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Design007-Nov2024

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NOVEMBER 2024 I DESIGN007 MAGAZINE 61 • Follow manufacturers' guidelines: Adhere to the recommended component spacing and layout guidelines provided in datasheets or reference designs, especially for high-power or RF components. • Separate power and signal sections: Keep power supply components (e.g., regulators, decoupling capacitors) away from sensitive signal traces to reduce noise interference. • Consider thermal management: Place components that generate heat, such as power ICs or large resistors, where heat can dissipate efficiently. Use thermal vias, or copper pours for better heat management. Routing Rules • Use short, direct traces: Keep traces as short and direct as possible, especially for high-speed signals, to reduce resistance, inductance, and signal integrity problems. • Route power and ground first: Prioritize power and ground planes or traces. Use wide traces or copper pours for power lines to reduce voltage drop and noise. • Minimize via use: Reduce the number of vias as much as possible to maintain signal integrity and avoid unnecessary imped- ance changes. Vias also increase cost and complexity. • Follow the 90/45-degree rule: Avoid sharp 90-degree angles in trace routing, especially for high-frequency signals, as these can cause impedance discontinuities. Use 45-degree angles for smoother transi- tions. Also, a 45-degree angle is easier for the fabricator to manufacture. • Route differential pairs together: For dif- ferential signals (e.g., USB, HDMI, Ether- net), route both traces of the pair together with consistent spacing and equal lengths to maintain signal integrity. • Respect signal integrity and crosstalk: To prevent crosstalk, maintain adequate spacing between signal traces, especially for high-speed or high-frequency signals. Use ground planes or shielding where needed to protect sensitive signals. 3W Rule of Thumb A common rule of thumb for spacing is the 3-width (3W) rule, which oen has sev- eral inherent problems. Let's see what I mean. Use this as a learning lesson regarding other rules of thumb you may use. e 3W rule says that when designing a PCB, the space between two signal traces should be three times the width of the traces. e thought is that doing so reduces or removes crosstalk between those two signals. While this rule is easy to remember, it has some problems. First, it oversimplifies things because increasing the distance between traces doesn't always stop interference, especially for fast signals. Also, the rule doesn't consider how the layers of the PCB are set up; having a ground plane can reduce interference better than just adding space. Additionally, the 3W rule doesn't help when looking at traces on different layers of the PCB. Following this rule can waste space, especially on crowded boards, because not all signals need that much space. Finally, it treats all signals the same, but some signals, like slow ones, don't need as much distance. Overall, while the 3W rule is a good guideline, it's better to consider each situation carefully instead of using one rule for everything. As you can see, I frequently use rules of thumb in PCB design. While they should be used with caution, for the reasons I've men- tioned, just like a cooked piece of spaghetti thrown against the wall (or ceiling), these rules stick. DESIGN007 John Watson is a professor at Palomar College in San Marcos, California. To read past columns, click here.

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