Flexible Printed Circuits (FPCs) offer a great opportunity for the packaging engineer and electronic designer. These versatile electronic wiring systems can be shaped, bent, twisted, and folded into infinite dimensional configurations…limited only by an engineer’s origami creativity. In this sense, they offer significant design advantages over a rigid, two-dimensional, inflexible printed circuit board (PCB). This extra dimension can make flexible circuitry a design engineer’s dream, but with the addition of flexibility come some “rules” that must be followed (sounds like an oxymoron?) to ensure a robust design is achieved.
Different manufacturing methods and material sets are used for FPCs and one immediate difference is the dimensional properties. Rigid printed circuits are generally more dimensionally stable than printed circuits. the standard polyimide film used as the building block in 98% of the flexible circuits produced. This greater dimensional variability means that a flexible circuit requires different design rules than its rigid PCB relative. Unfortunately, much of the available design software uses rigid PCB design rules and this can create manufacturing and functional problems for the flex circuit. Some in the industry refer to preparing a flex circuit design for the factory as “flexing” the design.
The following list details five of the most common ways that “relaxing” makes a design more robust, more producible, and manufacturing-ready.
- Solder mask or cover film openings: During manufacturing, flex circuits can demonstrate dimensional changes after exposure to processes such as pumice washing, copper plating, and/or etching. While some changes may be allowed for, flex circuit design rules generally require larger tolerances to accommodate subsequent registrations for cover film, underlays, or die-cutting. Additional consideration is required for adhesive release that occurs during lamination of the dielectric from the cover film. What complicates the prediction of compensating design features is the myriad of processes and sequences required to produce a custom flex circuit. The bottom line is that the openings in the cover film generally need to leave more room in a flex circuit design.
- Spacing between solder pads and adjacent tracks: Here is the trade-off, i.e. design compromise, to be made based on item #1. When the openings of the cover film or solder mask get larger, the edges of the tracks of the Adjacent conductor could be exposed if placed too close to a solder pad. This can cause short circuits if jumpers are soldered between the connector pins or pads. The physical size of the circuit is another factor that can affect recording capacity. In general, more space is needed between a solder pad and an adjacent conductive trace to accommodate the placement tolerance of the solder mask or cover film.
- Conductive stress points: Because flex circuits are used in both bend-to-install and dynamic-flex applications, trace configurations that are acceptable on a rigid PCB can create problems on a flex circuit. Conductor traces with sharp corners and sharp joints at the base of solder pads become natural “stress points” when the area near them is flexed. This can result in fractures or trace delamination. A good flex circuit design will have a smooth radius to the conductor pivot points (instead of sharp corners) and a gentle radius from the trace to the pad fillet instead of a sharp angle. Selective joining of reinforcements will prevent bending in the welded regions and is a common design practice.
- Stacked traces: Traces on opposite sides of the dielectric should not “stack” directly on each other. Traces in tension (on the outside of the radius of curvature) can break when the circuit is bent if they line up directly parallel with a trace on the opposite side. Tracks in tension are forced further from the neutral axis of the folded region and may fracture, especially with repeated bending. A good design practice is to keep the copper on the neutral axis of a curve by designing this region as a single conductive layer. Where this is not possible, a proper design will “stagger” the traces between the top and bottom copper layers to prevent top and bottom alignment.
- Welded joints too close to the flex point: A solder joint is formed by an intermetallic bond of the solder alloy to the copper trace. While the copper trace is normally flexible, the regions that have been soldered become very rigid and inflexible. When the substrate is bent near the edge of the solder joint, the solder pad will crack or delaminate. Either situation will cause serious functional problems.
The bottom line is that designing a flex circuit using standard PCB software can lead to some serious manufacturing and reliability issues. It is best to work with your flex circuit supplier or a flex circuit design expert to “flex” the design before manufacturing begins, or create the design directly from a netlist. This will ensure that the design can be manufactured to meet your needs.