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94 DESIGN007 MAGAZINE I JANUARY 2021 The basic idea of a flexible circuit is argu- ably more than a century old, based on the interpretation of a 1903 patent by Albert Han- son, which circuit industry maven and histo- rian Dr. Ken Gilleo found in his research on industry origins some years back. Fundamen- tally, 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 imag- inings of Hansen in his day. Moreover, the materials and processes used to produce flex- ible circuits have been on a steady evolution- ary 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 descrip- tion. Flexible circuits can be simple one-metal- layer devices to replace discrete wire assem- blies, or complex multilayer or rigid-flex assem- blies 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 Flexible Circuits Versus Flexible Hybrid Electronics—Where's the Line? Flexible Thinking by Joe Fjelstad, VERDANT ELECTRONICS