Flexible circuits Rigid/flex

Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which a thin copper foil is... 

Chip-on-flex is similar to chip-on-board except that the die, chips, or CSPs are wire bonded, flip chip attached, or epoxy connected to a flexible circuit instead of to a rigid interconnect substrate such as a PWB. 

Flexible circuits. Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which is adhesively bonded a thin copper foil. The copper is then photoetched to form a circuit pattern using normal photolithography processes. A plastic film (coverlay) is then adhesive bonded to the etched... 

As flexible circuits have various mechanical stresses, their conductors also have requirements for more flexibility and toughness than traditional ED copper foils of standard rigid circuit boards. An RA copper foil is the solution for high flexing endurance. It is necessary for dynamic flexing use. RA copper foils are more expensive than ED copper, especially at thicknesses of less than 18 micron. Therefore, high-ductility electrodeposited (HD-ED) copper foils have been developed that have greater flexibility than ED copper foils and a lower cost than RA copper foils. The differences between the copper foils are shown in Table 61.11. 

A combination of rigid and flexible circuits can be called rigid/flex, flex/iigid, orrigidfllexible. However, they are called multilayer flex when flexible materials are used instead of glass/epoxy. The term rigid/flex is used throughout this chapter. 

Multilayer rigid/flex is basically a combination of rigid multilayer and flexible circuit processes. 

Several adhesive resins, especially acryhc-based adhesives developed for flexible circuits are not stable with the permanganate solution that is common for rigid multilayer boards, which means that a very narrow window is allowed for the desmearing of the rigid/flex process. If the materials are dipped too long in the solution, the adhesive layers swell and the rehabihty of the through holed will be damaged. 

The basic concept of quality assurance for flexible circuits is the same as that for rigid boards, in the sense that any critical defect must be eliminated before shipping. This includes failures due to opens and shorts dimensional degradation of pad arrays, serious defects on conductors, substrates, coverlay, and so on. These defects need to be inspected for each circuit, whether it is flexible or rigid. Technically, however, there are several differences in quality assmance for flexible circuits because of their additional functions, such as dynamic flexing capability. 

As the name suggests, these are combinations of flexible PCBs and rigid PCBs in a single nnit. The flexible portion of the circuit is made first and inclnded in the lamination process of the rigid portion of the assembly. This process eliminates wiring harnesses and the associated connectors. Applications include avionics and portable eqnipment snch as laptop compnters. As a rule, a flex-rigid assembly is more expensive than an eqnivalent combination of PCBs and cables. 

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