Guide

Basics of PCB Layout: Components, Traces, and Ground Planes

PCB Layout

Printed Circuit Board is the most compact form factor for any electrical circuit. The compact nature of the PCB tends to have all the Active, and passive components close to each other so there might be a lot of cross-talk and signal integrity issues if not placed and connected without PCB board layout guidelines. This is the most and primary factor influencing the crucial performance of the PCB working. The nature of the routing or placement of components or ground plane and polygon pours cannot be changed once after printing the PCB. Therefore, it is essential to consider all the X factors before the manufacturing process.

The Main Components to be Considered  in PCB Layout

PCB board layout guidelines are diverse and dependent on every crucial component in a PCB. Understanding the purpose of every component, trace, and return path is the best practice. The basics of PCB board layout guidelines focus on the Component placement, Routing, Ground planes, or return paths.

To determine the placement of the component following factors have to be considered, Functionality, Signal flow, Power requirements, thermal management, mechanical considerations, Manufacturing capability, Routing considerations, and electrical considerations in the circuit board layout. A PCB layout services (printed circuit board layout services) are needed to acquire the PCB layout.

  • Functionality

Functionality Consideration in PCB Layout

In the circuit board layout, the primary consideration while placing is to ensure that each component is placed in a flow where the circuit flow is intended. Each component functionality on the PCB is to be facilitated with signal flow, power distribution, and other functional requirements. In the circuit board layout, components with high signal priority need to be placed close to each other to avoid signal degradation and electromagnetic interference. The below illustration shows the isolation of circuits on a single Printed Circuit Board not just considering one placement factor but also ground, trace, routing, and external accessibility. A combination of digital and analog ICs is placed between the two areas to isolate the ground or consider a separate ground for mixed-signal circuits.

  • Thermal Management

In the circuit board layout, heat-emitting components like power MOSFETs/ IGBT/ Transistors or Processors need to be placed with sufficient gaps to dissipate the heat using a coolant fan, Heat sink or thermal pads. Thermally unstable or sensitive components have to be positioned away from any of the above-mentioned heat-generating components. Potting is also done in a few cases to provide heat dissipation.

  • Mechanical Considerations

Mounting holes on Printed circuit boards and mounting points for heavy components should be positioned in the earlier stage and populating the control or dependent circuit around them has a good leverage. Providing sufficient clearance for placement, access, mountings, and assembly processes in PCB layout design is also vital. Vibration and stress acting on a PCB continuously also causes degradation in the performance, in some cases potting material is used to add on strength.

  • Manufacturing Capability

The manufacturing process generally has two main factors; one is the assembly process and the other is the soldering process. The placement of every component plays a major role wherein there is no hindrance to the automated assembly process. Many of the manufacturability constraints are entirely based on the factory setup. If the design considerations are made in the initial stage, then the cost of manufacturing is significantly reduced. Most of the assembly errors are due to misalignment or solder bridging which should be followed as recommended by IC manufacturing datasheet.

  • Routing Considerations

Considering minimal trace lengths for high-priority signals is a factor that influences noise, interference, and degradation of the signals. In high-speed signal designs, we have to look into impedance matching and length matching. The space allocated for the routing is also a factor during placements.

  • Electrical Considerations

Components should be placed close to each other to avoid interference or noise. A sensitive component like an analog amplifier should be placed away from any source Magnetic components like Transformers, motors, inductors, or even high-frequency digital circuits. Power loss happens on PCB if high current traces are long because of trace resistance.

PCB Layout Guidelines for Traces

In PCB Layout, traces involve creating copper connections established for the circuits according to the schematics. Three major factors in PCB layout design influencing Routing are Signal integrity, Power distribution, and electromagnetic interference. As we know to reduce signal degradation and interference the trace lengths are kept shorter. In high-current parts the traces are kept wider and polygon pours are used to connect them.

In PCB Layout, trace routing is one of the most common types of routing which is nothing but a conductive path on a PCB. Components in PCB layout design like resistors, capacitors, inductors, ICs, and connectors are placed according to the circuit flow and connected using trace options. The trace width in PCB layout design plays a vital role as using the appropriate width impacts the signal parameters.

  • Signal Integrity

One primary consideration in PCB Layout when routing is maintaining the signal integrity throughout the PCB. Signals should not be distorted or affected by noise while routing, this can only be achieved by trace length, impedance matching, signal termination, and cross talk. If a trace in PCB layout design induces voltage into an adjacent plane or a trace it is called cross talk which can be mitigated by pouring the second layer as the ground plane. The below image shows the larger loop path of a circuit to be completed.

Larger Loop Path of a Circuit

Whereas when the return path or the trace is placed closer to each other then the Loop inductance is reduced as shown in the image below.

Return Path or the Trace is Placed Closer

  • Power Distribution

In PCB Layout, proper power distribution is achieved by providing sufficient space for the power to get distributed to all the related circuits without getting crossed over with other high or low-power circuits. Power distribution for multilayer PCB is connected using a polygon poured on a separate layer. Where all the components get access to the power in the 3rd or 4th layer through via. In PCB design and layout, we have to ensure that High-voltage power polygon pours do not cross over and have different isolated ground reference points as designed in the schematics. If this polygon pours tend to go close to each other or when crossed over, high-frequency noise is injected from one power plane to the low potential one. The main application of polygon pour is there is minimal voltage drop as the width of the current carrying path is higher.

PCB Layout Guidelines for Ground Planes

In PCB board layout, polygons or Planes are generally used as a grounding layer as grounding for every PCB Design. To reduce the loop inductance to a low impedance, the return path in PCB design and layout is set by poring a ground plane adjacent to all signal and power layers in the PCB board layout. This method of routing in PCB board layout ensures the stability of signals and the proper functioning of circuits by providing a return path. The purpose of grounding is to reduce noise by providing a current path to dissipate into the earth. Ground planes and traces should be placed as close as possible.

Circuit Board Layout Based on Ground Functionality

Circuit Board Layout Based on Ground Functionality

Functionality of the ground is based on the signal type that a circuit handles; Analog circuits are sensitive and when crossed over these grounds tend to capture higher potential noise and get reflected on sensitive signal lines. Here the Analog circuit ground is separated from the digital ground and high current or power grounds. These grounds are not completely isolated but they are connected using a via or by connecting them to the point of filter circuits or power ground.

Power Ground Functionality

Power ground in PCB design and layout plays a vital role where the entire circuit is connected to a power supply or connector. These grounds serve as a common ground for the entire circuit and are generally connected using the star grounding technique. SMPS design generally uses a power ground where the PMIC control circuitry grounds are connected to the point of the Power ground and the GND pin of the IC.

Grounding Techniques

  • Star Grounding

Star Grounding

As the name implies it is a technique to connect all grounds to one particular point where the actual power ground or GND pin of an IC or Filter circuit is connected, this isolates all the sub-circuit grounds and ensures no cross-over return path.

  • Ground Grid

Ground Grid

A large ground plane is poured over the entire circuit on adjacent layers not isolating any of them to maximize the current flow and improve overall ground performance. Comb ground structures are not used in multi-layer PCB which has the same type of signal functionality, the signal return path is longer.

  • Ground Stitching

In a few cases, the PCB trace width is not sufficient to carry enough current, so other layers of the ground are used via stitching which distributes the current flow throughout the layers. This is another method to ensure a low impedance path in multilayer PCB.

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