickness. The best way to minimize the effects of these variables is by using impedance calculators online.
Another factor that impacts impedance is the width of a signal trace. The wider a trace is, the lower its impedance will be. High Multilayer PCB In fact, a trace that is too narrow will experience a voltage drop across the length of the trace. This is a significant issue for high-speed signals.
If the PCB manufacturer has to reroute some of these signals because their widths are too small, the overall performance of the board will suffer. To avoid this, the PCB designer should choose a width that meets multiple design requirements.
When defining a trace’s impedance, the designer should take into account the signal speed and the desired propagation delay. If several high-speed signals are routed simultaneously, they should be as closely matched as possible (within a specified mismatch tolerance). In addition, it is important to leave a large margin of error in the trace widths. This allows the manufacturer to easily locate these signals and make any necessary adjustments.
Copper Thickness
The copper used in a circuit board is essential for conducting current to and from the components. It is also the primary thermal conductor that dissipates heat from the components into the surrounding environment. This is why the thickness of a PCB’s copper layer can have a significant impact on its performance and durability. High-power applications often require thicker copper layers to handle the increased amount of current and provide adequate heat dissipation.
Copper thickness is a significant factor when it comes to designing a multilayer PCB because it has a direct impact on the stackup, impedance control calculations and trace width/spacing. A change in the copper weight of a single layer can have a cascading effect on other layers and may require adjustments to the conductor clearance.
It is important to discuss the fabrication capabilities and tolerances of your manufacturer when determining the appropriate copper thickness for your application. A qualified fabricator should be able to assist you in selecting the best materials for your application and help you avoid design issues that may cause manufacturing delays or cost overruns.
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Vias
As the layers increase in multilayer PCBs, there is an increasing need for vias to connect the varying traces on each layer. There are three different types of vias – standard (also known as through-hole), blind and buried. The standard vias are visible on the top and bottom of the board and allow electrical signals to travel through all stacked planes on a layer. Blind vias do not make a hole on the other side of the board and are used to reduce the size of the copper area on inner layers.
For high-speed signaling, it is important that signals take a direct route between layers. The presence of a via in the path forces the signal to take a detour, which can cause the signal to attenuate. The design of a multilayer PCB requires that the designers plan the layout of the vias carefully to ensure they are properly placed for high-speed signaling.
A good multilayer PCB must also have adequate heat dissipation to prevent overheating and short circuits. The layout of the PCB must incorporate thermal vias and copper pours to regulate and disperse the generated heat, especially in the power and ground planes. This will help ensure a robust and reliable system. Moreover, the PCB must be designed with manufacturability in mind to avoid expensive rework and delays during production. This can be achieved by using proper panelization techniques, well-thought out solder mask design and balanced copper distribution.
Lamination
When designing multilayer PCBs, designers must balance cost and performance. The best way to do this is by choosing a high-performance laminate such as FR4. This material has a low dielectric constant and high TG, which make it well-suited for high-frequency circuits. It also has a high strength-to-weight ratio and is flame resistant. FR-4 is also easy to manufacture, which makes it a good choice for mass production.
During the cap lamination process, a stack-up of prepreg materials and copper foils is assembled and then pressed together under heat and pressure. This process ensures that the layers are bonded together and prevents delamination. The core layer is usually made of FR-4, which provides structural integrity to the circuit board. The inner layer is then plated with copper and sandwiched by prepreg and stainless-steel patch layers. This structure is then cured under heat and pressure, which results in the final PCB.
It is important to keep in mind that the inner layers of a multilayer PCB can be subjected to thermal stresses, especially when there are multiple power and ground planes on the board. To avoid this, it is important to follow DFM guidelines and use the correct copper thickness. It is also crucial to select the right layer configuration for your application. For example, sensitive signals need to be separated with their own ground planes. This will allow them to maintain their return path, even when the circuit is powered on.