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 unaccompanied by an engineer’s origami creativity. In this regard they have the funds for significant design advantages again a two dimensional and unwavering rigid printed circuit board (PCB). This to the fore payment dimension can make flex circuits a designer engineer’s point, but subsequent to the assistant of flexibility arrive some “rules” that compulsion to be followed (sounds in the melody of an oxymoron??) to make unconditional a robust design is achieved.
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Different manufacturing methods and material sets are used for FPC’s and an mordant difference is the dimensional properties. Rigid printed circuits are generally more dimensionally stable vs. the customary polyimide film used as the building block in 98% of the flex circuits produced. This increased dimensional variability means a gymnastic circuit requires swap design rules than its rigid printed circuit board relative. Unfortunately, much of the design software understandable uses rigid PCB design rules and this can create manufacturing and operational problems for the lithe circuit. Getting a lithe circuit design ready for fab is referred to some in the industry as “flexizing” the design.
The list out cold details five of the more common ways “flexizing” makes a design more robust, more producible, and ready for fabrication.
Solder mask or coverfilm openings: During fabrication alert circuitry can disconcert dimensional fine-look after drying to processes furthermore pumice scrubbing, copper plating, and/or etching. While some have emotional impact can be accounted for, athletic circuitry design rules generally require larger tolerances to accommodate subsequent registrations for coverfilm, stiffeners, or die cutting. 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 fabricate a custom nimble circuit. The bottom heritage is the openings in the coverfilm generally habit to meet the expense of access more room in a flex circuit design.
Spacing together in the midst of solder pads and bordering traces: Here is the tradeoff, i.e. design compromise, which will be made based as regards item #1. When the coverfilm or soldermask openings are made larger, the edges of the adjacent conductor traces could be exposed if they were routed too oppressive to a solder pad. This can cause shorts if solder bridges along in addition to connector pins or pads. Physical size of the circuit is choice factor that can engagement registration adroitness. In general more character is needed in the company of a solder pad and an adjacent conductive reference to accommodate the coverfilm or soldermask placement tolerance.
Stress points in conductors: Because flex circuitry is used in both fold to install and on the go flexing applications, smack configurations that are enough in a rigid PCB may create problems in a alert circuit. Conductor traces past brilliant corners and acute junctures at the base of solder pads become natural “inflection points” bearing in mind the place oppressive them is flexed. This can outcome in mention crack or delamination. A pleasing on the go circuit layout will have a mild radius for conductor viewpoint points (instead of cunning corners) and a gentile radius from the suggestion to the pad fillet on the other hand of a brilliant angle. Selective growth of stiffeners will prevent bending in soldered regions and is a common design practice.
Stacked traces: Traces upon opposite sides of the dielectric should not directly “stack” upon each subsidiary. Traces in nervousness (upon the outside of the shape radius) may crack subsequent to the circuit is bent if they directly align in parallel taking into account a trace upon the opposite side. The traces in fright are bothered farther from the genderless axis of the folded region and can fracture, especially moreover repeated bending. A fine design practice is to save the copper in the neuter axis of a alter by designing this region as a single conductive buildup. When this is not attainable, a proper design will “stagger” the traces along plus peak and bottom copper layers to prevent extremity and bottom alignment.
Soldered joints too close to change reduction: A solder joint is formed by an intermetalic bond of the solder alloy to the copper relish. While the copper savor is normally gymnastic, regions that have been soldered become enormously rigid and unbending. When the substrate is bent stuffy the edge of the solder joint, the solder pad is either going to crack or delaminate. Either issue will cause omnipotent lively issues.
The bottom stock is that designing a flex circuit also confirmed PCB software can consequences in some supreme manufacturability and reliability issues. It is best to problem an deed as soon as your variable circuit supplier or a amendable circuit design competent to either “flexize” the design prior to arrival fabrication or create the layout directly from a net list. This will assure that the design can be manufactured to meet your needs.