A PCB stackup design is the arrangement of repeating conductive and insulating layers within a PCB most often to accommodate signal flow through vias. The stackup determines how signals are routed and how the board handles power distribution and electromagnetic interference (EMI) on and between layers. Especially important for high-speed boards, is the following of important PCB stackup design guidelines. Doing so enables designers to effectively manage signal integrity issues, reduce crosstalk, and ensure board functionality.
Important Guidelines for PCB Stackup Design
Designing your board’s stackup requires the consideration of several factors, as listed below.
Common PCB Stackup Layer Configurations
Although circuit boards can be built to include dozens of layers, generally, stackups come in even numbers (e.g., 2-layer, 4-layer, 6-layer, and 8-layer). 2-layer configurations are used for the simplest designs or where minimal restriction on board routing real estate exists. However, demand for smaller devices and systems makes higher layer counts more typical for many applications.
PCB Stackup Design for 4-Layer Boards
Four-layer PCBs are often used in applications with moderate complexity and performance. A typical stackup for a 4-layer board includes two signal layers and two plane layers for ground and power.
- Top Layer (Signal 1): High-speed signals and components.
- Inner Layer 1 (Ground Plane): Continuous ground plane for signal return paths.
- Inner Layer 2 (Power Plane): Dedicated power plane to distribute power uniformly.
- Bottom Layer (Signal 2): Additional routing for signals
PCB Stackup Design for 6-Layer Boards
A 6-layer PCB offers more flexibility for routing and better control over signal integrity, making it more appropriate for high-speed applications.
- Top Layer (Signal 1): Critical high-speed signals.
- Inner Layer 1 (Ground Plane): Solid ground plane.
- Inner Layer 2 (Signal 2): High-speed differential pairs.
- Inner Layer 3 (Signal 3): General routing for less critical signals.
- Inner Layer 4 (Power Plane): Dedicated power plane.
- Bottom Layer (Signal 4): Additional signal routing.
4 and 6-layer stackups are often used in similar systems and share PCB stackup design considerations, as shown below.
PCB Stackup Guidelines for 8-Layer Boards
For even more complex designs, such as those involving high-speed data transmission and dense component placement, an 8-layer PCB stackup is ideal. This stackup provides increased flexibility for routing and superior control over EMI and impedance.
- Layer Arrangement: a typical arrangement is Signal 1 (top), Ground, Signal 2, Power, Signal 3, Ground, Signal 4, and Signal 5 (bottom). The increased number of layers allows designers to separate power and ground planes, dedicating entire layers to routing critical high-speed signals, increasing signal integrity.
- EMI Control and Shielding: The additional ground planes in an 8-layer stackup offer superior EMI control by “sandwiching” high-speed signals between ground planes, creating isolated paths.
- Improved Power Distribution: The 8-layer stackup has multiple dedicated power and ground planes, which stabilize current distribution and minimize voltage fluctuations —particularly useful in high-power applications. Additionally, the layered structure improves thermal dissipation, enabling the PCB to handle higher power levels.
- Precise Layer Separation for Controlled Impedance: An 8-layer PCB allows designers to control the separation between signal and ground planes.
- Applications: 8-layer PCBs are commonly deployed in high-speed computing devices, telecommunications equipment, and RF systems where signal integrity, low EMI, and efficient heat dissipation are crucial, i.e., boards with dense routing and high-frequency signal transmission.
Today’s compact circuit board designs necessitate that essential PCB stackup design issues be understood and guidelines followed. This is essential for rigid and rigid-flex stackups to ensure functionality and performance objectives are met, while adhering to dimensional and installation constraints.
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