Rigid flexible PCB Layout guidelines
Rigid-flex PCB boards are unique in terms of the integrated construction of both rigid and flex circuit technologies. Unique construction comes with unique requirements that should be reviewed and implemented during the rigid-flex PCB Gerber layout phase of the design process.
The first two regulations relate to minimum space requirements, as measured to the flex transition zone(s) within the design of plated through holes (PTH) and external layer copper features. The following two regulations deal with the mechanical flexibility and reliability of the flex areas when the parts are bent into the required shape.
From the differences mentioned above, for the layout and use of rigid-flex PCB circuits, the following can be deduced:
- For single-sided flexible circuits, the bending radius is approximately 6 x the thickness of the flexible material, and for double-sided circuits, it is approx. 12 x the thickness.
- Choose track widths and spacing in the flexible region to be as large as possible (> 150 µm)
- The flexible region should have parallel tracks which are the same width, with the same leakage resistance, and which run at 90° to the bend line
- Tracks should extend at least 1 mm into the rigid region
- On flexible layers, include hatched copper areas which are as large as possible
- The tracks on double-sided, flexible parts should be symmetrically offset
- Holes should be at least 2 mm away from a flexible part
- Choose solder surfaces to be as large as possible, the diameter of solder eye pads should be at least twice the diameter of the hole.
- Solder must not come closer than 1 mm to the flexible area
- Add about 1 mm all round to the dimensions of openings in non-photo structure covering films
- To be able to assemble the components on a board, to solder & check them, a stiff frame with defined breaking points should be used. Blanks can contain single or multiple boards.
- Always use smooth (round) milling transitions to the connected parts of the circuit
Fully Bonded Vs Air Gap Constructions
Higher layer count rigid-flex PCB designs may need to utilize an Un-bonded Air Gap construction, a technique that configures layers as separate pairs, as opposed to a single fully bonded unit. The thinner individual pairs provide a higher degree of flexibility.