Flexible Thinking: Flexible Circuits Vs. Flexible Hybrid Electronics—Where’s the Line?

The basic idea of a flexible circuit is arguably more than a century old, based on the interpretation of a 1903 patent by Albert Hanson, which circuit industry maven and historian Dr. Ken Gilleo found in his research on industry origins some years back. Fundamentally, it was an ordered pattern of wires on a wax paper-like insulator. This is still a suitable definition to this day.

However, today’s flex circuit technology is many orders of magnitude beyond its humble beginnings in ways likely far beyond the imaginings of Hansen in his day. Moreover, the materials and processes used to produce flexible circuits have been on a steady evolutionary path with advances coming routinely. Not all the advances are ground-breaking but each one adds to the designer’s tool chest to create the product needed to solve a general problem or particular issue.

The term flexible circuit is the fundamental and most universally accepted term for the class of products that meet the simple description. Flexible circuits can be simple one-metal-layer devices to replace discrete wire assemblies, or complex multilayer or rigid-flex assemblies that are often populated with all manner of electronic components, both through-hole and surface mounted normally using solder. The most common materials used are copper for conductors, which are commonly etched from a thin copper foil, and either polyimide or polyester for the flexible base material to which the copper foil is bonded. In most cases flexible circuits are designed to conform to the surfaces of the housing in which they are contained. Dynamic flexing is required in a relatively small percentage of applications; however, it is important that the designer be familiar with dynamic flexing guidelines as naturally occurring vibrations encountered in use can result in flexing-related failures if their frequency and amplitude are sufficiently high.

That covers in broad brush strokes what a typical flexible circuit is, so what then is a flex hybrid electronic assembly or FHE? The term “flex hybrid electronic” is a relatively new term that has been coined to rebrand a lower-cost type of flexible circuit historically called a polymer thick film circuit; it seems to have caught on likely due to the large amount of government and industry in-kind funding to promote use of digital inkjet printing technologies and low-cost substrates along with conductive adhesives to make novel low cost electronic assemblies. Polymer thick film circuits are also typically produced by printing but by screen printing conductive silver-based inks onto a thin, typically polyester polymer film. The features are generally coarser, and the silver conductors have historically tended to be more resistive and thus less suitable for higher power or higher performance applications, but they have proven to be very well suited to applications were power is lower and higher voltages are used. One of their most common applications has been in the manufacture of membrane switches and keyboard circuits, but the technology has been adapted to the manufacture of numerous other products.

Another distinction between the two is that FHE not only has greater demonstrated interest in the use of inkjet printing but also increased focus on the integration of electronic components and function into the designs facilitated by sequentially inkjet printing conductive, resistive, insulating and even semiconductive materials to realize unique flexible circuit designs. This includes the prospective printing of resistors, capacitors, and inductors of some precision. There is also demonstrated interest in building in sensors and thinning of semiconductor ICs to make them flexible as well.

In summary, the line separating polymer thick film flexible circuit assemblies from flexible hybrid electronics exists, but it is not hard and bright. The introduction of new flexible circuit manufacturing technologies and materials including stretchable substrates has created a surge of interest in their use. This is due in no small part to the versatility they offer. As a nearly half-century veteran of the electronics interconnection industry, including participating in a startup that sought to produce flexible circuits on a moving web in 1990, I have a bit longer view and perhaps a bit more appreciation of the evolution of flexible circuit technology.

The advances being offered coming from and being exploited by those newer entrants to the industry have injected a new vitality to a venerable type of flexible circuit in the form of FHE. That energy and creativity is resulting in FHE proving itself to be highly useful in ways heretofore unseen in the past. For that, my hat is off to them.    

This column originally appeared in the January 2021 issue of Design007 Magazine.

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2021

Flexible Thinking: Flexible Circuits Vs. Flexible Hybrid Electronics—Where’s the Line?

01-21-2021

The line separating polymer thick film flexible circuit assemblies from flexible hybrid electronics, exists but it is not hard and bright. The introduction of new flexible circuit manufacturing technologies and materials including stretchable substrates has created a surge of interest in their use.

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2020

Flexible Thinking: Thermal Management—Electronic Technology’s Rodney Dangerfield

09-17-2020

Thermal engineering has, unfortunately, often been treated with less respect than it deserved. Dealing with the heat generated by electronics was often not given full consideration until after the design was completed and prototyped, and the problem manifests as a failure. Joe Fjelstad emphasizes why keeping devices cool is a vital objective.

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Flexible Thinking: Designers at the Edge

07-15-2020

Designers often play it safe in the center, but step out on the edge and you’ll likely see things much differently. Joe Fjelstad shares his thoughts.

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Flexible Thinking: Lead-Free Solder—Panacea or Pandemic?

06-26-2020

Solder has been used as the primary means of interconnecting electronic components for more than seven decades. For the benefit of all those who are new to the electronics interconnection industry, Joe Fjelstad shares how we got to this point.

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Flexible Thinking: When Expectations and Results Don’t Line Up

05-15-2020

Around 20 years ago, I had the good fortune of receiving a recommendation to read the book The Four Agreements by Don Miguel Ruiz and subsequently picking it up. It is a short and simple book that the author says is based on ancient Toltec wisdom.

