Estimated reading time: 5 minutes

Flexible Thinking: Flexible Circuits—A Road Less Traveled

Referencing a famous poem may seem an odd way to start a technology column, but I find it fitting; my long engagement with flexible circuit technology has taken me places, both mentally and physically, that I would never have seen or experienced had I not developed an interest in what was once a marginal interconnection technology. Flexible circuits were a road less traveled when I first encountered them but taking that path has made all the difference in my career.

For the last couple of decades, this column—as well as the musings of other flexible circuit advocates in the industry— have extolled the myriad advantages flexible circuits offer as an interconnection medium. As flexible circuit aficionados, we have collectively stressed the unique abilities of flexible circuits to make three-dimensional interconnections, which free the designer to create a never-ending variety of electronic products. We have continued to point to the versatility flexible circuits offer as a means of interconnecting electronic elements that must move relative to each other, such as is required for inkjet print heads, the read-write heads of disc drives, the hinges of laptop computers, and many others. We have also described how effective flexible circuits can be as a medium for controlling the transmission of electronic signals from point to point. With the massive number of opportunities residing in these domains, it might seem quite unnecessary to explore those that lay beyond; yet today, that is exactly where I intend to take you.

A Vast Highway of Opportunity

Heater Circuits
There are countless design opportunities that lie outside the generally perceived realm of flexible circuits, but far too often we simply don’t see them. For example, consider the flexible heater circuit. Thin flexible heaters are used in numerous invisible applications. These heater circuits are often found in such items as fog and frost-free mirrors. They are a simple, yet very practical flexible circuit application that can be found anywhere—from the automobile to the bathroom—where relatively low temperatures suffice to accomplish the objectives. These low-power circuits can be made relatively inexpensively using materials such as polyester films and polymer thick film inks. However, higher temperature flexible circuit heaters are also possible. Such high-temperature heaters can be made using highly resistive metal films or foils such as nickel-chrome, Iconel, or stainless steel—in combination with high temperature materials, such as polyimide, that are capable of withstanding and putting out significant wattage and heat.

Smart Cards
Another area where flexible circuit technologies have played a significant role over the years is in the manufacture of smart cards. This is not a new application for flexible circuits, but it is clearly an emergent technology as we press on into the age of the internet, where nearly every individual item ever manufactured can have a unique, trackable identity. Smart cards have become the backbone of the internet of things (IoT). Such devices generally have a chip attached to provide their identity. Metal coils etched into the copper foil of the flex circuit material are typical features on such devices, used to both send and/or receive wireless data. Because of the huge and growing volume of such circuits, roll-to-roll processing—a process that flexible circuits are naturally suited to—is generally employed to manufacture these increasingly ubiquitous circuits.

Wearable Electronics
Yet another area of gathering potential for flex circuits is wearable electronics, an industry that has exploded in recent years due to the growing interest in personal health monitoring. Typically, these devices include both electronic devices with thinned integrated circuit chips and passive devices that conform to the anatomical features of the wearer. Such devices can monitor many bodily functions, including heart rate, respiration, electrolytes, blood sugar, and many others; this data can then be transmitted to a smartphone, for example. These expanded capabilities have inspired developers to label these more integrated assemblies “flexible hybrid electronics” (FHE). They have been integrated into another area of investigation: stretchable circuits, which use elastic or elastomeric substrates; this technology opens doors to even more practical and fanciful electronic product innovations.

This brief column may have fallen short of covering the usefulness of flexible circuits in its entirety, but for those not yet familiar, it has hopefully provided a taste. The simple reality is that flexible circuits offer an unending range of opportunities to solve problems. This column has been an exercise in trying to inspire you to consider flexible circuits as an alternative “road” to your next electronic design destination and implementation.

Circling back to where I started this column, I’d like to share the poem that has inspired me many times over my life as a courtesy to those who may never had an opportunity to read and enjoy it.

The Road Not Taken

by Robert Frost

Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;

Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same,

And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.

I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.

Joe Fjelstad is founder and CEO of Verdant Electronics and an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 185 patents issued or pending. Download your copy of Fjelstad’s book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series, “Flexible Circuit Technology.”

This column originally appeared in the December 2022 issue of Design007 Magazine.