plated through hole, terminations are provided on both sides of the circuit, thus allowing components to be placed on either side. Depending on design requirements, double-sided flex circuits can be fabricated with protective coverlayers on one, both or neither side of the completed circuit, but are most commonly produced with the protective layer on both sides. One major advantage is that it allows easy crossover connections. Many single sided circuits are built on a double sided substrate because of the crossover connections. An example of this use is the circuit that connects a mousepad to the motherboard. All connections on that circuit are located on only one side of the substrate, except a small crossover connection that uses the other side. Multilayer flex circuits Flex circuits with three or more layers of conductors are known as multilayer flex circuits. Commonly the layers are interconnected by means of plated through holes, though this is not required because it is possible to provide openings to access lower circuit level features. The layers may or may not be continuously laminated together throughout the construction with the exception of the areas occupied by through-holes. Discontinuous lamination is common in cases requiring maximum flexibility. This is accomplished by leaving unbonded the areas where flexing or bending is to occur. Rigid-flex circuits Rigid-flex circuits are a hybrid combining rigid and flexible substrates laminated into a single structure. Rigid-flex circuits are not rigidized flex constructions, which are flex circuits to which a stiffener is attached to support the weight of the components. A rigidized or stiffened flex circuit can have one or more conductor layers. The terms represent quite different products. The layers are normally interconnected by plated through holes. Rigid-flex circuits are often chosen by military product designers and increasingly in commercial products. Compaq Computer chose the approach for laptop computer boards in the 1990s. While the computer's main rigid-flex PCBA did not flex during use, subsequent Compaq designs utilized rigid-flex circuits for the hinged display cable, passing tens of thousands of flexures during testing. By 2013, the use of rigid-flex circuits in consumer laptop computers had become common. Rigid-flex boards are normally multilayer structures; however, two metal layer constructions are sometimes used. Polymer thick film flex circuits Polymer thick film (PTF) flex circuits print circuit elements onto a polymer film. They are typically single conductor layer structures, however two or more metal layers can be