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96 DESIGN007 MAGAZINE I JANUARY 2021 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 technolo- gies and low-cost substrates along with con- ductive adhesives to make novel low cost elec- tronic 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 sil- ver 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 manu- facture 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 compo - nents and function into the designs facilitated by sequentially inkjet printing conductive, resistive, insulating and even semiconduc- tive materials to realize unique flexible circuit designs. This includes the prospective print- ing 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 flex- ible 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, includ- ing 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 flex- ible 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. DESIGN007 Joe Fjelstad is founder and CEO of Verdant Electronics and an i nternational authority and inno- vator in the field of electronic interconnection and packaging technologies with more than 185 patents issued or pending. To read past columns or contact Fjelstad, click here. Down- load your free copy of Fjelstad's book Flexible Circuit echnology, 4 th Edition, and watch his in-depth workshop series "Flexible Circuit Technology." 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.