There are many factors that engineers need to consider. This article organizes some detailed issues that need attention in PCB layout. Let’s see if you are aware of all of them!
01Layout of Special Components
- Heat-generating components should be placed in positions favorable for heat dissipation, such as the edges of the PCB, and away from microprocessor chips;
- Special high-frequency components should be placed closely together to minimize the length of their connections;
- Sensitive components should be kept away from noise sources such as clock generators and oscillators;
- The layout of adjustable components such as adjustable inductors, variable capacitors, push-button switches, and potentiometers should meet the structural requirements of the entire device for easy adjustment;
- Heavier components should be secured with brackets;
- EMI filters should be placed close to the EMI source.
02Placement of CrystalsCrystals are made of quartz and are easily affected by external impacts or drops, so during layout, it is best not to place them at the edge of the PCB, but as close to the chip as possible.The placement of crystals should be away from heat sources, as high temperatures can also affect the frequency offset of the crystal.03Decoupling Capacitor Rules
It is necessary to add decoupling capacitors on the printed circuit board to filter out interference signals on the power supply, stabilizing the power supply signal. It is recommended that the power supply be connected to the power pin after filtering through capacitors.04Placement of Decoupling Capacitors for ICs
Decoupling capacitors need to be placed near the power ports of each IC, as close as possible to the IC’s power pin. When a chip has multiple power pins, decoupling capacitors should be arranged for each pin.05Electrolytic Capacitors Away from Heat SourcesWhen designing, PCB engineers must first consider whether the environmental temperature for electrolytic capacitors meets requirements, and secondly, ensure that the capacitors are as far away from heat-generating areas as possible to prevent the internal liquid electrolyte from drying out.06Spacing Between SMD ComponentsThe spacing between surface mount components is a critical issue that engineers must pay attention to during layout. The spacing should not be too large (wasting PCB area) or too small, to avoid solder paste printing adhesion and difficulties in soldering repairs.
Spacing can refer to the following specifications:
- Same components: ≥ 0.3mm
- Different components: ≥ 0.13×h+0.3mm (h is the maximum height difference between neighboring components)
- For manual soldering and SMD, the distance requirement between components: ≥ 1.5mm.
07Consistent Width of Component Leads
08Retain Pads for Unused Pins
For example, in the case where two pins of a chip are not used, the physical pins still exist. If the two pins are left floating as shown on the right, it can easily cause interference.If the chip pins are internally unconnected, adding pads and grounding them can help avoid interference.09Use of Vias with CautionIn almost all PCB layouts, vias must be used to provide electrical connections between different layers. PCB design engineers need to be particularly careful, as vias can introduce inductance and capacitance, and in some cases, they can also cause reflections due to changes in characteristic impedance when vias are made in traces.It is also important to remember that vias increase the length of traces, requiring matching. If differential traces are used, vias should be avoided as much as possible. If unavoidable, vias should be used in both traces to compensate for delays in the signal and return path.
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