Estimated reading time: 5 minutes

Elementary, Mr. Watson: Pushing Design Boundaries

Imagine constructing a suspension bridge, where the flexibility of the bridge's structure is crucial for withstanding dynamic forces such as wind, traffic, and seismic activity while maintaining stability and functionality.

In this analogy, the bridge's design flexibility represents the ability of PCB designers to adapt and optimize the layout, materials, and components to meet the demands of high-speed design. Just as a suspension bridge must flex and adjust to varying conditions, a high-speed PCB design requires flexibility to accommodate signal integrity, power distribution, and thermal management.

Let's look at the consequences of the other side. If the bridge's design were too rigid and lacked flexibility, it would be vulnerable to stress fractures, instability, and even collapse under changing conditions. Similarly, an overly rigid approach with excessive constraints in high-speed PCB design can lead to signal degradation, impedance mismatches, and reliability issues, especially as environmental conditions or operational requirements change.

Furthermore, regarding our analogy, a well-designed suspension bridge balances strength with flexibility, allowing it to absorb and distribute forces efficiently while maintaining structural integrity. Likewise, a high-speed PCB design that balances constraints and flexibility can optimize signal transmission, minimize interference, and adapt to evolving design requirements without compromising performance or reliability.

Overconstraint: What a concept. Our first thoughts would be: What are we hurting by overconstraining a design? Isn't it better to be safe than sorry? What is meant by overconstraint? It means to apply excessive constraints. In engineering and mathematics, it's used when there are too many simultaneous equations to result in an exact solution. For example, fitting a line to many points is overconstrained because a line cannot be drawn simultaneously through all of the points. In PCB design, overconstraints always occur, including dimensional, electrical, manufacturing, and timing constraints. The list goes on.

Designers often unknowingly fall into a significant pitfall by overconstraining designs, especially related to high-speed designs and materials. I've observed several reasons why novices (and I would add experienced) designers have overconstrained designs.

Risk aversion: Designers may err on caution, imposing overly conservative constraints to mitigate potential risks or uncertainties in the design process. However, excessive caution can lead to unnecessary limitations that hinder performance or increase costs. Furthermore, overconstraints will limit the fabrication of your design.

I recently spoke with a fabrication house. They confirmed that for the past 10 years, the ECAD software improvements and capabilities have outpaced advancement in the fabrication process. A great example is that most ECAD tools can now easily go down to 1/10000 of an inch tolerance in certain design specifications. The fabrication could never meet the stringency of tolerance.

There is a point where PCB designs are over-engineered, and you cross over to overconstraints. I believe this directly results from unclear specifications and a lack of understanding of design objectives. With more strict specifications, designers may need more time to meet rigorous specifications or requirements within tight deadlines, leading them to impose overly aggressive constraints without thoroughly evaluating their impact on performance, manufacturability, or cost.

The need for more experience: Designers less familiar with high-speed design principles may apply constraints based on general guidelines or outdated practices without fully understanding their implications. Inexperienced designers may be more prone to overconstraint due to a lack of knowledge about alternative approaches or optimization techniques.

Perceived design best practices: Designers may adhere rigidly to perceived best practices or industry standards without considering alternative approaches or innovative solutions. While established guidelines can provide valuable guidance, mindlessly following them without critical evaluation can lead to overconstraint. A widespread belief is that we have always done it this way in the past, so why change it now? If it's not broken, don't fix it. How often have we heard that?

Let us look at a common overconstraint in PCB regarding materials. Overconstraints in high-speed PCB design regarding materials can occur when the material properties are overly restricted or specified too conservatively, leading to unnecessary limitations or constraints that hinder design flexibility or increase manufacturing costs. Here's how several overconstraints can manifest in the choice of materials:

Excessive dielectric permittivity requirements specifying an overly high dielectric constant (εr) for the PCB substrate material can lead to overconstraint. While certain high-frequency applications require specific dielectric constants to control signal propagation, unnecessarily strict requirements can limit the choice of available materials and increase costs without providing significant performance benefits. Commonly, even high-speed design would work with a lower high dielectric constant (εr). This is something you should keep in mind when selecting materials.

That could go back to a long-standing principle in PCB design I have followed for many years: Just because you can do something doesn't mean you should.

Strict tolerance on dielectric thickness: Requiring an excessively tight tolerance on the thickness of the dielectric layers can restrict the selection of substrate materials. While maintaining uniform dielectric thickness is critical for controlled impedance, overly stringent tolerances may limit the range of available materials and increase fabrication costs. Tolerance is something that is an afterthought with most designers. They are doing everything they can to solve the puzzle, and setting and understanding tolerances for the design often doesn't happen.

Narrow range of available materials: Limiting the selection of materials to only a few options without considering alternative substrates with comparable performance characteristics can lead to overconstraint. New materials or composite substrates may offer improved performance or cost-effectiveness, but strict adherence to a narrow range of materials may overlook these alternatives.

Unnecessary constraints on Dk and Df: Overconstraining the dielectric constant (Dk) and dissipation factor (Df) of the substrate material can once again limit the choice of available materials. While controlling these parameters is crucial for signal integrity and minimizing signal loss, overly strict requirements may unnecessarily restrict material options and increase manufacturing costs.

Excessive material cost constraints: Requiring high-cost materials without considering lower-cost alternatives that meet the performance requirements can lead to overconstraints. While premium materials may offer superior performance characteristics, they may not always be necessary for achieving the desired electrical performance, and cost-effective alternatives may suffice.

To avoid overconstraints in high-speed PCB design regarding materials, it's essential to carefully evaluate the performance requirements, consider alternative materials with comparable characteristics, and balance performance goals with cost considerations. Collaboration between design engineers, material suppliers, and manufacturers can help identify optimal material choices that meet the project's requirements without unnecessarily limiting design flexibility or increasing costs.

John Watson is a professor at Palomar College, San Marcos, California.