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22 FLEX007 MAGAZINE I JULY 2018 nally targeted at 50 ohms, have been success- fully employed in transmission line applica- tions, and are often used for single-ended interconnections. Higher characteristic imped- ance designs can be built with microstrip con- struction, but flexibility normally suffers. Stripline circuits are the third basic type of controlled impedance construction. These cir- cuits feature three metal layers with ground layers on both sides. Excellent signal integrity can be achieved with this construction; how- ever, they do not tend to be very flexible due to the extra dielectric and metal foil used. Strip- line circuits are often designed to 100 ohms and are frequently used for differential pair interconnections. There is also a stripline construction in which ground lines surround the signal lines to approximate coaxial cable constructions. These structures are of greatest interest where crosstalk is a concern and where maximum signal integrity is required. To improve per- formance, some designers stitch the top and bottom grounds together through the ground traces with plated through-holes, but this can be a costly process. Important Mechanical Design Concerns Flexible circuits are much thinner and gener- ally less mechanically robust than their rigid counterparts. Therefore there are some impor- tant issues related to the mechanical require- ments in flex circuit design. First is the matter of the plated though-hole. Through-hole reliability is critical to the elec- trical performance of a flexible circuit. While there are no hard rules relative to plating thick- ness in the though-hole, a higher nominal value is generally going to provide better long-term performance and greater resistance to barrel cracking and other thermal and stress-induced failures. With respect to positioning of holes in the body of the flex circuit, there should ideally be a minimum of 50 mils of clearance between the edge of any such hole and the edge of the circuit or transition areas between rigid and flex. Placing vias closer to the flex and rigid interface can cause issues with possible chem- istry leaking under the rigid portion and caus- ing shorts or contamination. In rigid-flex circuits, it is recommended that some strain relief be provided at the interface Figure 3: Cross-hatched ground layer.