Flexible Thinking: How to Get From Here to There

"One day Alice came to a fork in the road and saw a Cheshire cat in a tree. 'Which road do I take?' she asked. 'Where do you want to go?' was his response. 'I don't know,' Alice answered. 'Then,' said the cat, 'it doesn't matter."

—Lewis Carroll, author of Alice’s Adventures in Wonderland

Lewis Carroll is said to have written Alice’s Adventures in Wonderland originally for the entertainment of the seven-year-old daughter of a family friend named Alice Liddell. While it has since delighted countless children (and adults, I am certain) with the imaginative and fanciful world it presents since it was first published, the book is also full of playfully thoughtful dialogue that often seems to have meaning deeper than the humorous exchanges belie.

The quoted excerpt includes one such exchange, and it is an instructive life lesson for all who read and take it in fully. To begin the 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.

From Concept to Product

In my book Flexible Circuit Technology, 4th Edition, I mapped out a number of steps for getting from concept to product using flexible circuits in “Chapter 6: Implementing Flexible Circuit Technology.” The pathway was not paved in stone because the evolution of flexible circuit technology is ongoing, and new materials, processes, and processing equipment continue to come online, opening the doors to new opportunities and prospective ways of getting the job done. However, the basics of the process remain the same. So, here in brief are some of the process touchstones that I think are important.

First, there is the product concept. What specific design should be used for elements of the product that make flexible circuits attractive? This question needs to be asked to make sure that flexible circuit technology is truly required for the product. Because the thinness of the circuit is frequently an objective, flexible circuits are usually tapped to provide the sought-after benefit. However, sometimes—even often—a thin reinforced laminate circuit will suffice, and typically, at a lower cost. Thus, do a quick reality check; there is no need to make a product more expensive than necessary.

Second, flexible circuits have the obvious ability to interconnect electronic elements (e.g., modules, displays, connectors, etc.) that are distal from one another, commonly in three-dimensional space. Moreover, these electronic elements may be required to move relative to each other when in use (e.g., disk drives, read-write heads, printer cables, etc.) or during maintenance or upgrading of a system to facilitate access to the elements of interest. Understanding the mechanical requirements associated with this movement will influence design choices, including the type of materials used and the type and weight of metal foil (most frequently rolled annealed copper).

In addition, there are many seemingly insignificant details (e.g., copper foil grain direction) that need to be addressed to ensure that the product will perform to expectations for the duration of its anticipated lifetime. The type and thickness of the cover-layer used in the circuit also plays into this equation as well as the objective of keeping the copper foil at the center of the construction, especially in areas designed for bending or flexing. The first topic is somewhat macro, and the second is a bit more micro and nuanced, but both topics are germane to this subject.

Designing Flex Right the First Time

Circling back to the bigger, yet often overlooked, items associated with the process of getting a flex circuit designed right the first time, there is a need to consider the operating environment for the end product. For example, consider the temperature excursion range and humidity expectations over its life and use as well as the processes that will be used in its fabrication. These factors will impact and likely limit the choices of materials used. Solder remains a commonly used method for assembly. And with higher temperature lead-free solders in use today, it is necessary to choose a material that will stand up to requirements.

Another matter to consider relative to field use is the mechanical stresses and strains that may be encountered by the product. Designers use flex with great frequency in dynamic applications, such as those mentioned earlier (e.g., disc drives read/write heads, printer cables, etc.), and they pay special attention to the circuit features that pass through the dynamic bend areas of the design.

However, products that are deemed to be non-dynamic applications can and sometimes do fail due to copper fatigue failure as a result of shock and vibration endured in a field application. These are not the visible flexural cycles that can be seen with the unaided eye, but microscopic flexural cycles that are characteristic of vibration. While in the former case, the cycles tend to be high amplitude and moderately low frequency, in the latter case, the frequency tends to be high and the amplitude low. I have seen such failures and heard of others over the years.

Conclusion

The intent of this column was to make you aware of some of the issues that need to be considered as you navigate the waters that stretch between product conception and manufacture using flexible circuit technology. Flexible circuits are a very attractive interconnection technology, but you must be attentive to the many factors that can spell the difference between success and failure.

In closing, I invite you to download a free copy of Flexible Circuit Technology, 4th Edition to fill in more of the detail needed to make the journey. The book can be found and downloaded at flexiblecircuittechnology.com.

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 150 patents issued or pending.

This column was originally published in the April 2019 issue of Flex007 Magazine.

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2019

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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A Few Simple Lessons in Designing Reliable 3D Flex

04-11-2019

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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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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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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