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Flexible Thinking: DFM or Design With Manufacturing?

04-15-2020

The great Irish author, playwright, and humorist Oscar Wilde once defined a cynic as an individual who knows the price of everything and the value of almost nothing. Unfortunately, over the decades, that same analysis could often be applied to procurement agents in electronic product companies around the globe. The reward for a purchasing agent is too often derived not from getting the best solution for their company but the best price

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Flexible Thinking: Profitability—A Vital Design Requirement

03-27-2020

The decisions designers make will impact virtually every manufacturing step in the fabrication and assembly of electronics products. Joe Fjelstad explains how applying “design for” guidelines can help create products that can be made both reliably and profitably when applied.

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Flexible Thinking: Power and Thermal Management—Dealing With the Heat

02-15-2020

Without power, electronics are useless. With power, miracles happen. Managing that power is critical in both design and operation in terms of heat generation and energy conservation, especially for battery-powered devices. Moreover, often in electronic products, designers find themselves providing power to an electronic module or system at multiple different voltages and currents.

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Flexible Thinking: Looking Back and Looking Forward

01-27-2020

The month of January is upon us once again. The month is named after the Roman god Janus. According to Wikipedia, Janus is the god of beginnings, gates, transitions, time, duality, doorways, passages, and endings. He is usually depicted as having two faces: one on the front of his head, and one on the back since he looks to the past and future.

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2019

Flexible Thinking: The Value of Experience

12-15-2019

For many people, December is a month in which to reflect on the experiences and lessons encountered and learned over the past year. As the years pass, I am increasingly thankful for the many experiences that have brought me to this point. In sitting down to collect and share my thoughts, what first came to mind was a timeless story about the value of experience. It goes something like this.

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Flexible Thinking: Additive Manufacturing of PCBs

11-23-2019

We are seeing increasing interest in technologies that will allow one to make electronic substrates in near real-time using additive processing techniques and 3D printers. It is a true game-changer in product development. The surge in interest in additive manufacturing technologies shown in recent times—as indicated by the significant increase in published articles and press releases—suggests that the electronic interconnection manufacturing industry could be on the verge of a manufacturing renaissance.

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Flexible Thinking: Standards—An Industrial-strength Glue

10-21-2019

Standards are frequently viewed as cumbersome nuisances and impediments to progress by those pressing for rapid change. The process of writing, getting approval, and promulgating standards can be arduous and frustrating. It has a lot of similarities to the creation and passage of laws in various government bodies in that there are many opinions and interested parties who engage in the process to make sure that it results in a product that does not damage or favor one solution or party over another.

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Flexible Thinking: Making Flexible Circuits Stretchable

09-05-2019

It is my opinion that the initial driving impetus for the development of stretchable circuits was a bit different than normal, meaning that military and aerospace have traditionally driven the development of arcane electronic interconnection technologies as they did with the development of both flexible and rigid-flex circuits. In contrast, it was a consumer-driven market that appears to have been the gate opener in the form of wearable electronics.

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Flexible Thinking: How to Get From Here to There

04-26-2019

To begin any process, you must first know where you are going. This is true for any project or life pursuit, I believe, and I often try to bring it to mind as I start any new project. With respect to developing products that might benefit from flexible circuit technology, this is no less true. Find out why.

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Flexible Thinking: Ways to Conserve Flex Circuit Material in the Design Process

02-25-2019

In summary, the decisions made by the flex circuit designer when laying out a flex circuit will have an impact that lasts the entire process. By considering how the circuit might fit onto a panel before submitting the design to a manufacturer, it may be possible to save a considerable amount of material and money.

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Flexible Thinking: A Few Simple Lessons in Designing Reliable 3D Flex

02-11-2019

There is an old and familiar adage that goes something like this: “If the only tool in your tool chest is a hammer, you tend to see every problem as a nail.” We all have a tendency to stick close to the familiar and use the tools we know to create solutions to problems confronting us; we’re only human.

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Flexible Thinking: A Few Simple Lessons in Designing Reliable 3D Flex

01-15-2019

We all have a tendency to stick close to the familiar and use the tools we know to create solutions to problems confronting us; we're only human. Unfortunately, using only familiar tools limits our ability to come up with optimal or even superior solutions. This article will help you avoid some of the traps conventional wisdom doesn't always give guidance on.

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2018

Flexible Thinking: Achieving Continuous Flexible Circuit Innovation

12-07-2018

Since their introduction, flexible circuits have continued a steady climb from relative obscurity to center stage in the world of electronic interconnections. Today, they are among the most popular choice for solving challenging electronic interconnection problems. Those who use this technology on a regular basis are familiar with the many reasons for the popularity of flex.

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Flexible Thinking Redux

07-02-2018

Flexible circuits are known by a few different names depending on one’s global location and language: flexible printed circuits, FPCs, flex circuits, flexi circuits, flexibles, bendables and a few others that are application-specific such as flexible heater circuits and controlled impedance cable constructions. While flex circuits are an original and foundational interconnection technology for electrical and electronic products (one of the first patents for electrical interconnections, issued at the turn of the last century, was arguably a flexible circuit), over the years there have been several forays into technological extensions of the basic idea.

