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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.