How to prepare a board file for layout.
The proper workflow to ensure manageable routing even for daunting layouts.
The importance of good documentation when wrapping up designs.
The schematic and board are the start and endpoint of PCB layout design steps, respectively
Preparation is the cornerstone of success in the PCB design process. Improvement and growth will come with experience and a straightforward workflow without surprises will enable designers to implement best practices. Here, we will walk you through the PCB layout design steps you need to know.
An Overview of the Layout Process
Beginning PCB Layout Design Steps
In the beginning PCB layout design steps, designers need to ensure the CAD library parts and schematic are ready to go:
- Symbols: Although it is an accepted practice to work with “placeholder” parts, inaccurate schematic symbols cannot convey meaningful circuit information. Make sure that symbols have the correct pins, part numbers, and component attributes.
- PCB Footprints: Layout requires correct component size and shape, pin locations and numbering, and land pattern sizes. Land patterns can either be adapted from common packages or designed by hand. Incorrect footprints may result in defects and require redesign time.
- Schematic Placement and Connectivity: Symbols should be organized with high-visibility text. Route nets with clearly defined net names and pin numbers.
Next, gather as many of the board details together before commencing layout:
- Board Outline: The board dimensions should be set prior to any placement or routing. Adjusting the size or shape of the board mid-design will negatively impact productivity.
- Holes and Zones: Incorporate mechanical information such as keep-out areas, cutouts, or mounting holes. Doing so prevents having to duplicate placement work that failed to heed mechanical features.
- Layer Stackup: Work with the manufacturer to determine the correct layer stackup before starting, then shift to a field solver/calculator to determine the target impedance for the conductor types used on the board. Having to add, delete, or repurpose layers for a board mid-design can cause serious delays.
Establishing communication with the manufacturer is critical to success. The best laid-out board cannot be manufactured if its features are unproducable. Becoming acquainted with the manufacturing process and any special exceptions that need to be made for a particular board design will help shape design rules.
The first steps in successful design are making sure your schematic and library parts are ready
Placement and Routing: The Heart of Layout
With the design set, it’s time to place and route the board. While placement must occur before routing, there is a give-and-take between the two steps: placement provides the framework for routing, but performance and signal integrity of the traces generally trump placement. Therefore, designers should not be afraid to readjust placement during routing to coax out a superior layout. With that in mind, some additional component placement considerations:
- Use schematic signal paths and component groupings to guide placement. For example, a small terminating resistor may fit perfectly next to a driver, but it needs to be placed at the end of the signal path, not at the beginning.
- Place components to keep trace lengths as short as possible (excluding specific length requirements).
- Decoupling capacitors should be as close as possible to the device pins that they are tied to in the schematic.
- Be mindful of all mechanical constraints to avoid overlap between components and the enclosure.
- Evaluate the placement in accordance with manufacturing, test, and thermal constraints.
Another aspect of placement and routing is planning out the power delivery network (PDN). Components need good power coverage while signal routing requires an adequate return path on a continual ground plane. Balancing these constraints almost always comes with some performance tradeoff; take the time to figure out the best course of action to preserve design intent.
Fortunately, with proper component placement and PDN design, the routing process can be simplified. While routing is rarely trivial, knowing that placement has been optimized will go a long way to decrease the challenge.
Take the time to carefully route escape patterns for BGAs and other fine-pitch devices. Use the design rules and constraints to standardize trace width and spacing as well as via holes and pads. Once the routing is complete, run the design rule checker and clean up any errors. A fully routed board can now be used for a wide range of analyses, especially signal and thermal modeling.
By going through each step of the design, you will soon be routing boards like this
Passing the Design to the Manufacturer
Layout may be complete, but the designer still needs to create and send out the manufacturing documentation. Depending on the design and manufacturer’s requests, certain file types may be added or omitted. However, almost all manufacturing file packages will at least contain the following:
- Fabrication and assembly drawings
- Design documents
- Bill of Material report
- XY location files for automated assembly machines
- XY location files for testing
While the difficult part of the layout may be over, designers should take heed that poor documentation corresponds to delays, manufacturing challenges, extra costs, and poor quality. Designers must ensure their design intent carries through to the manufacturer with clear, unambiguous, and thorough documentation.
Layout designers can also improve their productivity and the final product with comprehensive toolsets that offer a myriad of design options, simulations, and manufacturing checks for PCB layout design steps. Cadence’s PCB Design and Analysis software offers design teams an all-in-one design for manufacturing solution. For layout specifically, OrCAD PCB Designer assists teams with a powerful yet easy-to-use environment with a high degree of customizability.
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