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42 The PCB Magazine • March 2017 The dielectric material sets used for fabricat- ing a flexible circuit are what distinguishes a flexible circuit from a rigid printed circuit. Al- though both these products are often referred to in the industry as "boards," a better meta- phor for a flex circuit might be a noodle! Glass epoxy is the most common dielectric used to fabricate most rigid printed circuits. Polyimide film (also known as Kapton®, a DuPont trade name) is the material of choice for the clear ma- jority of flexible circuits. Developments in ma- terial science have resulted in several versions of polyimide tailored to meet specialty require- ments in unusual environments including solar arrays and space applications. Glass epoxy is produced in some very thin constructions and it can be bent for some simple applications but continuous flexing, twisting, or multi-planar folding is best accom- plished with a polymer film. Polyimide films can be bent numerous times without degrad- ing its electrical and mechanical properties and has proven reliable in applications literally re- quiring millions of bend cycles. This inherent flexibility provides a wealth of design options for the electronic packager, but a material disad- vantage is they are not as dimensionally stable as the glass epoxy materials used to produce rigid board circuits. Per DuPont [1] , the dimensional stability of polyimide film depends on two factors—the normal coefficient of thermal expansion and the residual stresses placed in the film during manufacture. This stability measure is only representing the effect of the film itself. Stability becomes a more complex characteristic when the film is exposed to the elevated temperatures and pres- sures to attach copper either through an adhe- sive lamination cycle or through processing to create an adhesiveless laminate. But creating a laminate, and then fabricating a circuit, in- Understanding Dimensional Stability in Flexible Circuits FEATURE COLUMN: ALL ABOUT FLEX by Dave Becker ALL FLEX FLEXIBLE CIRCUITS LLC