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Guidelines before beginning PCB layout.
PCB design layout guidelines for location and route.
PCB design completion; Layout guidelines for completing the job.(Video) Introduction to Basic Concepts in PCB Design
Trace routing should follow the PCB design layout guidelines for that board
A PCB layout is much more than it may appear. A successful PCB layout physically arranges the circuitry to provide the best electronic performance of the board while still being fully manufacturable. This requires careful management of library parts, CAD setups and parameters, component placement, trace routing, and Power Delivery Network (PDN) design. In addition, layout designers must ensure that their work is fully documented and that the final product is ready for integration into the main electronic system for which it was designed.
This is a lot of work, especially for engineers who arenew in PCB layout process. To support this workflow, it is good practice to have a comprehensive set of PCB layout guidelines available for reference. Industry and company standards dictate the details of the design, but layout guidelines are important to help engineers navigate the board development process from start to finish. Here are some basic PCB design layout guidelines that can be used to develop your own PCB design guidelines.
Before starting the layout
Before the layout process begins, there are several tasks that need to be completed to ensure the success of the design, starting with the PCB footprint libraries used.
Ifbuild librariesIt is important for your PCB layout to use industry standards such as IPC or the manufacturer specifications for package sizes and dimensions. However, individual, company-specific or technological requirements may dictate changes in some parts. For example, footprints in RF designs may require smaller pad sizes than a standard digital design. Here are some more guidelines for creating your own PCB component footprints:
- Make sure any library parts you build have acceptable face pattern sizes, spaced according to the standard for that part.
- PCB footprints must contain all necessary elements, such as B. Part outlines, silk-screen marks, and reference designators.
- A good rule of thumb is to make sure your manufacturer can build the parts you design before committing them to the final design.
Another alternative is to use PCB footprints from external CAD library providers. Part manufacturers often have their own components pre-built for your design system, and some tools have browsers to conveniently download these parts.
Board outline and layer structure
You should work with the mechanical designers to get a good outline shape before you start laying out the board. Although the form factor of the design can be changed later, changes could force extensive redesigns of the circuit to fit the new form. Also, most CAD tools accept data imports from mechanical design systems, making your work much easier. But even with imported data you need to make sure that the board outline is correct and contains all the necessary CAD elements, such as e.g. B. Restricted zones that your design requires.
Board-Layer-Stapelshould also be completed before starting the layout. Again, these can be modified later, but the potential impact on existing circuitry can ruin your design schedule and budget. Board layer stackups should also be fine-tuned for your specific design to ensure the correct layer configurations for impedance-controlled routing and other signal integrity requirements. It is also important to select the board materials at this stage so that the correct track width and other design calculations can be made according to the physical properties of the materials. These properties include dielectric constants, insulating qualities, moisture absorption values, and dissipation factors.
CAD parameters and settings
It's not uncommon for designers to work with itdefault settingsthat come with their CAD systems. However, most CAD systems give the user a wide range of control over colors, fill patterns, shading, and font sizes and widths. You can also change how specific objects are displayed, prioritize one design element over another, set up grids, and set placement and routing preferences. These settings are designed to make you more efficient at your job, and you could end up saving time by taking the time to tweak your settings in advance.
Setting up the display parameters of your CAD system is an important first step in PCB layout
PCB Component Placement Guidelines
With the CAD library, board outline, and other setup tasks complete, the design is ready for layout. The first step in this process is to place the PCB component footprints on the board. There are three main requirements that must be met when placing components on the board: circuit performance, manufacturability, and accessibility.
High-speed circuits need their components to be as close together as possible for short and direct signal paths, but they are not the only components with this requirement. Analog circuits and power components must also be placed so that their sensitive or high-current lines are as short as possible. This helps decrease and increase inductanceSignal and Power Integrity. However, these components may need to be splayed apart to allow for bus routing or thermal separation in some cases.
It is important to keep production costs as low as possibleplace componentsin such a way that they are as easy to produce as possible. For example, components that are too close together may not be able to be placed automatically or have difficulties with automated soldering processes. Larger chip components that precede smaller parts during flow soldering can create a shadowing effect, resulting in poor solder joints. Imbalanced copper between the two pads of small chip components can cause uneven heating, causing one pad's solder to melt before the other and pulling the other side up and off its pad.
Printed circuit boards often have to go through manual testing and rework, which requires access to the parts that need to be worked on. If other larger components overshadow these parts, it can make editing more time consuming or cause collateral damage to adjacent parts. Likewise, connectors, switches, and other human interfaces that are not accessible can slow down the manufacture of the PCB.
