Flexible Thinking: Blue Skying It With Aluminum Rigid-Flex

Aluminum is an amazingly versatile metal and has found its way into countless products since its discovery during the reign of Napoleon III of France. At the time, it was more valuable than gold, and at hosted dinners, the emperor and his honored guests dined using aluminum cutlery while the others had to make do with gold utensils.

It took some time for scientists to calculate that it was by far the most abundant metal found in the earth’s crust at 8.3%, ranking third among all elements found in the crust, ranking behind oxygen (46%) and silicon (26%). 

Today, aluminum is one of the most affordable metals and arguably, also one of the most useful. Aluminum has a long list of desirable properties. It is lightweight, and an excellent electrical and thermal conductor. It can be Fjelstad_May_Fig3.jpgeasily machined, stamped, chemically milled, and formed. Being a metal it is, by nature, dimensionally stable. It also expands isotropically (unlike composite materials which typically expand at different rates in X, Y, and Z dimensions). Aluminum can be anodized, which converts its skin to alumina (Al2O3), which is ceramic, and thus offers a potential to create an in situ distributed capacitance layer.

The metal can also be electrophoretically coated uniformly with polymer and enamel insulating materials (think automobile paint uniformity). It also has a CTE which is close to that of copper (22 ppm/°C for aluminum vs. 18 ppm/°C for copper) and is non-toxic (not a RoHS target). It is historically low cost at about $1 per pound. Those in PCB manufacturing will know that aluminum has for many years been a process consumable as drill entry material when drilling printed circuits to mitigate copper burr formation. Typical laminates, exclusive of copper foil, can be two to three times more costly on a by-weight basis and much more expensive to create.     

With such a long list of benefits, it would seem aluminum might be an ideal candidate for making printed circuits (including rigid-flex circuits), except for one hitch. Being a great thermal conductor makes aluminum a lousy choice for soldering components in place due to the high risk of creating cold solder joints, and/or if one is to avoid thermally damaging components by using excessive heat during the soldering process.

What If Soldering Could Be Avoided?
Following is a “blue sky” thought experiment/description of one possible way to build a rigid-flex assembly with an aluminum base and no solder. Rigid-flex circuits were originally designed to be used in rugged military applications and thus are perhaps ideal as a candidate for being adapted to the use of an aluminum core. While such an assembly could be comprised of any suitable metal carrier (brass, bronze, steel, etc.), aluminum, for the many reasons cited at the outset, is arguably preferred.

The process that will be described is basically the reverse of traditional manufacturing, in that, rather than building a rigid-flex board and soldering components to the finished product, the aluminum cores which support the components are processed in a manner to accept the chosen components so that their terminations face outward from the body of the aluminum sections on one or both sides of the metal cores and later plated with copper to interconnect their terminations. It is advantageous, after creating cavities which hold the components in place, that the assembly be processed by anodizing or electrophoretic coating with a suitable dielectric to make the surfaces nonconductive. The “component board” can then be coated with a flexible dielectric. The components are preferably of a common thickness, but it is not imperative as there are ways to deal with height differences. More importantly, components should be fully tested and burned in, though tested bare die are a possible option.

At this point, a layer of flexible insulation is to be applied to one or both sides of the core (Figure 1). The assembly can now be processed as if it were a rigid printed circuit using a build-up process to make the circuits. Side-to-side connections can be made by drilling and plating through-holes but will require a process step to coat the exposed aluminum and a second drill through the dielectric to accept the electroless and electrolytic copper plating.   

An advantage of not soldering to make connections is that the component lands can be treated as microvias and the free space can be used for increased routing. When all the required interconnections and circuits are added, the assembly can be partially machined in areas where bending is performed to thin the metal carrier sufficiently to allow the residual metal to easily bend. If left, the residual metal can serve as a ground layer for the assembly. If isolation is required or desired, the residual metal can be etched using chemistry appropriate for the metal used as a carrier. Alkaline chemistries such as dilute sodium hydroxide and sodium gluconate solutions are suitable for aluminum, but other chemistries such as ferric chloride are more universal in their potential application. There are many proprietary chemistries which can be used as well. The patented process concept is illustrated in Figure 1. 

Fjelstad_May_Fig1_cap.jpg

Accepted flex circuit design practices suggest that the number of circuit layers through the areas where flexing or bending is required should ideally be limited to one or two metal layers. That aside, the processes described offer significant potential benefits for increased reliability and reduced overall cost.

Conclusion
The continuing challenge of making lead-free solder work in electronic assembly has given rise to new ways of looking at circuit assembly and fabrication, using methods that could eliminate solder and its myriad issues altogether. What has been offered for the reader to ponder is just the tip of the iceberg. My mind (and perhaps now yours) lights up daily with ideas and ways to integrate packaged chips with the interconnections they require to carry out their mission. Improved approaches to assembly such as those that become possible using the Occam process offer both the independent designer and OEM a new option for products that are highly reliable and cost-effective. Such products will skirt the numerous concerns that have vexed manufacturers for the last decade.    

