Flex circuits that actually flex in regular use can, however, be considered a special case; very often, the intent is to provide an efficient interconnect between two boards that do not lie in the same plane. In such cases, the flexible part may only flex during final assembly of the product and never do so again.
Flex-rigid can be viewed as an alternative interconnect strategy or as cable replacement. One flex section replaces, at a minimum, a cable plus two connectors – which may be worthwhile for bill of materials reasons – and frees the volume the connectors would have occupied. This is significant for such products as wearables, where every mm3 within the enclosure is used, and in systems where two or more PCBs must be folded into place in final assembly. PCB fabricators report that products in which space and weight are at a premium are amongst the fastest growing adopters of flex-rigid technology.
Other benefits include: improved reliability, with fewer connectors and associated solder joints; and control of the signal path between the circuitry on the boards at either end of the flex.
Today, the design process is assisted by 3D capable PCB design software; if you are trying to make maximum use of space, you need to ensure there is no interference between component profiles. Increasingly, design software can also model the flexed state of the non rigid part to establish parameters such as bend radius – and do so dynamically, to check the entire path of system in which the two rigid elements move, in normal use.
Mature technology
Flex-rigid fabrication is a mature technology, with well developed rules for success. The first is to establish and maintain a dialogue with the board fabricator and to ensure that any modifications to design rules or design checks (DRC) are imported into the layout package in use and adhered to.
Many of the guidelines for a successful outcome of a flex-rigid design flow from the properties of the materials being employed. The base material for the rigid portion is most likely to be FR4, while the flex is typically a polyimide film (often called Kapton, after DuPont’s material) with copper foil applied and a coverlay film imposed in place of the solder mask.
Some thought is needed about how the material properties match the task and this quickly shows where care should be taken. The copper-to-film bond is a junction between dissimilar materials; the tighter the bend radius, the greater the stresses at the boundary and the higher the risk of delamination. The tracks, although thin, are copper and if the bend radius is too small, repeated flexing carries the risk of stress fracture.
The transition between rigid and flex is also an important area; as with cable termination, bending forces should not be imposed at the transition as this can create very small bend radii over a small distance. Again, dynamic modelling of folding in 3D is helpful and the composition of the layer stack-up is key for reliability and manufacturability.
Many common errors can be avoided by attention to a few key points in the design process. One area is the need for a larger copper ring around any hole drilled in the flex and, as a related point, the need to leave a larger separation between drill hole/pad and adjacent tracks. The flexible film is just that and the effect of that flexibility is to increase normal (inescapable) tolerances. Therefore, if a drilled hole misses concentricity with its pad by a small margin, the hole will still plate and connectivity through the via will be unaffected – but there is a risk of a short to a track passing too close.
Minimising vias in the flex region is desirable in itself; the cost of vias is higher than in the rigid, assuming the design requires a double sided flex. This is one area where the configuration of the stack-up that makes up the complete PCB is critical. The manufacturing process is in part subtractive – there will be layers over and under the flex in fabrication that are removed selectively to release the flex segment before the process is complete. The stack-up should be configured to ensure the polyimide layer is fully supported through any drilling. Visualising a drill bit attempting to penetrate a thin, deformable film will immediately show why.
Transition
Merely taking vias in the flexible area back into the rigid portion – the flex layer runs throughout the total area of the PCB and provides flexibility only where it is exposed – is not sufficient. For manufacturing reasons, the transition from flex to rigid is not at the same point throughout all layers, so any (possibly buried) vias in the flex layer must step back into the rigid area by a larger margin than might be obvious. Where tracks join circular pads in the copper of the flex layer, teardrops should always be applied; the gradual transition from linear to circular eliminates a possible stress point at the junction.
Use of copper on the flexible layer or layers also adds to the list of ‘special rules’ and visualising what is happening when the flexible segment is called into action will show why.
A common error is to have too much copper on the flexible layer – large areas of metal foil bonded to the polyimide will inhibit flexing. If continuous copper is needed, then an appropriate level of cross-hatching will be required. For similar reasons, tracks should be routed perpendicularly to the bend line. All copper should be derived the original foil as plating on the flexible surface tends to produce crystalline metal that is too brittle to guarantee long life.
A variant on the rigid/flex formula is the application of stiffening to a portion of the flexible layer. In effect, this delineates a region of the assembly that is rigid, but which does not carry components. Stiffeners can provide support during assembly and can constrain the shape of the flexed circuit or they can provide structure and minimum thickness where the flex layer terminates by entering a zero insertion force connector. Although only providing mechanical support, the same set of rules applies to avoid creating problem areas.
Those designing flexible PCBs have often found value in making paper models of their boards – ‘paper dolls’. Today, 3D representation of the design can provide that insight into real world geometry, while applying the parameters the board fabricator requires for high yield. By drawing accurate component profiles from its database, engineers can be certain from the outset that the flexed and folded product will fit together as intended, with no interference.
Author profile:
Robert Huxel is technical marketing manager, EMEA, Altium
