Issue link: https://iconnect007.uberflip.com/i/1276973
AUGUST 2020 I DESIGN007 MAGAZINE 101 Copper features' spacing to the edge of the flex circuit is a very important parameter which varies with each fab vendor, depend- ing on their capabilities. As an example, some production rules require 0.2 mm (8 mil) clear- ance from the FPC edge to copper traces, planes, and SMT pads. Via-to-edge clearance requires more separation; I recommend 0.45 mm (18 mil). A beginning FPC designer may use a solder mask as a cover layer instead of coverlays. Sol- der mask is a very thin layer; however, it will turn a flex circuit into a rigid spring. Here is another reason why communication with FPC production is very important. FPC stackups usually have dielectric cover- layers. The coverlay's purpose is to seal in and protect the FPC's copper pattern. Since cover- lay is a relatively thick material, all clearances to other features will be larger than if solder mask is applied. Therefore, a beginner FPC designer should learn all about the restrictions and limitations of coverlay applications pro- vided by the FPC manufacturer. It is important to understand coverlay materials and methods that FPC production applies when creating access openings for component pads (drilling, punching, etc.). Common production capabili- ties require the coverlay's web to be at a mini- mum distance of 0.25 mm (10 mils) and 0.3 mm (12 mils) between any opening in coverlay to the FPC outline. Fine-pitch components will get a gang opening in a coverlay. Due to the fragile nature of flexible circuits, in FPC layout, the designer should always address the problem of a potential material tear. Figure 6 shows possible techniques for flex tear relief: slots, slots, holes, corner tabs. Connector areas and heavy components assembled on FPCs require stiffeners. Some areas rich with components may also consider having a stiffener underneath. Stiffener mate- rial can be polyimide, FR-4, or stainless steel. Figure 5: From left to right: A trace with rounded corners, an FPC profile showing rounded corners, and SMT pads with oval and rounded corner pads. Figure 6: Tear relief techniques.