One habit behind all five
Rigid-board design tolerates a lot: mask covers what it covers, a via can go anywhere, a pad can sit next to a trace, and nothing moves after assembly. On a flex circuit every one of those assumptions becomes a failure mode, because the part is expected to bend, and often to bend thousands of times.
1. Coverlay openings drawn too tight
Coverlay is a film with adhesive, cut and laminated, not a printed layer. Registration, die-cut tolerance and adhesive squeeze all move the opening, so a window sized like a solder mask opening ends up either covered in adhesive or shifted onto the neighbouring trace. Generous openings, and generous spacing between them, are the fix.
2. Solder joints inside the bend
Solder is far stiffer than copper. A joint near a bend turns the pad edge into a hinge and the copper cracks there first. Joints belong in stiffened or rigid areas; the bend stays bare.
3. Pad spacing that ignores the larger opening
Once the coverlay opening grows, the distance between adjacent pads has to grow with it. Otherwise the enlarged windows expose the edges of neighbouring conductors and the assembly process bridges them. The 0.002″ line and space capability is not an invitation to use it between connector pads.
4. Sharp corners and acute junctions
A 90° corner in a conductor is a stress riser on a rigid board and a crack starter on a flex. Junctions taper, corners are rounded, and the transition into a pad is teardropped.
5. Copper stacked across the neutral axis
Two copper layers directly opposite each other in a bend put one of them in tension on every cycle. The strain is proportional to the distance from the neutral axis, so the fix is geometric: offset the layers, thin the copper in the bend, or use a single conductive layer there.
Catching them before tooling
All five are visible in DFM, and all five are cheap to fix at layout stage and expensive afterwards. A review that walks the bend region, the connector tails and the coverlay artwork catches most of them; the rest show up in bend testing, which is why a first article is worth cycling before volume.
The substrate, coverlay and stiffener ranges are on the flexible PCB page; where the flex meets a rigid section, the rigid-flex rules apply.
