Multilayer PCB Design Challenges
Among their standout attributes, multilayer PCBs are adept at mitigating electromagnetic interference. This is due to their meticulous layering, which allocates sensitive signals, power planes and ground lines to different layers.
The various layers are then combined with the core and prepreg in a lamination process under high temperature and pressure. The result is a functional and reliable circuit board with no air trapped between the layers.
Power and Ground Planes
Ground planes are flat copper planes designed to give the PCB a steady supply of voltage. They connect to power supplies and function as built-in capacitors, suppressing high-frequency noise and reducing the amplitude of voltage fluctuations on critical components. Ground planes are typically used in multilayer PCBs and help maintain signal integrity.
Ensure your ground plane has no gaps, voids, or breaks, as these can interfere with current flow and create EMI issues. It’s also a good idea to check the thickness and trace width of your ground planes to determine if they’re adequate for your system’s requirements.
Evaluate the current levels required by your circuits and components, as well as their intended voltage range. This will help you design your power planes with appropriate thickness and trace widths to handle the necessary voltage levels Multilayer PCB without exceeding the permissible temperature rise or causing voltage drops.
Consider utilizing multiple power planes for your multilayer PCB, especially for higher-speed applications. This helps you to isolate different voltage levels or power domains to reduce noise coupling between these regions and improve signal quality. You can further optimize your power plane design by implementing copper pour utilization, via stitching, and plane partitioning. This helps you to improve the effective plane area, minimize impedance, and create shorter return paths for current flow.
Controlled Impedance Traces
Signals pass through signal traces in a PCB to convey the information necessary for the board’s operation. However, these signals don’t travel freely, as they have to wait for a certain amount of time before they reach their destination. This delay depends on several different factors, such as the materials and the distance between signal traces. Controlled impedance is the practice of matching substrate material properties with trace dimensions and locations to achieve a specific value or range of impedance.
The first step in controlled impedance is to identify critical signals, such as high-speed data lines and clock signals. The next step is to determine the required impedance based on these signals’ specifications and standards. This information can be gathered through circuit schematics and other documentation.
Depending on the impedance requirements, a variety of design and manufacturing techniques can be used to ensure that the traces will meet these specifications. For example, the designer can specify the width, thickness, and spacing of the traces, as well as the dielectric materials and layers that are used to create them. The manufacturer can also use a process known as impedance watching to make sure that the traces are made to the specified tolerances.
Sierra Circuits can also manufacture test coupons with the same layer stackup as the main board, so that the manufacturer can verify that the traces will be within the impedance tolerances by using a tool like Time Domain Reflectometry (TDR). This method is particularly important for differential pairs since they have to have their lengths matched.
Buried or Blind Vias
A PCB via is a small hole that connects different internal layers of a multilayer board. Unlike through-hole vias that can be made to penetrate the outer layer, blind and buried vias only connect to specific layers.
Typically, a hole in a PCB is filled with copper to make a connection. However, it can be difficult to plate these holes, which can lead to signal attenuation and other issues. A better solution is to use blind and buried vias, which provide an alternative way to connect layers of a multilayer PCB.
This technique is commonly used in HDI (high density interconnection) circuit boards because it can save on space for component leads. It also eliminates the need for plating through-hole vias, which waste valuable space on layers that are not connected to the end-use device.
When a buried via is designed, it has a ratio between the depth of the hole and the drill diameter that must be met to ensure the Multilayer PCB manufacturer correct connection. This is why it is important to work closely with your custom PCB manufacturer during design.
While using buried and blind vias allows you to increase the function of your circuit board, it is crucial that the manufacturing process is carried out in accordance with its requirements. A failure to do so can result in costly production delays and faulty devices.
Thermal Management
PCBs need to disperse heat evenly across their entire surface areas in order to prevent thermal stresses and maintain circuit integrity. Uneven heating causes localized hot spots that result in damage and reduced lifespan. Understanding and addressing these issues is the key to successful multilayer PCB designs.
To keep temperature levels balanced, designers must understand their circuitry’s needs and select the appropriate layer configurations for each component type. For example, sensitive signals require a specific stripline layer configuration, while analog and digital circuitry need to be separated with their own ground planes. Additionally, high-power components need to be isolated from heat-sensitive parts to ensure optimal performance.
Choosing the right materials is also important. For instance, higher-frequency applications may require advanced materials like Rogers or polyimide to guarantee signal integrity and thermal stability. Lastly, proper trace width and spacing is essential for preventing crosstalk. Additionally, using buried or blind vias can reduce cost and weight by eliminating the need for through-hole components.
The final step in the fabrication process involves laminating all layers together with copper foil sheets and insulating materials like FR4. This is accomplished by applying heat, pressure and vacuum in a heated hydraulic press to bond the layers air tight. Then, precision drilling processes create holes for electrical connections between layers. These processes help create a finished multilayer PCB with the benefits of higher density, improved functionality, and enhanced signal integrity.
