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84 DESIGN007 MAGAZINE I JANUARY 2020 curves are not an option, two 45° corners are preferred over one 90° corners (Figure 2). When possible, route small conductors on the inside of a tight bend. Small conduc- tors (<0.007") will tolerate compression bet- ter than stretching. Placing these conductors on the inside of a bend will reduce or elimi- nate tension forces. Do not stack conductors on top of each other on multiple layers creating an I-beam effect. Stacking conductors will es- sentially increase the overall circuit thickness, thereby decreasing flexibility and the circuit's ability to bend reliably. Conductors Flexible circuit conductors are manufactured using a photo-etch process, which starts with a full sheet of copper. Conductors are formed by masking the desired conductive paths, and then chemically removing all unwanted cop- per, leaving the desired circuit patterns. As the etchant dissolves the unmasked copper, it also attacks the edges of the conductors, resulting in what is referred to as "under-cut." As copper foil thickness increases, so does the amount of undercut. This makes it very difficult for the flex manufacturer to create very small conductors on very thick foil. There are also variations in the etching process (pri- marily etchant strength, which varies with the amount of copper in the solution). For this rea- son, the designer must factor in a processing allowance for strand width (and spacing). For optimum etch yields, conductor widths should be at least 5X greater than the thickness. It is advisable to maximize conductor width wherever possible. For example, if your design requires 0.005" conductor width to squeeze be- tween pads in an isolated area, the conductor should flare back out to 0.010–0.012" once the conductor clears the tight area (Figure 3). This will improve the manufacturing etch yields, which in turn means a lower overall circuit cost to you. Pad Fillets It is a good idea to insert fillets on pads at each location where a conductor enters a pad. Pad fillets will reduce or eliminate potential stress concentration points. Figure 1: Conductors should be routed through bending or flexing areas when possible. Figure 2: When soft curves are not possible, corners of 2–45° are better than corners of 1–90°. Figure 3: If a conductor must shrink to fit between pads in an isolated area, the conductor should flare back out after clearing the constricted area.