Overview of PCB Layout and Wiring Design

In hardware design, PCB design is a very important and indispensable step. For some simple products, PCB design may simply involve connecting all components and networks accordingly. However, for high-speed circuits and RF circuits, the design of the PCB directly affects whether the product functions properly and whether it meets market requirements. Below, I will introduce the PCB design process, PCB layout, PCB wiring, and PCB design checklist from four aspects.

PCB Design Process

The quality of the PCB directly determines the quality of an electronic product, so a good PCB design process is crucial. Many engineers believe that PCB design is simply about arranging all components and then connecting the relevant pins together. This is a narrow view; a good PCB design process begins during the schematic design, such as how to choose the right scheme and select suitable electronic components. Specific details are as shown in the figure below:

It specifically includes schematic design, schematic netlist output and import, mechanical structure diagram import, layer structure design and editing, signal integrity (SI)/power integrity (PI) pre-simulation, PCB layout, design constraint rule import, PCB wiring, signal integrity (SI)/power integrity (PI)/electromagnetic compatibility (EMC)/thermal post-simulation, design for manufacturability (DFM) checks, and generating production files (Gerber). These tasks may be completed by a single engineer or multiple engineers working together. Of course, not every product’s PCB design process is the same; specific products can refine, add, or delete aspects of this process as needed.

Next, I will further introduce several important steps in the PCB design process.

PCB Layout

In design, layout is an important step. The quality of the layout will directly affect the effectiveness of the wiring; therefore, it can be said that a reasonable layout is the first step to a successful PCB design. Simply put, PCB layout involves arranging all components according to functional structure, modularization, meeting DXF requirements, and ensuring smooth layout and wiring principles.

Consider the overall aesthetics A product’s success depends on both its internal quality and overall aesthetics; both must be satisfactory for the product to be considered successful. On a PCB, the arrangement of components should be balanced, orderly, and should not be top-heavy or bottom-heavy.

Image: Well-laid-out PCB

The aforementioned points are just some general directions and requirements; in fact, there are many factors to consider in PCB layout. For example, the layout principle often follows the guideline of “first large and then small, first meet structural requirements and then aesthetic requirements, first difficult and then easy,” meaning that the important core circuit, high-speed circuit, RF circuit, core components, and interface circuits should be prioritized in the layout, followed by auxiliary circuits. When conducting PCB layout design, the following principles can be followed:

1. The layout should refer to the schematic block diagram and arrange the main components according to the main signal flow direction. The layout should meet the following requirements as much as possible:

• When there are no special requirements, keep the total wiring length as short as possible, with key signal lines being the shortest;

• The layout of decoupling capacitors should be as close to the IC power pins as possible, based on capacitor size, and minimize the loop formed with power and ground;

• Reduce signal return paths and avoid cross-segmentation.

2. The arrangement of components must first meet functional requirements and also facilitate subsequent debugging and maintenance. Small components should not be placed around large components, and there should be enough space around components that require debugging; too compact arrangements may prevent soldering.

3. For parts of the same structural circuit, use a “symmetrical” standard layout as much as possible; optimize the layout based on uniform distribution, balanced center of gravity, and aesthetic standards.

4. Components of the same type should be placed in the same direction along the X or Y axis. Likewise, polar discrete components should maintain consistency in the same direction, facilitating production and inspection.

5. Heat-generating components should be evenly distributed to aid in heat dissipation for both the single board and the entire device. Temperature-sensitive components, excluding temperature detection components, should be kept away from high-heat components. In addition to temperature sensors, transistors are also heat-sensitive components.

6. High-voltage and high-current signals must be completely separated from low-current and low-voltage weak signals.

7. Separate analog signals from digital signals; high-frequency signals should be isolated from low-frequency signals; ensure sufficient spacing between high-frequency components.

8. When arranging components, consider placing devices that use the same power supply together to facilitate future power path design and separation from other power planes.

For the positions of some special components, the following principles should generally be followed in the layout:

1. DC/DC converters, switching elements, and rectifiers should be placed as close to the transformer as possible, and rectifier diodes should be as close as possible to voltage regulation elements and filtering capacitors to minimize line lengths.

2. Electromagnetic interference (EMI) filters should be placed as close to the EMI source as possible. Minimize connections between high-frequency components and try to reduce their distribution parameters and mutual electromagnetic interference. Sensitive components should not be placed too close to each other; inputs and outputs should be as far apart as possible.

3. For adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the layout should consider the overall structure requirements. Frequently used switches should be placed within easy reach, as allowed by the structure. The layout of components should be balanced and appropriately spaced.

4. Heat-generating components should be arranged at the edges of the PCB for better heat dissipation. If the PCB is mounted vertically, heat-generating components should be placed on top of the PCB. Temperature-sensitive components should be kept away from heat-generating components.

