# A Few Simple Lessons in Designing Reliable 3D Flex

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. Unfortunately, using only familiar tools limits our ability to come up with optimal or even superior solutions. Hopefully, what follows will help you avoid some of the traps conventional wisdom doesn’t always give guidance on.

But first, having written extensively over the years about flexible-circuit technology, I still try to find fresh ways to think about flexible circuits and their construction and use. In the process of thinking about what I might say here, a fun book I read in high school more than half a century ago came to mind. The title of that book is Flatland: A Romance of Many Dimensions by Edwin A. Abbott [1]. In full disclosure, and as one might guess from the extended title, the book is not about geometry but is a rather entertaining read. I found it to be a useful book for stimulating thought.

Figure 1: View of an iPhone's internal electronic elements layout upon opening the device (folding interconnections between the halves enabled by flex circuits). (Source: Tech Insights)

In the book, almost every male character in Flatland has two major dimensions—X and Y—and many possible sides (thus, they have area), but they can only be perceived as lines when looked at from the reference of the planar world in which they exist. Women, on the other hand, are depicted as being basic lines, and are required to make noise as they move about through Flatland should they be encountered head on and appear as a simple point. The protagonist (a square) encounters both Lineland and Pointland, but the encounter with a sphere from Spaceland is where the entertainment really begins. Trying to convince others of another dimension is no mean task, one learns. Many humans are resistant to accepting the teachings of science. The book proved useful over the years to a number of prominent scientists (including Carl Sagan) seeking to help the layperson understand the multidimensional space they envisioned and were attempting to explain.

That function of envisioning the not-always-easily perceived is summoned here, but for a much less complex purpose than defining space and time. My specific purpose is, “How to tease a three-dimensional circuit from a two-dimensional copper-clad panel and do so reliably?” That should be job one in nearly every flex-circuit design. So how does one carry out the task and make 2D into 3D reliably? Here are some suggestions. Be warned in advance that there are no pat answers to the question posed. Each design inevitably has its own set of requirements and challenges, but there are a few steps that are common:

1. Define the end-product shape, size, and volume: This is the primary constraint for the design—the canvas for the painting. An individual or team has the job of defining the shape and volume of the product. Often, that job is constrained in some manner either by limitations of space where the electronics are going to be put to use or by a mandate from marketing or management. Steve Jobs, it has been said, was intimately involved in the design of the iPod and iPhone in terms of the size and shape and other design features along with its functionality. He did not execute the design and manufacture by himself, however. He created a significant challenge for designers, and flexible circuits were vital to achieving the goal. In fact, the use of flexible circuits was arguably crucial to making it happen as they were indispensable to the interconnection of the various high-density connecting elements of the electronics in the product (Figures 1 and 2).

Figure 2: Flexible circuits are plentiful inside the iPhone and vital, enabling elements of the phone’s construction. (Source: Tech Insights)

2. Identify all components and make selections carefully: Many years ago while working for a major aerospace company, I was part of a group tasked with making a circuit that needed to fit in a very small predefined space. To say that it was difficult was to understate the case. The devices chosen for use with the design were rated for reliability, but they were very large. It was as if someone had measured the volume of each of the individual components and their collective volume, and, also knowing the volume of the space available and seeing they matched, they decided it was possible. It wasn’t quite that bad but it seemed that way. Moreover, the design chosen was a rigid-flex, which was not very common at the time and the processing was not well understood. The product was built and had several overlapping flex arms designed to interconnect to bulkhead connectors inside the space. Trying to assemble the circuit was a nightmare, and because there was little additional length on the arms, many of the circuits broke during the assembly process. It was eventually solved by a combination of selecting different components and getting some relief on the size of the box by making it slightly larger. The takeaway is that the early decisions can have a knock-on effect of significance.

3. Use paper doll mockups of the proposed layout: Available CAD tools can lay out circuit patterns and allow them to be modeled for interferences in the application. However, a simpler method—at least for first-pass approximation—is to create a paper model of the assembly design to see what traps might lie in the road ahead. This will be instructive in determining roughly how much extra length might be required to provide a service loop, which not only facilitates assembly but also creates some intrinsic strain relief in the circuit. Later, when the layer counts that might be required and the construction are defined, it can be repeated using materials of similar thickness and stiffness to the anticipated construction. This will expose any previously unforeseen potential for buckling or kinking of the circuit during assembly.

4. Use no right or acute angles in the flex circuit outline: Stress risers are anathema in nearly every mechanical design, and with flexible circuits having a foot in an electrical, electronic, and mechanical world, stress risers are no less important. The first and arguably most important thing one must do is to make sure that right or acute angles be avoided everywhere possible in the circuit outline. The interface between the rigid and flex circuit is one spot where it is not really possible, and this should ideally be dealt with by providing some strain relief at the interface. Commonly, this is a bead of some resin or elastomer along the edge. I have seen failures occur at this interface, and I suspect other veterans of flexible circuits have as well over the years.

This was not meant to be an exhaustive treatise on the topic. As I am certain every other flex circuit engineer has, I have a number of anecdotal experiences that involve nuanced elements of the design and assembly process that have resulted in failures, but these are some important ones in my opinion. I would add that when you experience failure—and it is likely you will at some point—embrace the moment. These are teaching moments and lessons that will not be forgotten.

References

For more detailed discussions on this and other flex circuit topics, download your free copy of Flexible Circuit Technology, 4th Edition at www.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.

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