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92 DESIGN007 MAGAZINE I DECEMBER 2021 strength than the flexible polymer elements of flex circuit construction but they also are capable of plastic deformation and will perma- nently deform plastically when bent, provided its elastic limit is not exceeded to the point of fracture. Many polymers (elastomers are generally excluded, though they can also take a set over time) will also permanently deform if their elastic limit is exceeded. In the case of flex cir- cuit laminates, however, the elastic limit of the flexible carrier film is normally many times greater than the elastic limit of metal. us, when the composite structure we call a flex circuit is bent and holds shape to some degree, the metal has plastically deformed while the polymer is still likely to be in its elastic range. To prevent the polymer from snapping back aer bending the flex circuit, it must over- whelm the elasticity of the polymer. Copper is stronger and higher in elastic modulus, how- ever if the circuit traces are thin and small or few relative to the width of the bend area (i.e., the copper is a low percentage of the surface area through the bend), the remnant elastic strain in the polymer may cause the flex circuit to try and return to its original flat shape. One solution is to uniformly or selectively widen circuit traces through the bending zone to maximize the copper in the bend area as illustrated in Figure 1. Alternatively, if there are electrical/electronic concerns about the trace widths for controlled impedance reasons, fill the spaces between traces with non-functional metal. However, to complete the thought, if the signals are controlled impedance, this method may alter characteristic impedance of the sig- nals and should be checked or modeled. Also, if the design has a weight budget, the impact of the weight increase due to copper should also be considered. As alluded to earlier, if widening the traces alone is not sufficient, then one of two analo- gous methods can be employed. One can either use a thicker metal foil or use a thinner flexible base material. e objectives remain the same as they were in the first case: make certain that the metal can overwhelm the polymer in order the hold the final shape. ere are advantages and disadvantages to both paths. Making the copper thicker may make etching a bit more difficult; it will also take longer to etch and use more chemistry. On the other hand, mak- ing the polymer thinner could make handling a bit more difficult and the strength of the final assembly will not be as great as the alternative method. In many cases, slight overdriving of the cir- cuit in the bend area can help to create a stable and more permanent shape but be careful that the guidelines for the minimum bend radius for the flex construction are not violated. Remem- ber that when permanently bent and plastically deformed, the copper is normally thinned in Figure 1: To create a permanent bend, copper traces can be made wider through the bend area. Notice that the circuits are tapered before they enter and after they egress the bend area. An alternative is to use thicker copper, in which case, sculptured flex methods may be best. (Source: Flexible Circuit Technology 4 th Edition)