An extremely important guideline is that placement should start with developing a basic layout of the parts on the board. This allows you to be strategicDivide the different circuit areason the board to avoid overlapping of analog and digital signals.
PCB Layout Guidelines: Effective component placement results in the best trace routing
PCB design layout guidelines for routing
It is imperative for PCB designers to design their PCB to achieve the best possible signal and power integrity. The components should be arranged in the optimum position for a short and direct conductor path. At the same time, the board must be designed in such a way that all networks can be laid completely. Trying to balance these requirements can be quite a challenge in high-density designs. The first PCB design layout guideline is to set up theDesign Rules and Restrictionsfor trace routing.
Design Rules and Constraints
Technically, the configuration of the draft rules and constraints should have been included in the parameters and settings. But since most of the rules apply directly to trace routing, we've included that guideline here. Rules and constraints are used to control trace widths and spacing, and can be set up for individual nets, groups of nets called net classes, or as a default for all unspecified nets. Design rules are also used to control which vias are selected for different nets, trace lengths and custom lengths, and which board layers are allowed for routing specific nets and routing topologies. In addition, design rules are also used to control component spacing, screen printing rules, mechanical spacing, and a host of other constraints.
It is imperative that rules and constraints are established for PCB design prior to PCB layout to ensure the correct physical parameters for performance and manufacturability are met.
Signal and Power Integrity
For maximum performance andsignal integrity, PCB layout designers must meet specific requirements for routing traces of various circuits. This is where the design rules and constraints help by allowing the designers to enter the physical routing parameters into the CAD system for routing. Although the exact values will change depending on the board's requirements, designers will usually establish rules to ensure the following guidelines are met:
- Short and direct high speed transmission line routing.
- Trace width, spacing, and allowable board layers for controlled impedance routing.
- Fixed track lengths and length tolerances for equal-length routing.
- Different track widths and spacing requirements.
- Width and spacing for sensitive signals such as clock and control lines.
- Via types for different networks.
- Track widths and spacing for analog circuits.
- Trace widths and copper weight for high current power circuits.
Another important guideline to remember is that when routing traces in mixed-signal designs, avoid crossing areas of digital circuitry with analog traces, and vice versa.
Guidelines for active power and ground planes
In modern high-speed designs, the best grounding strategy is usually to use one or more continuous ground planes on an internal layer. This offers the best protection against EMI and ensures clear signal paths, improving overall signal integrity. Avoid routing traces across ground cavities for areas where the ground plane is broken due to unique circuit board contours or features. Without a continuous and adjacent ground plane for the signals to use as a clear return path, your design can introduce a lot of unwanted noise. Here are some power supply and ground plane guidelines to keep in mind:
- Ground planes must be adjacent to signal layers in the layered board structure for high-speed routing. This will help shield the high-speed routing from interference and perform wellreference planefor the signal return paths.
- Heat relief pads must be used and carefully managed for power and ground connections to the aircraft. The spokes of the relief pads must be wide enough to handle high currents while preventing these connections from acting as a heat sink.
- Carefully plan power connections and split power planes to ensure power is adequately delivered to all connected parts throughout the board.
Avoid routing analog and digital circuits together in a mixed-signal design
Test guidelines for screen printing and PCB
With the PCB design complete, it's time to turn your attention to finalizing the layout by cleaning that upscreen printed layersand adding test points. Reference designations, part numbers and other company information are marked with ink on the circuit board through a screen printing process. Designers typically use "silkscreen" layers in their CAD systems to design these markings.
To ensure that screen printed layer markers are legible, designers follow these guidelines:
- Line widths should not be less than 6 mils.
- The font size should not be smaller than 50 mil.
- Rename component reference identifiers according to a corporate grid pattern to make it easier to locate specific parts on the board.
- Move and rotate the reference designators so they are easy to read.
- Add polarity and pin 1 markers if needed.
Test points are essential for PCBs that are mass-produced for automated placement validation. Every net in the design should have a test point, whether that test point is an existing through-hole pin, via, or an additional surface mount test point pad. Test points should be at least 50 mils from other board objects such as components or pads and at least 100 mils from the edge of the board. However, these values are likely to change from vendor to vendor, so first check what your vendor's test point requirements are.
PCB Fabrication File Guidelines
The final PCB design layout guideline is to create the fabrication files for fabrication and assembly and send those files to your suppliers. Output files are often automatically generated by scripts developed by designers or your CAD department. Most PCB design CAD tools, such as Cadence's Allegro PCB Editor, have built-in authoring tools that can be used. Many tools also have functionality that communicates directly with PCB manufacturers via the IPC-2581 format. These unique features allow you to automatically transfer a manufacturing database of fabrication and assembly files without having to create and submit each file individually.
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