When building PCBs for frequencies over 5 GHz and currents above 2A, trace design becomes paramount. To ensure signal integrity and reliable power delivery, optimize the trace width, spacing, and impedance of high-frequency and high-current traces.
When designing high-speed and high-current PCB traces:
In this article, we will first cover 5 best practices for high-speed traces and move on to 5 essential guidelines for high-current traces. At the end, you will find 5 DFM guidelines.
Signal integrity becomes significant as signal speeds increase. Let’s dive into the factors you must check before designing high-speed signals for signal integrity.
The critical length (l c) is the trace length above which a trace must be designed as a PCB transmission line with controlled impedance. If a trace is longer than the critical length, it might cause signal reflections if the impedance is inconsistent. Conversely, if the trace length is less than the critical length, the signal can be transmitted without distortion.
For analog signals, the critical length is given by:
l c = 𝛌ₘ /𝟔 = 1/ 4t pd f m
For digital signals, the critical length is given by:
l c = t r / 2t pd = (t r . V) /2
Where 𝛌ₘ = maximum wavelength of the signal, t pd = propagation delay, fₘ = maximum signal frequency, t r= rise time and V= signal speed
Signal transmission behavior when the trace length is below or exceeds the critical length.
Shortlines refer to signal paths where the trace length is less than the critical length divided by 1.5. In these cases, signal delay and impedance mismatching are negligible.
Let’s consider a practical example to illustrate the calculation of a shortline:
First, calculate the critical length, l c= t r/2 ×v = 1/2 ns ×6 inches/ns = 3 inches. Therefore, signal traces < 3/1.5 = 2 in are considered shortlines for this design. This means that for any trace length below this threshold, the effects of signal delay and impedance mismatching are minimal.
3 dB bandwidth signifies the frequency range over which the transmission line can effectively transmit signals without loss. All signal frequency components propagate without distortion when the trace has a sufficient 3 dB bandwidth. If a trace does not have adequate bandwidth, the higher-frequency components of the signal might get attenuated, leading to signal degradation and loss of integrity.
When a trace has a sufficient 3 dB bandwidth, it allows all frequency components of a signal to propagate without distortion.
The 3 dB bandwidth is inversely related to the signal rise time. It can be calculated using the following formula:
3 dB bandwidth = 0.35/t r
Rise time (t r) is the time it takes for the signal to transition from a low to a high state (typically measured from 10% to 90% of the signal’s maximum amplitude).
For instance, if the signal rise time is 1 ns, using the above formula: 3 dB BW = 0.35/1 ns = 0.35 GHz = 350 MHz. It means the trace can effectively transmit signals with frequencies up to 350 MHz without significant attenuation.
Serpentine routing and meandering are length-matching techniques used in PCB designs to ensure signals reach their destinations simultaneously. This is particularly important in high-speed and RF designs where minimizing propagation delay is critical. This technique involves creating a series of curves or bends in a transmission line to increase its physical length without altering its electrical properties. It helps maintain the required electrical length for signals, ensuring synchronization across different traces. Meandering incorporates similar principles,
