Are there any application where flex pcb's (rigid/flex) are really being used and required? or is this a more luxury product which is not entirely mature yet to the level where it can be implemented in every day products.
As the comments suggest, the main use of flexible PCB techniques is interconnect: linking one (rigid) PCB to another. They can be produced in arbitrary shapes more easily and cheaply than thin cable looms.
Lots of laptops and tablets will have flexible PCB connectors for things like the screen.
Placing components on flexible PCBs has the problem that the components themselves are not flexible, so they tend to strain the joints. Unless a "stiffener" is attached behind the PCB.
Yes, they are used frequently. A few examples (by no means a comprehensive list):
- Replacing wiring harnesses in equipment. They are cheaper at moderate volume than a hand-assembled loom, and generally are smaller and lighter. The last point (lighter) has made them very popular in aerospace in recent years, where every pound counts against your takeoff weight and fuel economy. I actually had difficulty getting flex boards built in the US recently because many of the US-based fabs are chock full to capacity with defense and aerospace customers.
- Smartphones - many phones use a rigid-flex board for the small PCB where the buttons and the USB port connect. This part is rigid, and the flexible portion is terminated with exposed contacts to plug into a ZIF connector. It's more reliable than two ZIFs, easier to assemble, and saves two parts (the cable + the additional ZIF connector).
- Digital cameras - probably one of the first mass-market applications of flex PCBs. Digital cameras are pretty space-constrained (since much of the volume is taken up by the optical path), which means you need to do a lot of flex circuit origami to get parts to fit while still being able to interface with all the buttons and sensors. Here's a picture of the guts of a really old Olympus camera showing a flex board.
- Medical implants - these things are very space constrained, and for good reason. The smaller you can build a pacemaker/ICD/neurostimulator, the less invasive the surgery will be for the patient. Rigid-flex is becoming very popular for these applications: they let you use the interior volume efficiently, and remove the need for connectors between boards (improves space utilization, reduces possibility of assembly error, and increases reliability).
I'm not sure why you have the impression that flex and rigid-flex technologies are tremendously more expensive than conventional rigid PCBs. Yes, they do cost a bit more: design is trickier (especially in continuous flex applications), prototyping runs are a lot more expensive, and the final product does cost a little more.
But at high volume, the difference isn't huge: I received a volume quote from a major manufacturer in Asia for a rigid-flex board which came in at well under $5 per unit. Probably don't want to make toys for that price (you're focused on shaving off fractions of pennies in that market), but it definitely makes sense for the right product such as a smartphone. And pure flex boards (especially ones that are only two layers) are very cheap - you can produce them for under a buck in high enough quantity.
Rigiflex PCBs can eliminate connectors (the primary source of problems in most electronics) and provide a really good interconnect solution in physically 'difficult' situations.
One design I had required an external connector to be mounted such that I needed a 90 degree interface to the primary processing circuitry (a digital video and audio recorder in this case); I could have done a plugin board but as this was in a high vibration environment (fast jet) that introduced a level of risk of operational problems.
A rigiflex solves that problem at a stroke and eliminates the space requirement for an actual connector.
They are also extremely useful for interconnect where the mounting locations for each end are not precisely defined (I had that issue with some POP4 fibre carriers that had to connect to a large motherboard - there were 8 of them).
I do not normally mount components on a flex section (although that is being done more often).
There is a bit of a cost impact, but the overall reliability is usually better if it can eliminate a connector.
As an added bonus, the high speed performance of many flex materials are superior to epoxy style FR-4.