Currently, solderless flexible circuit assembly technology is making headway in several ways using printed inks and conductive adhesives in place of copper traces and solder. These are important first steps, but it is still uncertain if the inks will ever compete with copper metal. Still those efforts hold out the promise of significant potential for the future. They are helping to broaden the range of material options and significantly expand the horizon of electronic interconnection design. Once interconnection technologists and product designers begin to open-mindedly climb this new “Occam technology tree,” they will find branches that take them to many new types of processes, methods, and structures that they would not have anticipated or seen before they began the climb. The specific structure and process example provided here has not yet been reduced to practice, but it is possible. All that is missing is a willingness to try something, and unlike building a new semiconductor fab, it won’t cost billions of dollars to try it out. It just requires a few more people to take up Steve Jobs’ challenge to the industry issued at Apple some 20 years ago to “think different.”

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

 

 Download your free copy of Fjelstad’s book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series “Flexible Circuit Technology.” 

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2022

Flexible Thinking: Blue Skying It With Aluminum Rigid-Flex

05-13-2022

Aluminum is an amazingly versatile metal and has found its way into countless products since its discovery during the reign of Napoleon III of France. At the time, it was more valuable than gold, and at hosted dinners, the emperor and his honored guests dined using aluminum cutlery while the others had to make do with gold utensils. It took some time for scientists to calculate that it was by far the most abundant metal found in the earth’s crust at 8.3%, ranking third among all elements found in the crust, ranking behind oxygen (46%) and silicon (26%).

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Flexible Thinking: The Fascinating History of Wearable Electronics

04-14-2022

Wearable electronics have been capturing much attention in the press, both technical and business, over the past few years. Articles for consumption by the public, as well as technical research papers on the topic, have been increasing steadily in recent times. However, wearable electronics are far from new. Moreover, the term “wearable” is quite fungible and encompasses a broad spectrum of prospective embodiments.

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Flexible Thinking: Flexible Circuits or Flexible Electronics?

03-11-2022

The term “flexible circuit” has been ensconced and accepted in electronic interconnection technology lexicon for several decades. In broad brush strokes the term has embraced every type of printed circuit produced on flexible base materials, regardless of the nature of the conductors used; metals, such as copper; or conductive inks, such as silver or other conductive particle filled polymers. The latter type of truly printed circuits, have, for many years, been referred to as “polymer thick film” circuits.

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2021

Flexible Thinking: Shaping Flexible Circuits

12-16-2021

It is axiomatic that bending and folding are fundamental to flexible circuits. Reality is that most flex circuit applications are ones where the circuit is made to conform to the confines of the package that contains it to meet product design objectives. This is a matter that is often glossed over but it is often very important to get the flex circuit to take on a reasonably permanent shape to facilitate its installation into a housing.

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Flexible Thinking: A Brief Retrospective of 50 Years in the PCB Industry

12-02-2021

I recently reflected upon the notion that this year marks my 50th year in the printed circuit industry. It was a bit of a shock when I looked at the calendar and realized that I have been kicking around this industry for a half-century. I was fortunate enough to find my way into the PCB industry through the analytical lab of a PCB company in Mountain View, California in the early days of Silicon Valley. The name of the company was Printex and it was one of the premier PCB fabricators in the United States.

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Flexible Thinking: Flexible Circuits—A Catalyst for Technological Evolution

11-04-2021

With only a wee bit of prejudice, I would argue that flexible circuits are among the most adaptive and adaptable of all electronic interconnection technologies and perhaps the most catalytic as well. The driver of change has been that the industry is continuously being pressed to develop newer and better products with more functions and at lower cost. There is likely a tendency to think that change is the result of consumer demand, but as Steve Jobs observed many years ago, the consumer doesn't necessarily always know what they want until they see it and can sense or experience the value.

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Flexible Thinking: Intellectual Property—How it Works for the Benefit of All

09-23-2021

Innovation is the lifeblood of technological progress. It has been the driving force in electronics for over a century. In general, intellectual property (IP) refers to innovations, those creations of the human mind. Patents protect those creations. Joe Fjelstad explains.

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Flexible Thinking: The Importance of Asking 'Why Not?' When Inventing

08-18-2021

With such an impressive list of benefits, it seems it might seem as though flexible circuit technology has reached its improvement limits. However, the principle of continuous improvement does not rest and it demands that we persist in our efforts do and make things better over time.

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Flexible Thinking: The Calf Path—Redux

07-19-2021

When I first read the poem many decades ago, it immediately struck me with its simple yet profound wisdom. Since that fortunate discovery, the poem has informed often my conscious thinking. I'm sure it will for you as well.

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Flexible Thinking: Star Trek Memories

06-15-2021

Columnist Joe Fjelstad not only watched Star Trek with fascination, he grew to become his own inventor, thanks to his father—an aerospace engineer. "The passion for flight, especially rocketry, entered my veins early," he writes.

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Flexible Thinking: Process Flow for Occam QFN Test Vehicle

05-19-2021

Joe Fjelstad teaches the Occam process through a series of steps and images. These solutions can significantly reduce the number of process steps required to manufacture an electronic module or assembly (perhaps by as much as one-third) and in the process making electronic assemblies more reliable and less costly by fundamentally focusing on the elimination of solder and the soldering process.

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Flexible Thinking: IC Package Footprints—Why So Many and How Many Is Enough?

02-12-2021

Joe Fjelstad takes a historical look at the formation of integrated circuits and what that means for today's PCB designs.

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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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