5. When laying out the power supply, arrange components to facilitate the routing of power lines. During layout, minimize the area of the input power loop. Where possible, avoid routing power lines all over the board, as this can create large loop areas. Proper positioning of power and ground lines can reduce electromagnetic interference. Improper coordination of power and ground lines can create many loops and potentially generate noise.

6. High and low-frequency circuits have different interference characteristics and methods for suppressing interference. Therefore, during component layout, digital circuits, analog circuits, and power circuits should be separated by modules. Effectively isolate high-frequency circuits from low-frequency circuits or divide them into small sub-circuit modules connected by connectors.

7. The layout should also pay particular attention to the distribution of strong and weak signal components and the direction of signal transmission paths. To minimize interference, after separating analog and digital circuits, keep high, medium, and low-speed logic circuits in different areas on the PCB. The PCB should be divided according to frequency and current switching characteristics. Noise components should be kept at a distance from non-noise components. Temperature-sensitive components should be distanced from heat-generating components. Low-level signal channels should be kept away from high-level signal channels and unfiltered power lines. Separate low-level analog circuits from digital circuits to avoid common impedance coupling between analog circuits, digital circuits, and common power return lines.

PCB Wiring

When the schematic netlist is imported into the PCB design software, all the pins connecting the components are initially connected by “mouse lines,” which do not have network property significance. As shown in the figure below:

Image: Mouse line connection of PCB

This requires engineers to connect them according to the corresponding design constraint rules. Only when all networks are interconnected do they exhibit electrical characteristics. Wiring serves this purpose, which is to connect all signal networks, power networks, and ground networks appropriately.

During PCB wiring, design constraint rules must be used. These rules include signal network line widths, differential pair spacing, differential pair matching length requirements, spacing requirements between transmission lines, total length of transmission lines, and segment length requirements for transmission lines, etc. The figure below shows the requirements for PCIE design on a certain Intel platform:

Image: Requirements for PCIE design on a certain Intel platform

After completing the layout and wiring according to the respective requirements, a well-structured PCB layout is obtained, as shown in the figure below:

Image: Well-connected PCB layout

Once PCB design is completed, production files can be generated according to production requirements, typically including PCB production files, PCBA production files, stencil files, etc.

PCB Design Checklist

Before officially generating PCB production files, a detailed inspection of the PCB design is generally conducted, including checks for DFM, SI, PI, EMC, Thermal, reliability, and more. How to check? Some companies use tools for inspection, while others rely on engineers to conduct checks themselves. Regardless of the method, it is based on certain rules for inspection and analysis, commonly referred to as PCB design checks.

