Guidelines prior to starting the PCB layout.
PCB design layout guidelines for place and route.
Circuit board design completion; layout guidelines for finishing the job.
Trace routing should be done according to the PCB design layout guidelines for that board
There is a lot more to a printed circuit board layout than how it may appear. A successful PCB layout will have its circuitry physically arranged for the best electronic performance of the board while also being fully manufacturable. This requires diligently managing library parts, CAD setups and parameters, component placement, trace routing, and the design of the power delivery network (PDN). Additionally, layout designers must ensure that their work is fully documented and that the final product is ready for inclusion in the main electronics system it was designed for.
This is a lot to work through, especially for engineers who are new to the PCB layout process. To help with this workflow, it is a good practice to have a comprehensive set of circuit board layout guidelines available for reference. Industry and corporate standards will 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 set of guidelines for circuit board development.
Before Layout Starts
Several tasks need to be taken care of before the layout process starts to ensure the design’s success, starting with the PCB footprint libraries that will be used.
When building libraries for your PCB layout, it is important to use industry standards such as IPC or the manufacturer specifications for package sizes and dimensions. However, individual, corporate, or technology needs may also dictate changes in some of the parts. For example, footprints in RF designs may require smaller pad sizes than a standard digital design. Here are some other guidelines for building your own PCB component footprints:
- Ensure that any library parts you build have acceptable land pattern sizes spaced according to the standard for that part.
- PCB footprints need to contain all the necessary elements, such as part outlines, silkscreen markings, and reference designators.
- A good rule of thumb is to ensure that your manufacturer can build the parts that you are designing before committing them to the final design.
Another alternative is to use PCB footprints from external CAD library vendors. Part manufacturers often have their own components pre-built for your design system and some tools have browsers to download these parts conveniently.
Board Outline and Layer Stackups
You will want to work ahead with the mechanical designers to get a good outline shape in place before you start the layout of the board. Although the form factor of the design can be changed later on, any alterations could force extensive re-designs of circuitry to fit the new shape. Also, most CAD tools will accept data imports from mechanical design systems, making your job a lot easier. However, even with imported data, you still need to ensure that the board outline is correct and contains all of the necessary CAD elements, such as keepout zones, your design requires.
Board layer stackups should also be finalized before the layout starts. Again, these can be changed later on, 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 proper layer configurations for impedance-controlled routing and other signal integrity requirements. It is also important to select board materials at this stage, so that proper trace width and other design calculations can be made according to the materials’ physical characteristics. These characteristics include dielectric constants, insulating qualities, moisture absorption rating, and dissipation factors.
CAD Parameters and Settings
It is not unusual to find designers working with the default settings that come with their CAD systems. However, most CAD systems give the user a wide range of control over colors, fill patterns, shadowing, and font sizes and widths. You also will be able to change the display of certain objects, give priority to one design element over another, set up grids, and specify placement and routing preferences. These settings are intended to make you more efficient in your work, and you could save time in the end by taking the time up-front to refine your settings.
Setting up the display parameters of your CAD system is an important first step to PCB layout
Guidelines for Placing PCB Components
With the CAD library, board outline, and other setup tasks completed, the design is ready for the layout to begin. The first step in this process will be placing the PCB component footprints on the board. There are three main requirements that have to be met with the placement of components on the board: circuit performance, manufacturability, and accessibility.
High-speed circuits need to have their components as close as possible together for short and direct signal paths, but they aren’t the only components with this requirement. Analog circuitry and power components also need to be placed to make their sensitive or high-current lines as short as possible. This helps reduce inductance and increase signal and power integrity. However, these components may need to be spread apart to accommodate bus routing or thermal separation in some cases.
To keep production costs as low as possible, it is important to place components in a way that they are as easily manufacturable as possible. For instance, components that are too close to each other may not be able to be automatically assembled or may have difficulty with automated soldering processes. Taller chip components preceding smaller parts into wave soldering can create a shadow effect, resulting in poor solder connections. Unbalanced copper between the two pads of small chip components can create uneven heating, resulting in one pad’s solder melting before the other one and pulling the other side up and off its pad.
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 may make working on them more time-consuming or cause collateral damage to adjacent parts. Likewise, connectors, switches, and other human interfaces that aren’t accessible can also slow down the manufacturing of the circuit board.
One extremely important guideline is that placement should start with developing a basic floor plan of the parts on the board. This will allow you to strategize how to partition the different circuitry areas on the board to avoid overlapping analog and digital signals.
