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Elementary, Mr. Watson: Consider Physics When Designing Non-traditional Geometries

Electronics have profoundly impacted society, shaping how we live, communicate, work, and entertain ourselves. Nearly every aspect of our lives is affected by electronics in some way. If you want proof, sit and watch people in public; everyone is on a screen.

The communication revolution alone has transformed our lives, enabling instant global connectivity. Some would argue these advances come with their distinct disadvantages. We have never been more connected but more isolated without face-to-face conversations. Smartphones, the internet, social media platforms, and email have transformed how we interact, share information, and conduct business worldwide.

In this realm of modern innovation, PCBs have undergone a profound evolution in their functionality, shapes, and materials. The increasing demands for smaller, more powerful, and more efficient electronic devices have led to a paradigm shift in PCB design and manufacturing.

One of the significant impacts of innovation on PCB shapes is the move away from conventional rectangular and square boards toward more customized and irregular shapes. These unconventional shapes cater to the specific requirements of diverse applications, such as wearables, IoT devices, automotive electronics, and more. These nontraditional shapes allow for better integration into the final product's form factor, enabling designers to maximize space utilization within the device.

But there is something that is much more important here. Yes, PCBs are the foundational backbone of every modern electronic device, and they’re composed of relatively simple fiberglass, copper, and dielectric materials. Every PCB has two purposes: First, to provide a method of placing components, and second, to make all the required interconnections between those components. That, in its most basic terms, is a PCB. These purposes extend beyond mere mechanical support; they're the intricate conductors that orchestrate the symphony of electrical signals within a device. So, at their core, PCBs provide a structured framework, and, I would add, a controlled environment by arranging these components on a board and linking them with conductive pathways (traces). PCBs facilitate the flow of electricity precisely where needed, ensuring proper functionality.

Some novices believe that PCB design is just "connecting dots" in the schematic and routing in the PCB, but design encompasses so much more. A PCB is a controlled environment. Our PCB refers to the deliberate design and engineering practices implemented to regulate various factors influencing electron flow and electrical performance. This controlled environment ensures reliability, stability, and optimized functionality within an electronic system.

With the ever-increasing demand for smaller electronic devices, which in turn means higher-density PCBs, the PCB design process cannot be done in a vacuum. Looking at only one aspect, such as the board shape, without looking at everything else, sets us up for failure. There is a balance and what I would refer to as a symbiotic relationship within the entire PCB—the flow of electrons in copper and how it interrelates to the fiberglass and other copper areas. Any change in one of these factors will affect the physics of the PCB. Any changes in the "platform" (especially the board shape) further impact the design's physics, such as controlled impedance, signal Integrity, and EMC/EMI consideration. Throughout the PCB design, with any changes, there are documented trade-offs for changes; some are improvements, and others are not. Know these trade-offs and know if implementing a change would cause more problems.

I have heard various analogies to describe a PCB design, from a city with the buildings being components, and the streets being routing. But one of my favorites is considering a complex orchestra performance where various musicians with distinct instruments come together to play a symphony. In this analogy, the PCB represents the stage where this intricate performance unfolds.

Each electronic component on the PCB is like a musician, playing a unique part in the composition. The conductive pathways on the board act as musical scores, guiding the flow of electrons like musical notes flowing through the air.

Just as in an orchestra, where timing, coordination, and harmony are crucial, the PCB maintains a delicate balance. The precise arrangement of components and traces mirrors the musicians' seating arrangement and sheet music, ensuring that every electron (note) reaches its destination at the right time and in the correct sequence.

As a conductor guides and coordinates a diverse group of musicians, the controlled environment of the PCB orchestrates the electron flow. It carefully manages impedance, signal integrity, and interference, ensuring each musician plays in tune and at the right tempo, preventing overlaps or disruptions.

Just as an orchestra requires a well-designed stage and acoustics to produce a flawless performance, a well-engineered PCB provides the environment for electrons to move efficiently, harmoniously, and flawlessly through an electronic system, producing the symphony of functionality that powers modern technology.

Let me demonstrate my point. Consider the 62-mil PCB in Figure 1. It features a four-layer stackup of two signal layers on the outside and a ground and power plane on the two internal layers. A differential pair was routed on the top layer with the internal ground plane as its reference, with a distance of 10.2 mils. The trace width required to obtain a 100-ohm impedance matched pair is 6.41 mils. If everything works fine, the environment will be considered controlled.

Because of a lack of planning, mechanical changes are required to make the product smaller and sleeker. The PCB stackup width must be cut in half—down to 32 mils. These are the sort of changes that keep PCB designers up at night. A relatively simple change causes considerable problems in the design, especially in how these differential pairs work. Not making any changes in the PCB in terms of material, with the new thickness on the PCB, means that the trace width cannot exceed 4.226 mils. This change may also affect the required distance between the two traces to allow the correct coupling of the energy wave of each trace to allow them to work correctly and cancel any noise.

 In conclusion, meticulous considerations are vital when altering the PCB's physical aspects and impacting its physics. Validate material changes for their electrical properties, ensuring compatibility with signal propagation and impedance requirements. Evaluate the impact of shape modifications on electromagnetic interference (EMI) and signal integrity, conducting simulations or analyses to anticipate disruptions. Assess thermal properties to prevent overheating or impedance variations due to alterations. Follow industry standards and guidelines to maintain desired characteristics. Collaborate with electrical engineers and utilize simulation tools to predict and mitigate potential changes in impedance, signal loss, or EMI. Test prototypes rigorously to validate alterations before final implementation.

Do not decide to change the shape of the PCB without considering how those changes will impact your overall PCB design, the pros and cons, and whether you are opening the proverbial can of worms.

This column originally appeared in the December 2023 issue of Design007 Magazine.