PCB Layout Considerations: Design With Intent

Key Takeaways

• Placement will have the greatest impact on the overall design but will evolve throughout the layout.

• Power plane design will provide some early feedback and molding on initial placement.

• Routing will need to incorporate the ultimate goal of the board file.

With proper technique, dense circuit layouts become easily navigable

For the designer, the bulk of PCB design will concern the layout of the board. The layout covers all the major board design processes, from arranging the land patterns associated with the schematic symbols into the board design space to connecting all of the same net pads together by trace or plane. PCB layout considerations should balance general best practices as well as any specific challenges arising from the needs of the board. Designers and engineers must operate in tandem to form a satisfying board solution to the schematic representation.

PCB Layout Considerations Begin With Placement

PCB layout considerations will have a progressive give-and-take between placement and routing. While both tasks, along with a few others, comprise the greater layout, the interplay between those two will form the majority of this design stage. With the exception of features that have been called out at certain positions (e.g., the x and y distance away from the board edge for a connector), the fluidity of the board is important to allow the designer room to experiment and innovate. 

Placement will occur first–it is, after all, fairly difficult to route connections without pads to guide beginnings and destinations. The rat’s nest–the colloquial name for the lines of connections spanning the board–is addressed in a slow and methodical manner. Think of unwinding a large tangled network of electrical wires; progress at times may seem exhaustingly slow. However, optimization of the design will slowly allow for denser routing and placement patterns without negatively affecting signal integrity. 

One approach to the initial stages of layout is determining the circuitry or signals that are most crucial to the design. These may be indicated by the engineer or called out in design documents, but a good start is differential pairs, data lines, and clock signals. Power circuitry will also play a significant role during routing, and in many ways is its entire own subset due to both the importance of power/return paths and plane design. 

How Placement Influences Plane Design (And Vice Versa)

After initial placement, power and ground planes should be fanned out to establish the plane perimeter–this helps give the designer an early idea of how to begin dividing up the planes, if necessary. The shape and positioning of the planes will then influence routing on adjacent layers. To avoid sacrificing signal integrity, traces must reference the same plane over the entire course of its travel. Traveling over split planes results in a longer and less direct return path, increasing the inductance of the trace. 

In addition to providing a direct connection to the power planes sandwiched in the inner layers, the shape of the copper features also comes into play. First, the width of the features needs to be great enough to meet the current density demands of the board. Copper features that bottleneck or otherwise do not possess a minimum requisite width are likely to encounter issues related to power delivery during operation. Worse yet, significantly choked power design is liable for thermal failure events that could prove ultimately hazardous to the board and operators. 

At this point, assuming the placement is more or less set, components and fan outs can be nudged slightly from their position to promote the better design of power planes. For larger planes, a split plane may be preferable or necessary depending on how constrained the plane layers are. When power planes span multiple layers, it is important that the overlap contains several vias to ensure there are no significant areas of substandard current flow. Typically, this is best achieved at points where the power plane is being created in circuitry or arriving from an off-board connection. A star ground pattern that encompasses a significant copper area with multiple vias is ideal in this case. 

A ground plane pour on the bottom layer can help with thermal and electrical concerns

How Different Design Goals Affect Routing

Following an early template for plane design, signal routing will be the next design descendant of the layout process. The primary goal of routing will simply be to connect all the respective pads and pins together. However, a simple connection alone is not enough. Signals passing by high-frequency components like the switches found in some power circuitry must be spaced far enough to avoid induction from the aggressor signal. Roughly grouping the circuitry at the beginning of placement should descend naturally from the schematic and any supporting design files. 

Placement Impacts Testing and Solderability

During the initial placement stage, designers should use the schematic to guide the grouping of component blocks. Beyond this, designers should be cognizant of how the placement of components on the board may affect downstream processes. Take testing and solderability for example–if smaller components are placed too close to a tall component (like a connector), issues may arise due to the inability of a machine or human operator to access the former. The layout designer needs to understand that failing to account for the processing the board has to undergo during manufacture or rework is likely to increase turn time, as the board files will have to undergo revisions. In special cases like this, the layout designer may need to break from standard best practices. For example, while short trace lengths may be preferable for signal integrity, a longer trace to prevent production issues takes precedence. 

Layout for Production vs. Simulation

A major fork in the design process will arise from whether the board is being laid out for production or purely simulation. Simulation testing can be an element of design for manufacturing (DFM), but here the simulation is constrained by the real-world manufacturing processes. Designing for simulation involves looser guidelines–for example, trace widths far in excess of standard manufacturing abilities–as the major thrust of the work revolves around providing a fully connected board that does not necessarily adhere to realistic production. As the board in its current iteration is not being prepared for physical production, additional steps may be taken to resolve routing issues that would otherwise complicate manufacturing.

The PCB layout considerations of any board can be vast and vary intensely from one design to the next; for this reason, you should work with PCB design and analysis software that is able to handle the challenges of modern PCB design. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.