Proper layer arrangement is crucial in PCB design, especially for minimizing issues like crosstalk, EMI, and signal integrity problems. Here are some key considerations:
- Stack-up examples
- In the first stack-up shown in this webinar snippet, signal layers are placed adjacent to each other, which increases susceptibility to crosstalk and EMI. Additionally, the ground plane is far from the signal layers, further impacting performance.
- In the second example, the clearance between the power and ground planes is too large, and there’s a split in the power plane, leading to undesirable effects.
- The third stack-up, by contrast, is optimal. Signal and reference planes are placed as close as possible, signal layers are not adjacent, and split reference planes are avoided. Power planes are coupled with ground planes for low-inductance distribution.
Best practices for layer arrangement:
- Place signal layers close to ground planes to minimize impedance and enhance signal integrity.
- Avoid placing signal layers next to each other to reduce EMI.
- Use unbroken ground planes for microstrip configurations to help contain the electromagnetic (EM) field.
- Couple power planes with ground planes at a small clearance (e.g., 8 mils) to create a low-impedance power distribution network and reduce EMI during signal layer transitions.
- Why signal layers should be close to ground planes
A transmission line consists of a signal trace and its return path. For example, in a stripline configuration, ground planes above and below the signal trace confine the EM field within the dielectric. However, if the dielectric spacing is too large or the ground plane is split, the EM field becomes unregulated, leading to signal integrity issues. Reducing dielectric spacing and maintaining an unbroken ground plane improves impedance control and EM field containment.
- Avoiding common issues
- Adjacent signal layers: When EM fields from one signal layer pass through another to reach the reference plane, coupling occurs, inducing common-mode currents and increasing EMI. A redesigned stack-up can mitigate this issue. If redesigning is not feasible, route traces at an angle between layers, and add a ground trace or shielding to reduce the impact.
- Routing over split planes: Routing signals across split planes causes signal integrity problems, as the return path is disrupted. If routing on a power plane is necessary, ensure traces do not cross splits.
- Advantages of coupling power and ground planes
Placing power and ground planes close together (e.g., within 8 mils) creates a high-frequency capacitance effect, forming a low-impedance power distribution network. This setup reduces EMI, stabilizes voltage levels, and improves performance during signal transitions between layers.
- Via transition best practices
- Minimize via size and count to reduce the risk of signal mismatches.
- Use backdrilling to eliminate stub reflections and avoid unnecessary laminations.
- For via transitions between ground planes, include a ground via alongside the signal via.
- For via transitions between power and ground planes, place a decoupling capacitor at the transition point for improved performance.
By following these guidelines and implementing thoughtful layer arrangements, you can significantly enhance signal integrity, EMI performance, and overall PCB functionality.
Watch the full webinar here:
