18. Jul 2016 21:27, rigidflex
5 Typical Flexible Printed Circuit Design Errors Flexible Printed Circuitry (FPC) delivers a tremendous chance for the packaging engineer and electronic designer. These versatile electronic wiring systems could be shaped, bent, twisted and folded into endless dimensional configurations...limited only by an engineer's origami creativity. Within this regard they offer important design and style positive aspects more than a two dimensional and inflexible rigid printed circuit board (PCB). This added dimension can make flex circuits a designer engineer's dream, but using the addition of flexibility come some "rules" that have to have to be followed (sounds like an oxymoron??) to create certain a robust design and style is accomplished. Distinct manufacturing strategies and material sets are utilized for FPC's and an quick difference could be the dimensional properties. Rigid printed circuits are typically extra dimensionally stable vs. the common polyimide film used because the developing block in 98% in the flex circuits created. This elevated dimensional variability implies a flexible circuit requires unique design guidelines than its rigid printed circuit board relative. Regrettably, much of the design and style software program readily available utilizes rigid PCB design guidelines and this could make manufacturing and functional difficulties for the flexible circuit. Obtaining a flexible circuit style prepared for fab is referred to some inside the sector as "flexizing" the style. The list beneath facts 5 with the more popular methods "flexizing" makes a design and style more robust, far more producible, and prepared for fabrication. Solder mask or coverfilm openings:In the course of fabrication flexible circuitry can demonstrate dimensional modify soon after exposure to processes like pumice scrubbing, copper plating, and/or etching. While some transform may be accounted for, flexible circuitry design rules frequently require larger tolerances to accommodate subsequent registrations for coverfilm, stiffeners, or die cutting. Added consideration is necessary for the adhesive squeeze out that happens throughout lamination of the coverfilm dielectric. Complicating the prediction of compensating design capabilities would be the myriad of processes and sequences necessary to generate a custom flexible circuit. The bottom line would be the openings in the coverfilm usually have to have to let more room inside a flex circuit design. Spacing amongst solder pads and adjacent traces:Here could be the tradeoff, i.e. design and style compromise, which will be made determined by item #1. When the coverfilm or soldermask openings are made bigger, the edges in the adjacent conductor traces may be exposed if they have been routed also close to a solder pad. This could lead to shorts if solder bridges involving connector pins or pads. Physical size from the circuit is another element that may affect registration capability. In general far more space is needed in between a solder pad and an adjacent conductive trace to accommodate the coverfilm or soldermask placement tolerance. Strain points in conductors:Simply because flex circuitry is utilized in both fold to install and dynamic flexing applications, trace configurations which are acceptable within a rigid PCB may possibly make issues within a flexible circuit. Conductor traces with sharp corners and acute junctures at the base of solder pads turn into organic "stress points" when the area near them is flexed. This could lead to trace fracture or delamination. A good flexible circuit layout may have a smooth radius for conductor turn points (in place of sharp corners) in addition to a gentile radius in the trace towards the pad fillet in place of a sharp angle. Selective attachment of stiffeners will avoid bending in soldered regions and is usually a widespread design practice. Stacked traces:Traces on opposite sides of the dielectric need to not straight "stack" on each and every other. Traces in tension (around the outside of your bend flex pcb radius) may well crack when the circuit is bent if they directly align in parallel using a trace on the opposite side. The traces in tension are forced farther in the neutral axis from the folded area and can fracture, in particular with repeated bending. A very good design and style practice is always to preserve the copper inside the neutral axis of a bend by designing this region as a single conductive layer. When this is not achievable, a correct style will "stagger" the traces between prime and bottom copper layers to stop leading and bottom alignment. Soldered joints as well close to bend point:A solder joint is formed by an intermetalic bond in the solder alloy towards the copper trace. Whilst the copper trace is commonly flexible, regions which have been soldered become really rigid and inflexible. When the substrate is bent close to the edge in the solder joint, the solder pad is either going to crack or delaminate. Either situation will result in really serious functional problems. The bottom line is the fact that designing a flex circuit with typical PCB computer software can result in some severe manufacturability and reliability troubles. It is finest to rigid flex board function with your flexible circuit supplier or perhaps a flexible circuit design and style expert to either "flexize" the design prior to beginning fabrication or produce the layout directly from a net list. This will assure that the design and style can be manufactured to meet your requires.