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2017

Flexible Thinking: The Benefits of Employing a Standard Grid Pitch in Design

03-31-2017

The industry at large needed to jump on the learning curve and overcome its fear of the unknown. One of the most vexing concerns at the time (an arguably still today) is that terminations beneath the area array package were unseeable. Given the fact that then, as today, solder joints were a major cause of failure, there was much consternation over the quality of the joints.

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2016

Flexible Thinking: Process Engineering—PCB Manufacturing’s ‘Delta Force’

05-11-2016

Process engineers serve a vital function on the front line of printed circuit manufacturing. They are often, if you will, the “Delta Force” that subdues and controls that which is one of the mortal enemies of manufacturing…process variation.

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2015

3D Printing in Electronics - A Perspective

01-14-2015

Knowing the value of a product or technology is key to making the right decision. Appreciating the value of an element of business is evermore important as the rate of change surrounding an industry accelerates. This brings us to one of the current buzz subjects in our industry: 3D printing. Understanding what it is and what its value is to a company and that company's ability to improve its place in the industry is vital.

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2013

The E.I. Files: The Electronics Industry's Black Swans

07-31-2013

First proposed in 2007, there is a potential electronics industry "black swan" technology quietly being developed and refined. It is one that could greatly and positively impact, at once, the cost, reliability, and environmental friendliness of electronic manufacturing by simply eliminating the soldering process.

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Flex Circuits and Photonics: A Pairing for the Future, and the Here and Now

04-17-2013

Photons are making continuous headway into the world of electronics. One thing that the basic data carriers (electrons, microwaves and photons) have in common is that flexible circuits are being increasingly looked to for help in managing their data transmission function.

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2012

Stretching the Limits of Flex

11-29-2012

Those steeped in flexible circuit design and manufacture for any length of time fully appreciate the long list of benefits that only flexible circuits can offer. Some of the most fundamental benefits of flex circuit technology have been exploited since the earliest days of the technology. Joe Fjelstad explains.

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Flexible Thinking: Circuit Flexibility (and How to Achieve it)

10-04-2012

The most common interpretation of the word flexible, as applied to the flex circuits that the industry currently makes, is something capable of being bent repeatedly without breaking. Joe Fjelstad discusses a few other definitions of flexible that are worthy of consideration when using the term, for their ability to unlock new thinking patterns relative to what is flexible.

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Technology Roadmaps: Thoughts and Observations

09-26-2012

If one is without a sense of the direction their technology is headed, odds are that they will sooner find themselves on the road to ruin than the road to success. A technology roadmap is a critical tool in helping a company make informed decisions. By Joe Fjelstad.

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2011

Something Old, Something New: Stretchable Circuits and Elastronics

10-13-2011

The stretching of circuits to alternately increase and decrease the length of a circuit has proven useful for electronic products and assemblies for years. Stretchable circuit technology and elastronics are poised to take on challenges that cannot be easily met by flexible circuit technology alone. Keep them in mind next time you find yourself in need of a little more "spring" in your design. By Joe Fjelstad.

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Stretchable Circuits: The Emergence of "Elastronics"

07-14-2011

The stretchable circuit is an interesting and promising new branch on the flexible circuit tree. The stretching of circuits to alternately increase and decrease the length of a circuit has proven useful for many years. The European Union has funded research in this area through such initiatives as the STELLA project.

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2010

Flexible Thinking: An Alternative Approach to Rigid-Flex Assembly

11-18-2010

The fundamental approach to manufacturing rigid-flex has remained constant for the 40-plus years of rigid-flex history. But is there a better way? What if one could produce a circuit that was rigid throughout the manufacturing process and only become flexible in the final step? In other words, what if one could make a rigid circuit assembly, flex?

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Reasons Why The Flex Market Continues to Shine

10-07-2010

According to IPC market statistics, flexible circuits continue to be the brightest sector of the overall printed circuit market. The reasons for this are many but, at the end of the day, it generally boils down to the fact that flexible circuits are an excellent way to solve interconnection challenges in a cost-effective way.

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Flexible Thinking: Flexible Structures for Data Transmission

08-12-2010

Flexible circuit cables offer some significant advantages for facilitating the movement of data between elements of a system that must also be moved or flexed. However, there is a balancing act involved and there is more than one master to be served to create a system that is robust, reliable and easily manufacturable.

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Flexible Thinking: Supporting Components on Flex Circuit Assemblies

07-21-2010

With proper planning, stiffeners can be designed to aid assembly through the designed manufacture of a flex circuit that can be handled as if it were a rigid circuit board. Such constructions can be accomplished by using any one of several methods.

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A Simple Approach to Flex Manufacture, Assembly

04-29-2010

Flexibility, the single attribute that makes flex so attractive, also makes flex circuits more difficult to build. What if we could produce a circuit that was rigid throughout the entire manufacturing process and only become flexible in the final step?

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