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Beyond Design: Refining Design Constraints

Before starting any project, it is crucial to develop a thorough plan that encompasses all essential requirements. This ensures that the final product not only aligns with the design concept but is also manufacturable, reliable, and meets performance expectations.

High-speed PCB design requires us to not only push technological boundaries but also consider various factors related to higher frequencies, faster transition times, and increased bandwidths during the design process. These requirements should be considered during placement and routing; however, establishing clear rules for the software to adhere to and verify is essential.

These constraints should be established at the outset to manage various downstream processes. However, if they are overly stringent, the design may become unmanufacturable or unroutable. Fortunately, constraints can be modified on the fly if a specific rule is found to be too restrictive, as long as the designer can justify relaxing the specification while ensuring the product remains manufacturable.

In the past, we managed only a handful of critical signals. But the landscape has shifted, and now a substantial number of interconnects have reached critical status. Each design necessitates a specific set of constraints based on the technologies employed. While we can transfer basic design rules for trace width, clearance, and other parameters from one design to the next, individual constraints must still be defined. The reuse of constraints is also restricted by net and group naming conventions. Consistency in naming makes the porting process much smoother.

Neglecting signal and power integrity, as well as electromagnetic compatibility, can result in serious problems regarding system performance. This oversight may cause schedule delays, increased development costs, and the potential failure to produce a functional product. Establishing effective constraints when working with SI, PI, and EMC is crucial for the success of your design.

Furthermore, each specific interface, such as DDR, PCIe, or USB, has its own unique set of stringent constraints. To effectively address high-speed design challenges, it is crucial to comprehend the underlying issues and translate them into specific design constraints that will guide the entire design process. Developing these high-speed design constraints based on pre-layout simulations, as illustrated in Figure 1, is highly recommended. This proactive approach ensures that the design is optimized for manufacturability and performance, reducing costly iterations and later debugging.

Pre-layout simulations allow engineers to:

• Define routing constraints such as impedance control, delay matching, and skew requirements
• Analyze crosstalk, signal attenuation, and power distribution to prevent performance degradation
• Establish layer stackup planning for controlled impedance and high-frequency operation
• Optimize termination schemes and buffer selection before placement and routing

By integrating a constraint-driven methodology, designers can streamline the feedback loop between simulation and layout, ensuring that PCB routing adheres to predefined electrical performance requirements. This approach minimizes trial-and-error adjustments and enhances overall design reliability. 

Design rules must evolve alongside the latest devices and fabrication processes while maintaining a focus on design for manufacturability (DFM), which involves creating products that can be manufactured cost-effectively using existing processes and equipment. Adhering to IPC guidelines ensures that your designs are optimized for both manufacturability and mass production. However, there are times when it is necessary to bend the rules slightly to accommodate specific design requirements. This is acceptable as long as you can justify your decisions and accept the potential consequences.

As signal frequency and rise times increase, PCB design becomes increasingly complex. Designing intricate PCBs necessitates extensive knowledge, experience, and the use of simulation tools. However, it is not always essential to minimize trace lengths, couple differential signals closely, or eliminate crosstalk. The importance of the signal plays a crucial role in these decisions. Essentially, designers must identify the sensitive components within the circuit and recognize potential issues arising from coupling and reflections. Armed with this understanding, effective device placement can be achieved. Given the critical nature of placement in high-speed design, designers should consistently consider both placement and return current.

Establishing design rules for complex PCB designs requires careful consideration of manufacturability, signal integrity, power distribution, and thermal management in a structured approach:

Understand Fabrication Constraints

• Consult your preferred PCB manufacturer early to determine their capabilities (minimum trace width, via sizes, layer stackup)
• Start with the IPC standards, which ensure manufacturability

Define Electrical Design Rules

• Trace width and spacing: Ensure proper clearance to prevent crosstalk and meet power requirements
• Via design: Optimize via sizes and types (through-hole, blind, buried) for signal integrity
• Power and ground planes: Use solid planes to minimize noise and improve power distribution where possible

Component Placement Strategy

• Group components logically to minimize routing complexity
• Place high-speed components close to connectors and their associated ICs to reduce signal degradation
• Consider thermal dissipation: Avoid clustering heat-generating components

Signal Integrity

• Determine which signals are critical and attach rules to restrict routing and control impedance
• Define technology rules: Group signals by bus, component, and technology
• Set the minimum spacing to avoid crosstalk for each layer
• Ensure consistent spacing and symmetry for differential signals

Power Integrity

• Ensure the planes are low impedance for stable power delivery and add more planes if necessary
• Choose PCB materials with low-loss dielectrics for better high-frequency performance

EMC

• Minimize routing on the outer layers of the PCB to reduce radiation
• Set fanout rules to create BGA breakouts

If the design rules are established correctly, the design process should proceed smoothly. However, there are always exceptions to these rules. For example, when using ICs with 0.3 mm pitch BGA packages, you may need to reduce trace widths, clearances, and via stacks to accommodate routing. There is no one-size-fits-all solution when it comes to design constraints. Nevertheless, having a previous set of rules and reusing them for similar technologies can save time and reduce frustration during setup.

Figure 2 illustrates the typical constraints involved in planning and defining a high-speed DDR2 and DDR3 design. These constraints should be established at the schematic level and carried through to the layout process. This approach allows the engineer to clearly communicate their intent to the PCB designer, reducing the risk of misinterpretation. Additionally, reusing constraints from a previously proven design not only ensures consistent rules but also minimizes the potential for errors.

Entry-level EDA tools often depend on the expertise of PCB designers to identify potential issues as they emerge during the design process. However, today's complex designs necessitate a more constraint-driven, correct-by-construction approach. Once the rules are defined, downstream tools will adhere to them and validate compliance through various Design Rule Checkers (DRCs). For managing a high volume of constraints, a spreadsheet format proves to be more efficient, allowing for sorting, filtering, and duplicating constraints.

Effective constraint management enhances synchronization between schematic and layout, facilitates streamlined access to pertinent PCB data, reduces errors stemming from data integrity issues, and encourages increased reuse of PCB data.

Key Points

• Each design necessitates a specific set of constraints based on the technologies employed.
• The reuse of constraints is also restricted by net and group naming conventions. Consistency in naming makes the porting process much smoother.
• Develop high-speed design constraints based on pre-layout simulations.
• By integrating a constraint-driven methodology, designers can streamline the feedback loop between simulation and layout.
• Adhering to IPC standards ensures that your designs are optimized for both manufacturability and mass production.
• Designers must identify the sensitive components within the circuit and recognize potential issues arising from coupling and reflections.
• There is no one-size-fits-all solution when it comes to design constraints.
• Once the rules are defined, downstream tools will adhere to them and validate compliance through various DRCs.

Resources

• Beyond Design by Barry Olney: “High-Speed PCB Design Constraints,” “The Fundamental Rules of High-Speed PCB Design Part 1”
• “Why impose design constraints,” by Steve Hughes, Siemens EDA

This column originally appeared in the July 2025 issue of Design007 Magazine.