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12 The PCB Design Magazine • June 2014 by Thomas Stearns BranDer inT'l ConSulTanTS An article in the September issue of The PCB Magazine, How to Select a Flex Circuit Supplier, featured an excellent article on how to select a flexible printed circuit (FPC) vendor. Herewith two thoughts on why procuring FPC is differ- ent from other products. Firstly, FPC specifications and quality con- trols tend to be too tight; FPC is, by nature, a flexible, easily formed product of thin sheets of plastic film and metal foils bonded together with flexibilized adhesives. It is not carved out of billets of metal in a machine shop and it is not reasonable to assign thousandths-of-an- inch tolerances to every dimension. FPC appeared as a commercial product in the late 1950s. Back then, the manufacturing process consisted of fusion-bonding high-perfor- mance polymer films onto treated copper foil to form an initial laminate. This was imaged and etched to form the conductor pattern, then sub- jected to a second fusion bond process in which the covering film had pre-punched apertures to expose terminal pads, sometimes with an added piece of film attached to the backside of the pad to force it upward into the punched openings to stiffen and reinforce the area against solder heat and stress. Typical bonding temperatures reached 530°F or so, and distortion and residual stress were constant problems. But this was a product with outstanding chemical and environmental durability and superb electrical performance. There were no flexibilized adhesives here! feature Why Procuring Flexible Printed Circuitry is Different