Flex PCB's for panel mount parts?

[Processaurus]

Having panel mount parts- pots, switches, jacks flying on wires is the best from a reliability standpoint, no vibration or shock to break the solder joints like with PCB mount parts, one of the fringe benefits of doing DIY pedals and one-offs.  It does take FOREVER, which is why PCB mount pots and board mounted switches are popular with manufacturers, despite introducing failure points with the solder joints cracking when the part gets jarred.  1/4" jacks and DC jacks are especially accident prone, I've had to take apart so many amps to fix the flaky input jack- it's soldered to the PCB, so take off every knob and every nut on every pot to get the board out to reheat the input jack.

I saw OSH Park has started offering flex PCB's, with them starting to be accessible, I'm wondering if anyone has thoughts on doing pots (like the green little 9mm pcb mount pots), footswitches, jacks, etc. flying on some kind of flex PCB, and coming in to the main PCB with the flex pcb ending in a ribbon, to get soldered in or into one of those little compression SMT connectors (FFC/FPC connectors) like you see on consumer stuff, laptops.  Boss has their row of 4 pots on a little daughter board, connecting back with a ribbon cable- like that but the daughter board is flexi and and the ribbon connection tail could be part of the flex PCB.  Each part has some mechanical isolation form the others, like if they were flying on wires, but is fast to solder like PCB mount parts.  If you could do a double layer flex pcb, you could even do a ground plane (or flex friendly alternative) to do some shielding of the "wires".

Way overkill for oneoffs, be proud of your parts on wires, but it could be nice for kit makers and booteekers, or projects with a lot of jacks or rows of pots.  Any precedent for this in the audio stuff world, or reasons it wouldn't work out?

[amptramp]

We had a project once where we had to have some digital meters on a panel so we chose Analogic meters that were made with flexprints, exactly what you are talking about.  We had no end of open circuits with them because the solder connections take a lot of the force of flexing.  Even folding the flexprint once to put the electronics into the box was enough to do this.  When we had the same problem, we used separate flexprints for connection and normal rigid boards for component mounting.  That was more reliable.

[GGBB]

Not sure I understand how flex helps. The "flex" part of flex PCB material is only bi-directional - it does not help with forces applied parallel to the board which is I think the main problem. Off-board hardware avoids these problems entirely as you noted. Secondary boards enable factory automation to have off-main-board hardware. It is MORE work to mount hardware to secondary boards AND add ribbon cables (or any wiring) for the hand-builder (adds cost as well).

[mozz]

Having worked on lots of military equipment that has multi-layer flex prints, i hate them. I see no use except space saving. Often higher current traces will melt and some you bend more than once or twice they break. Some stuff was obsolete and when a contract was put out for new flex cables, they were often very thin and worse quality than the originals.

[Processaurus]

We had a project once where we had to have some digital meters on a panel so we chose Analogic meters that were made with flexprints, exactly what you are talking about.  We had no end of open circuits with them because the solder connections take a lot of the force of flexing.  Even folding the flexprint once to put the electronics into the box was enough to do this.  When we had the same problem, we used separate flexprints for connection and normal rigid boards for component mounting.  That was more reliable.

You mean you used a daughter PCB for the panel part, and used the little compression connectors with flex pcbs, like the flex pcbs were serving the role of a ribbon cable?

Having worked on lots of military equipment that has multi-layer flex prints, i hate them. I see no use except space saving. Often higher current traces will melt and some you bend more than once or twice they break. Some stuff was obsolete and when a contract was put out for new flex cables, they were often very thin and worse quality than the originals.

Would they break at solder connections or the copper traces would break at the bends?  Do you know if these flex pcbs were done with soldermask or coverlay (a laser cut sticker alternative to soldermask, more flexible).  I saw at PCBways they have a variety of different weights of copper and polymide (the plastic flex core), the ones you didn't like, I wonder if they were lighter copper or lighter core?  I would imagine the thinner ones would be more flexible.

Not sure I understand how flex helps. The "flex" part of flex PCB material is only bi-directional - it does not help with forces applied parallel to the board which is I think the main problem. Off-board hardware avoids these problems entirely as you noted. Secondary boards enable factory automation to have off-main-board hardware. It is MORE work to mount hardware to secondary boards AND add ribbon cables (or any wiring) for the hand-builder (adds cost as well).

The hope was the flex would allow a PCB mounted panel part to move a little when it takes abuse, like a panel part on wires, without stressing/cracking the solder joints like happens when the part moves but the PCB stays rigid in place.  It would allow for PCB mount parts of different heights. The side walls of our hammond boxes are at an angle, parts in the side could flex to match the angle. It would also allow little misalignments when assembling the parts or tightening them back into place.  My current M.O. with the PCB mount parts is to put them in the box, finger tight, wrestle the board onto all of them, then tighten the panel parts, then solder, hopefully never take it apart again.  When you put it back together again there are probably little differences in height that would stress the joints, DEFINITELY if the parts are soldered to the board first and then assembled into the box.  I see what you mean about yanking type movement on the flex PCB, the strain is on the solder joints. 

Wonder if it would solve any problems to glue the part to the flex PCB, for strain relief?  Though that would make the assembly less repairable, down the road...

Getting more complex, OSH Park doesn't have it but some other flex PCB manufacturers have stiffeners you can add to the areas of the flex PCB, like our FR4 PCB stock, I wonder if that would solve the problems Amptramp was having with the meters?  They recommend it for areas with solder joints (like SMT parts on the flex pcb), but like a PCB mount pots solder joints, it could keep that area from bending.  That's what I see usually when I take apart cell phones, like the side buttons are on flex pcbs with little rigid areas under the buttons.

[amptramp]

Most flexprints are made with Kapton polyimide which is flexible but rather stiff.  It is quite easy to make the material overstress a solder joint.  You can get flexprints with daughter boards already attached to the flexprint but you still have a problem with the copper traces on the flexprint itself fracturing where they are bent because the polyimide is much stiffer than copper.