PCB layout guidelines: effective component placement will lead to the best trace routing
PCB Design Layout Guidelines for Routing
It is essential for circuit board designers to layout their board to create the best signal and power integrity possible. Components should be arranged in the optimum position for short and direct trace routing. At the same time, the board must be laid out so that all of the nets can be completely routed. Trying to balance these needs can be quite a challenge in high-density designs. The first PCB design layout guideline is to set up the design rules and constraints for trace routing.
Design Rules and Constraints
Technically, configuring the design rules and constraints should have been included with the parameters and setups. But, since a large part of the rules apply directly to trace routing, we have included this guideline here. Rules and constraints are used to govern trace widths and spacings and can be set up for individual nets, groups of nets called net classes, or as a default for all non-specified nets. Design rules are also used to control which vias are selected for different nets, trace lengths and matched lengths, and which board layers are allowed for routing specific nets and routing topologies. Additionally, design rules are also used to control component spacing, silkscreen rules, mechanical clearances, and a host of other constraints.
Signal and Power Integrity
For maximum performance and signal integrity, PCB layout designers need to follow specific requirements for routing traces of different circuitry. Here is where the design rules and constraints will help--by allowing designers to input the physical routing parameters into the CAD system for routing. Although the exact values will change depending on the needs of the board, designers will usually set up rules to ensure the following guidelines are followed:
- Short and direct high-speed transmission line routing.
- Trace width, spacing, and allowed board layers for controlled impedance routing.
- Specified trace lengths and length tolerances for matched length routing.
- Differential pair trace widths and spacing requirements.
- Width and spacing for sensitive signals such as clock and control lines.
- Via types for different nets.
- Trace widths and spacing for analog circuitry.
- 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 Effective Power and Ground Planes
With modern high-speed designs, the best grounding strategy is typically to use one or more continuous ground planes on an internal layer. This gives the best protection from EMI and ensures clear signal paths, which will improve overall signal integrity. Avoid routing traces across any ground voids for areas where the ground plane is broken up due to unique board contours or features. Without a continuous and adjacent ground plane for the signals to use as a clear return path, your design may create a lot of undesirable noise. Here are some power and ground plane guidelines to keep in mind:
- Ground planes need to be adjacent to signal layers in the board layer stackup with high-speed routing. This will help shield the high-speed routing from interference and provide a good reference plane for the signal return paths.
- Thermal relief pads need to be used and carefully managed for power and ground connections to the planes. The relief pad spokes must be wide enough for high currents while eliminating those connections’ chances to act as a heat sink.
- Plan power connections and split power planes carefully to ensure that the power is adequately delivered to all the connected parts throughout the circuit board.
Avoid routing analog and digital circuitry together in a mixed-signal design
Silkscreen and PCB Test Guidelines
With the circuit board design completed, it will be time to turn your attention to finalizing the layout by cleaning up the silkscreen layers and adding testpoints. Reference designators, part numbers, and other corporate information are marked in ink on the circuit board through a silk screening process. Designers typically use “silkscreen” layers in their CAD systems for designing these markings.
To ensure silkscreen layer markings are readable, designers follow these guidelines:
- Line widths should be no smaller than 6 mils.
- Font sizes should be no smaller than 50 mils.
- Rename component reference designators according to a corporate grid pattern to help locate specific parts on the board.
- Move and rotate the reference designators so that they are easily readable.
- Include polarity and pin one markings where needed.
Testpoints are essential for circuit boards that will be mass-produced for automated assembly validation. Each net in the design should have a testpoint on it, whether that testpoint is an existing thru-hole pin, a via, or an added surface mount testpoint pad. Testpoints should have at least a 50 mil clearance to other board objects such as components or pads, and be at least 100 mils from the edge of the board. However, these values are likely to change from vendor to vendor, so be sure to check first what your manufacturer’s testpoint requirements are.
Guidelines for PCB Manufacturing Files
The last PCB design layout guideline is to create the manufacturing files for fabrication and assembly and send those files out to your vendors. Output files are often automatically generated by scripts developed by designers or your CAD department. Most PCB design CAD tools, like Cadence’s Allegro PCB Editor, have built-in creation tools available for use. Many tools also have functionality that communicates directly with PCB manufacturers through the IPC-2581 format. These unique features allow you to automatically transfer a manufacturing database of fabrication and assembly files without the need to create and send each file individually.
For more information on the fabrication data your vendors will be looking for, take a look at this E-book from Cadence.
If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.
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