Have you ever noticed that when we introduce some PCB designs or techniques like back drilling or teardrops, we can often see a sentence like “it can be applied to high-speed application”? But how much do you know about high-speed PCB design?
High-speed PCB design is specifically tailored to cater to circuits, digital or analog, that operate at frequencies typically exceeding 50 MHz. Crucial sectors such as 5G networks, IoT devices, and high-performance computing heavily depend on high-speed design to ensure rapid and efficient data exchange.
Considerations in high-speed design
Signal Integrity
Signal integrity refers to the signal’s ability to traverse the pathway from its transmitter to the designated receiver. This integrity is paramount, particularly in high-speed design, where signals are more susceptible to distortion and corruption. Despite the ideal aim of seamless transmission, various factors can influence signal integrity, including signal loss, reflections, crosstalk, and unwanted noise.
In high-speed design, strategic considerations are implemented to bolster signal integrity. A robust grounding and power plane, and controlled impedance traces work to minimize noise and disruptions. Additionally, incorporating wider traces, thicker copper, and active components plays a crucial role in mitigating signal loss, contributing to optimized signal integrity.
PCB Substrate
High-speed circuits, often characterized by signals with fast rise times and high frequencies, demand substrates that can provide optimal electrical and thermal performance. One critical aspect of high-speed design is the selection of PCB substrates with low Dielectric Constant (Dk). Dielectric Constant affects the speed at which signals propagate through the material. High Dk values result in slower signal propagation, which can lead to signal delay and increased attenuation.
Standard FR-4 substrate has a higher dielectric constant and dissipation factor than other substrate materials, but it also means it will slow down signals more and attenuate signals more. Although techniques such as a controlled impedance stack-up can mitigate the limitations to some extent, it is generally recommended to use some low Dk substrates like Rogers 4350B and Rogers 4003C.
Placement of large ICs
In high-speed design, the use of large ICs, such as microprocessors, FPGAs, and high-speed interfaces, is essential for advanced processing and signal handling capabilities. Proper placement of these large ICs is crucial for optimizing system performance.
First, large ICs are often central to the functionality of a PCB. Placing them close to relevant connectors can reduce signal trace lengths and improve signal integrity. Second, it is necessary to shorten the interconnections between the pins of high-speed circuit components. This is because longer interconnections introduce more distributed inductance and capacitance, which can lead to issues with signal reflection, oscillation, and other signal integrity problems. This can be achieved by reducing the distances between the pins of different components and routing the interconnections between components using the shortest paths.
Stack-up
PCB stack-up is another important aspect of high-speed design. Here are some additional tips for designing a high-speed PCB stack-up:
Use a reference plane: A reference plane is a continuous layer of copper that provides a stable ground potential for high-speed signals. It is important to improve signal integrity, impedance, and EMI reduction. Typically, it can use a ground plane or a power plane as the reference plane. For example, a five-layer stack-up could be Signal-Power Plane-Ground Plane- Signal- Signal.
Use Microstrip and Stripline Traces: Microstrip and stripline traces are effective for controlling impedance and minimizing EMI. Microstrip is on the outer layers with a reference plane beneath it, while stripline has reference planes on both sides.
