Flexible printed circuits have unique requirements for footprints owing to the nature of their application.

Here is another lesson I learned the hard way: taping out an FPC (flex printed circuit) using the usual components and finding it doesn’t really work that way. Several things separate a rigid board from a flex. One of the main tenets behind the different design rules is reducing the risk of the circuit peeling up when it gets flexed. Even without continuous flexing, a flex circuit can be under tension where it is folded, twisted, spindled or mutilated.

Ah, but the flexible section is generally not where we install components. Normally, a stiffener covers part of the flex, and components are on the other side. Therefore, it is rigid, right? Not really. Most stiffeners used on flex circuits have a degree of flex to them. Flex stack-ups are intended to be as thin as possible; it’s one of their advantages. Even stainless-steel versions have some give. Many are made of FR-4 or another layer of polyimide, not all that stout.

In short, this means we want something more like a Class 3 footprint in that the maximum size pad is preferred. More area gives it more bite on the surface. A typical rule for flex is to use a fillet to taper to the line width of the traces. Any abrupt angles are stress-risers and need to be avoided. Round things off rather than squaring them.

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Pros and cons – and costs.

It’s almost inevitable that a component that works well and lasts a long time will eventually be put on a list of parts not to be specified for mass production. Newer, better parts are on the way. The thinking goes that the microcontrollers and other devices on a board are already fine-pitch, so another one can be accommodated. That’s how we end up with those five-pin regulators with a tiny diamond-shaped pin trapped between four beveled rectangles.

Advantage: Component-to-component spacing. The via-in-pad trick enables high component density by enabling routing that is 100% internal to the board, with no exposed traces. The space normally set aside for the fan-out via can be used for the next component with the following stipulations:

• Test access is maintained
• Rework clearance (for desoldering)
• Electrical isolation (shielding)
• Thermal considerations (heat sink, heat pipe)
• Mechanical interference (headroom)
• Pick-and-place accuracy.

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