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28 FLEX007 MAGAZINE I APRIL 2018 With traces mapped directly over each other, added stress occurs on the traces located on the outside of a bend radius. The traces on the outside of a bend radius are in tension mode, and the layers on the inside are in compres- sion. Traces in tension tend to develop micro- fractures if they are flexed repeatedly. These can be particularly difficult to identify as the open circuit often occurs intermittently. Another mechanical stress point occurs at the end of a stiffener, which will tend to focus stress at the stiffener termination line. A sim- ple design feature solution is often to overlap the coverlay termination on the opposite side (Figure 2). Solder Joints/Vias too Close to Bend Points While copper foil is ductile, the alloy result- ing on a solder joint is much more brittle. These regions are particularly susceptible to trace fractures as the region defined by the junction of cover film and solder pad cannot take significant bending. Figure 3 shows a safe distance between the bend location and a sol- der joint or plated through-hole. This is the PREFERRED METHOD because there is not a common ending point of the coverlay and stiener. This form is NOT RECOMMENDED because it allows potential stress and cracking points where the coverlay and stiener end at a common edge. NO OVERLAP (Potential Stress Point) OVERLAP DISTANCE (Minimum .030") EXPOSED FINGER STIFFENER COVERLAY COVERLAY EXPOSED FINGER STIFFENER Figure 2: Overlapping the coverlay is the preferred method. Figure 3. Bend radius of a flexible interconnect should be approximately 10 times the material thickness and at least .050" away from the plated through hole. PLATED THROUGH-HOLES "R" (RADIUS) .050" (1.27mm) "T" (THICKNESS OF MATERIAL) R=10xT BEND RADIUS SOLDER JOINT Unbalanced Dielectric Thickness This issue is avoided by locating the cop- per traces in the "neutral axis" (i.e., by hav- ing equivalent material thicknesses above and below copper in regions to be repeatedly flexed). Polyimide film has been proven as a robust material for dynamic flex applications

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