I’m prototyping a 250 mm × 6 mm flexible PCB that carries three 3 × 3 mm DFN packages (hand-assembled). All DFNs will be on the top side; any stiffeners would go on the bottom, opposite the components. Design guides from many printed circuit board manufacturers insist on stiffeners under SMT devices, but for this tiny length/width I’d like to keep the flexible PCB fully flexible, is it absolutely necessary?
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If you put stiffeners slightly larger than the packages under each one, that will reduce the flexible length by less than 5%, which doesn’t sound horrible.
Is the real problem that these three packages are very near each other, and in a location that needs to do a lot of the bending? (For example, are they all in the last cm, and that part has to do as much bending as the rest put together?)
Hopefully someone with more experience will correct me, but I would expect the normal case to be that the packages themselves already prevent bending right there; it is just that the failure mode (without stiffeners) changes from “won’t bend” to “cracked solder joint”.
Or is this more of an aesthetic thing, where you want the look of a circle, rather than a dented circle? (A sleeve might help?) Or something I haven’t thought of…
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Yes, the three DFNs are placed close to each other, near the connector, and in the area of the flex that’s likely to bend the most. That’s exactly what I’m concerned about, whether the local stiffness of the ICs is enough to resist bending on its own, or if I risk cracked solder joints without additional support.
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If the section of the flex with the DFNs is expected to bend, especially near the connector, then adding stiffeners is strongly recommended. Without them, flexing can cause coplanarity shifts and stress the solder joints, especially under vibration or repeated handling, leading to failures like cracked joints.
A simple way to add mechanical support your flexible PCB is to bond polyimide (PI) or thin FR4 to the underside of the DFN area using pressure-sensitive or thermoset adhesives. This localized stiffening helps maintain solder joint integrity while keeping the rest of the flex cable fully flexible.
For more demanding cases (e.g., BGAs or thicker flexible PCB), adding a ground plane or using dual-rigid-flex construction could offer even better reliability, though it adds complexity and cost.
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If you need more bending than it can do with stiffeners, could you use wires instead of PCB in that section?
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I hadn’t thought about that, thanks for pointing it out! I’ll look into whether that approach is feasible for my flexible PCB prototype.