There are many factors that engineers need to consider. This article organizes some detailed issues that need attention in PCB layout. Let’s check if you are aware of them!
Layout of Special Components
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Heating components should be placed in a location conducive to heat dissipation, such as the edge of the PCB, and away from microprocessor chips;
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Special high-frequency components should be placed closely together to shorten the connections between them;
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Sensitive components should be kept away from noise sources such as clock generators and oscillators;
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The layout of adjustable components like inductors, capacitors, push-button switches, and potentiometers should meet the structural requirements of the entire machine for easy adjustment;
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Heavy components should be secured with brackets;
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EMI filters should be placed close to the EMI source.
Placement of Crystal Oscillators
Crystal oscillators, made of quartz crystals, are easily affected by external forces or drops, so during layout, they should not be placed at the edge of the PCB but as close to the chip as possible.
The placement of crystal oscillators should be away from heat sources, as high temperatures can also affect frequency deviation.
Decoupling Rules for Components
Adding necessary decoupling capacitors on the printed circuit board can 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.
Placement 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 port. When a chip has multiple power ports, decoupling capacitors must be arranged for each port.
Electrolytic Capacitors Away from Heat Sources
When designing, PCB engineers must first consider whether the environmental temperature for electrolytic capacitors meets requirements, and secondly, position the capacitors as far away from heating areas as possible to prevent the internal liquid electrolyte from drying out.
Spacing Between SMD Components
The spacing between surface mount devices (SMD) is a critical issue engineers must pay attention to in layout. The spacing should not be too large (wasting circuit board space) or too small, to avoid solder paste printing adhesion and difficulties in soldering repairs.
Spacing can refer to the following specifications:
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Different components: ≥ 0.13×h+0.3mm (h is the maximum height difference between neighboring components)
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For manual soldering and SMD, the distance requirement between components: ≥ 1.5mm.
Consistent Width of Component Leads
Retain Pads for Unused Pins
For example, if a chip has two unused pins, but the physical pins exist, if they 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 avoid interference.
In 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. When making vias in traces, the characteristic impedance can change.
It is also important to remember that vias will increase the length of the traces, requiring matching. If differential traces are used, vias should be avoided as much as possible. If they cannot be avoided, vias should be used on both traces to compensate for delays in the signal and return path.
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