Item	Check Content	Y/N	Remarks
General Inspection Items	Is the forbidden layout area set correctly? (Pay attention to the height limit area) Add: The oscillators, inductors, and transformers should have their forbidden areas drawn below them.
Is the structure updated correctly? Are the hole sizes and interface positioning and directions correct? Have any doubts about the interface direction been confirmed with the structural engineer?
Is the structure the final document?
Has the package been checked?
During revision design, has the package been checked and updated (original point changes causing fixed component displacement, etc.)?
Have self-built or temporarily replaced packages by traveling engineers been rechecked and corrected?
Is the photoplot setup correct?
Does each power supply have a source? Does the width meet the current carrying capacity? Are the number of vias sufficient?
Are the schematic and PCB netlists the latest, and is the import consistent?
Are there any unplaced components, unconnected networks, or redundant segments?
Has the IPC netlist been compared and confirmed to have no open circuits or short circuits?
Rule Settings	Is the stacking setting correct? (Including positive and negative films) Have rules been set according to the increased process manufacturing instructions?
Are the line width and spacing rules for differential lines and single-ended lines set correctly?
Is the high voltage safety regulation set correctly?
Are the length error and maximum length settings correct?
Is the protective ground set to a spacing of over 2mm?
Have all networks of the same category been assigned to the corresponding groups?
Are the corresponding rules enabled?
If there are isolation pads, are they set correctly?
Layout	Ensure that no components exceed the height limit in restricted areas.
Do components with sequential requirements (such as LEDs, buttons) comply with structural placement requirements?
Are TVS and ESD protection components placed close to interfaces?
Are digital, analog, high-speed, and low-speed sections separated in the layout? Does the analog layout ensure the shortest main path routing?
Are similar modules laid out similarly?
Are source and end matching components laid out correctly?
Are crystals, oscillators, and clock drivers placed reasonably?
Is the switch power supply laid out and wired according to requirements? (Is the loop minimized? Is single-point grounding done?)
Is the distribution of each power supply voltage capacitor uniform? (Each power pin should have a capacitor of less than 0.1uF)
Are temperature-sensitive components placed away from power supplies and other high-power components? (Are temperature sensing components positioned appropriately?)
Are wound inductors placed parallel together? (It is recommended to place them perpendicular to each other)
Has the RF circuit considered a linear or L-shaped layout?
Are isolating components (such as transformers) separated in the layout?
Are high-heat components also separated from each other to facilitate heat dissipation?
Ensure that no components are placed in the forbidden layout area.
Wiring	For phase-locked loop circuits, REF, are the wires at both ends thickened?
Are return ground vias added in areas with dense signal or power holes?
Are all power pin outputs wider than 20mil or the same width as the pin? (Including thermal pads, excluding pull-up and pull-down resistors)
Are all key signal lines routed without crossing adjacent plane layers?
Are RF lines and antennas handled correctly? (Thickened to control 50ohm impedance, with corresponding reference planes, ceramic antennas cut according to requirements, RF lines surrounded by shielding ground vias.)
Are analog lines and lines that do not require impedance (such as crystal clock lines, Reset, etc.) thickened to over 8mil?
Are there any redundant vias and lines, or redundant stubs in the routing?
Are there any right angles and acute angles in the routing?
Are there isolated copper areas and copper without networks?
Are polar components correctly placed? (Pay special attention to diodes, polarized capacitors, ESD, LEDs, etc.)
Is the wiring topology structure reasonable?
Are isolating components (optocouplers, common mode inductors, transformers, etc.) isolated or hollowed out?
Is the electrostatic protection ground isolated from the working ground (at least 2.5mm apart)?
Do power modules and clock modules have signal lines passing through them, especially under the switching power supply inductors?
Are adjacent signal layers routed in parallel? Parallel routing must be staggered or routed perpendicularly, not overlapping.
Are return ground vias added at layer changes for differential lines and important signal lines? It is best to symmetrically add two return ground vias.
Have sensitive signals been shielded with ground? Is there a via every 500mil?
Has shielding ground been added every 150mil on multilayer board edges?
Is there any excessive hole isolation causing the plane to be cut off, leading to insufficient current in the power plane?
Is there any inward shrinkage of the power plane compared to the ground plane?
Do all sections of the power network on the plane layer have copper connections?
Do ICs and connectors have power and ground pins, and are the lines thickened?
Is the copper area for high-heat components sufficiently large? Is there copper skin with heat dissipation windows on the surface layer?
Is there copper on the gold finger, with the inner layer copper extending halfway to the gold finger pad, and is there a complete solder mask on the gold finger?
Are there any wires passing through the pins of components (resistors, capacitors, inductors, etc.)?
Is there copper treatment in the blank areas on the surface layer?
Are the front and back sides of the two-layer board connected well? Pay special attention to whether the power and ground vias meet current-carrying capacity requirements at layer changes.
For serial port chips (e.g., 232, 485, 429, 422), are the capacitor lines thickened?
Is the power supply for clock circuits (including crystals, oscillators, clock drivers, etc.) well-filtered, and are there no stubs in clock routing?
When making equal lengths, are all networks in each signal group made equal in length?
Are important signal lines prioritized in wiring, routed on the optimal wiring layer?
When the voltage difference in the power plane is large, is the isolation band widened accordingly?
Are the number of vias for high-speed signal lines in the same group minimized and consistent, ideally under 2 vias?
Output Production File Check	Ensure that SMT components have a stencil opened and that all components have a solder mask opened.
Is the solder mask window consistent with the copper on the surface layer?
Are the device characters and silk screen marking directions correct? Are there any interference or text errors on the solder pad, and is the marking for pin 1 correct and clear?
Is the line width consistent with the production instructions?
Are non-metalized hole pads set correctly?
Is the labeling on the board correct? (Including drill layer instructions and error markings)

This is a conventional PCB design checklist, and the checklist used for each type of product is generally similar. It is usually recommended to create a specific PCB design checklist based on the specific requirements of one’s own products.

Author: Signal Integrity Public Account Jiang Xiuguo

Some content of this article is included in “100,000 Whys of Hardware – Development Process”

Book Recommendation: Cadence High-Speed PCB Design

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This book is divided into two parts: the development fundamentals section (Chapters 1-9) provides a detailed introduction to the use of Cadence Allegro software and the important components of mobile phone circuit boards, including component library management, Padstack creation, and detailed operations of PCB, starting with the hardware framework and structural components of mobile phones, leading into mobile phone circuit board design; the practical operation section (Chapters 10-13) gradually explains a practical case of designing a mobile phone circuit board, as well as how EDA engineers handle problems encountered during production and how to adjust routing based on simulation reports, helping readers quickly get started with PCB design.