Designing Flexi’s and Flex Rigids – What’s Involved?

Flexible and flex rigid PCB’s are offering solutions to product designers that are not just a luxury in this day and age but a necessity. The technology itself isn’t that new, flex PCB’s have been around for over 30 years. But in today’s world of extremely compact and high speed connections often there is no choice but to use them over conventional electronic assemblies using wired connections to connect between PCB’s and these are to large. This is the main reason for their increased use in the last 10 to 15 years.

But how does the PCB designer go about designing a flexi or flex rigid PCB with components on it and how can the design be made easier to manufacture?
The basic Do’s and don’ts around flexi circuits we can get a picture of what’s involved and why:

a) To avoid the flex material tearing in manufacture or use, put as large a radius as possible on all internal corners.

FCB1

b) Via’s should not be placed in bend area’s as they can crack.

FCB3

c) Tracks running through bend areas should be routed at 90º to the bend.

d) Tracks on flexi’s ideally should be routed with filleted corners to stop them breaking during flexing.

e) Copper pours on flexi’s should ideally be hatched, especially in bend area’s. Because of stresses in the solid copper pour fractures are likely especially during flexing.

FCB2

This simple guide gives the designer a good basis on which to design flexi PCB’S (FPCBs) well. The challenges don’t end here though. Ideally an FPCB will have stiffeners under areas where components are going to be in the circuit. You can bet there will be times where some bright spark will decide for one of many reasons that the FPCB will have components and no stiffeners.

This is not an easy task and the designer is likely to get the task of working out how this can be done. This usually means working out what kind of stiffening frame can be used with the fabricator and assembler and sticking the FPCB down.

The reason for sticking down and FPCB for assembly is clear when it’s understood that the copper on an FPCB is giving at least as much structural rigidity as the substrate itself. Stresses in the copper push and pull the shape into interesting and often unwanted shapes. Assembly would be impossible without sticking the FPCB down to something rigid.

Added to this the copper finish isn’t very durable and can crack if flexed. Keeping these component pads away from bend area’s is necessary to ensure they survive through to assembly, but if it putting the component in a bend area can’t be helped then sticking the FPCB down to secure it is another reason why sticking it down is not be a bad approach to use.

As always it’s never a bad thing to engage manufacturer’s in the design stage and with flex this is even more important, even if only at first until all of the design aspects are better understood.

© Circuit Mechanix 2017

 

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