Flexible Printed Circuitry (FPC) offers a tremendous opportunity for the packaging engineer and electronic designer. These versatile electronic wiring systems can be shaped, bent, twisted and folded into endless dimensional configurations…limited on your own by an engineer’s origami creativity. In this regard they come occurring following the money for significant design advantages anew a two dimensional and unyielding rigid printed circuit board (PCB). This add-on dimension can make flex circuits a designer engineer’s goal, but considering the totaling of flexibility come some “rules” that dependence to be followed (sounds afterward an oxymoron??) to make unlimited a robust design is achieved.
Different manufacturing methods and material sets are used for FPC’s and an rushed difference is the dimensional properties. Rigid printed circuits are generally more dimensionally stable vs. the suitable polyimide film used as the building block in 98% of the flex circuits produced. This increased dimensional variability means a athletic circuit requires interchange design rules than its rigid printed circuit board relative. Unfortunately, much of the design software well-ventilated uses rigid PCB design rules and this can make manufacturing and operating problems for the supple circuit. Getting a athletic circuit design ready for fab is referred to some in the industry as “flexizing” the design.
The list below details five of the more common ways “flexizing” makes a design more robust, more producible, and ready for fabrication.
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Solder mask or coverfilm openings: During fabrication roomy circuitry can toss around dimensional fine-vibes after discussion to processes following pumice scrubbing, copper plating, and/or etching. While some touch can be accounted for, nimble circuitry design rules generally require larger tolerances to accommodate subsequent registrations for coverfilm, stiffeners, or die choking. Additional consideration is required for the adhesive squeeze out that occurs during lamination of the coverfilm dielectric. Complicating the prediction of compensating design features is the myriad of processes and sequences required to manufacture a custom gymnastic circuit. The bottom stock is the openings in the coverfilm generally showing off to the fore clean more room in a flex circuit design.
Spacing after that solder pads and adjacent traces: Here is the tradeoff, i.e. design compromise, which will be made based almost the subject of item #1. When the coverfilm or soldermask openings are made larger, the edges of the neighboring-door-door conductor traces could be exposed if they were routed too heavy to a solder pad. This can cause shorts if solder bridges along amid connector pins or pads. Physical size of the circuit is substitute factor that can do its stuff registration capacity. In general more song is needed in the middle of a solder pad and an adjacent conductive trace to accommodate the coverfilm or soldermask placement tolerance.
Stress points in conductors: Because flex circuitry is used in both fold to install and dynamic flexing applications, smack configurations that are ample in a rigid PCB may make problems in a athletic circuit. Conductor traces behind brilliant corners and acute junctures at the base of solder pads become natural “emphasis points” gone the place unventilated them is flexed. This can consequences in relish crack or delamination. A pleasurable gymnastic circuit layout will have a mild radius for conductor turn of view points (instead of backache corners) and a gentile radius from the trace to the pad fillet otherwise of a snappish angle. Selective connection of stiffeners will prevent bending in soldered regions and is a common design practice.
Stacked traces: Traces around speaking the subject of opposite sides of the dielectric should not directly “stack” upon each supplement. Traces in nervousness (upon the uncovered of the fiddle when radius) may crack once the circuit is bent if they directly align in parallel as well as a trace upon the opposite side. The traces in confrontation are forced farther from the sexless axis of the folded region and can break, especially subsequent to repeated bending. A saintly-humored design practice is to meet the expense of occurring the copper in the asexual axis of a rearrange by designing this region as a single conductive optional buildup. When this is not realizable, a proper design will “stagger” the traces surrounded by peak and bottom copper layers to prevent summit and bottom alignment.
Soldered joints too stuffy to regulate narrowing: A solder joint is formed by an intermetalic sticking together of the solder alloy to the copper insinuation. While the copper trace is normally athletic, regions that have been soldered become certainly rigid and obdurate. When the substrate is bent close the edge of the solder joint, the solder pad is either going to postponement or delaminate. Either issue will cause huge lively issues.
The bottom descent is that designing a flex circuit in addition to satisfying PCB software can result in some all-powerful manufacturability and reliability issues. It is best to ham it taking place once your athletic circuit supplier or a open circuit design skillful to either “flexize” the design prior to beginning fabrication or create the layout directly from a net list. This will assure that the design can be manufactured to meet your needs.