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NOVEMBER 2024 I DESIGN007 MAGAZINE 49 stability of the circuit. Designing with a bias for copper is a practice particularly well-suited to single-layer flex circuit designs. Double-sided application is possible, and it can help in the manufacture of multilayer circuits by main- taining dimensional control (Figure 2). Staggered layer lengths: Rules of thumb have been applied in flex circuit manufactur- ing to create staggered layer lengths to ease the flexing multilayer and rigid-flex designs. e technique is accomplished by slightly add- ing to the length of each successive flex layer, moving away from the bend radius. A common and historical rule of thumb is to add length equal to roughly 1.5 times the individual layer thickness, but the value can vary based on the tightness of the bend and the number of layers. However, the objective is to prevent buckling of the center of bend layers (Figure 3). ing aspect ratios that are square (1:1) or even "over square" (1:2) when using thin dielectric materials. is translates to a 2-mil hole in a 2-mil dielectric in the first case and a 4-mil hole in the same 2-mil dielectric in the second case. Such small vias can be helpful, or even crucial, in meeting routing density requirements, such as those increasingly needed for the layers of redistribution wiring required for today's high pin count components. ere have been several rules of thumb and guidelines devised for flex circuit manufactur- ing over the years as it matured from art to sci- ence: Using copper in design: is has been a commonly practiced rule among flex circuit designers (Figure 1) because maintaining the extra copper helps to enhance the dimensional Figure 1: Designing with a bias for copper improves the circuit's dimensional stability but may not be practical for all applications, such as when weight is a concern 1 . Figure 2: Copper foil is maintained by design around circuits to provide better panel dimensional stability when manufacturing flex or rigid-flex multilayers 1 . Figure 3: Staggered length designs facilitate the bending of the flex circuit. However, the circuit can only be bent in one direction by design 1 .