Do’s and don’ts in flexible-circuit design ( Chapter two )

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Flexible circuits bent over 90° are subject to much higher compression and tension force, which boosts the chance of damage. If a circuit must bend beyond 90°, it should be bent once only and then mounted in such a way as to prevent the bend from opening again.

A rigid PCB can incorporate many features that should be avoided on a flexible circuit. Most of these “don’ts” concern features that cause discontinuities in the bending area. The stretching and compressing forces present in the bend area concentrate at discontinuities in material or construction. These higher concentrated forces can lead to fractures in the conductors or insulation.

In addition, it’s important not to change conductor width or direction in bend areas. An optimum flex-circuit design maintains a uniform width for all conductors that cross the bend area perpendicular to the bend.

This does not mean that there cannot be 0.010 and 0.030-in. lines side by side. It just means that each line should not change width in the bend area. Also, a change of direction of a conductor in a bend area can potentially cause a stress concentration point and should be avoided.

Don’t stack conductors on multiple layer boards. Many board designers like to run signal and return lines on top of each other on adjacent layers to minimize electromagnetic interference (EMI). Doing so makes the overall circuit thicker and creates an “I-beam” effect. If this method of EMI reduction is used, line pairs should be staggered to reduce the effect.

Flexible-circuit materials are reasonably tear resistant. But once a tear starts, it tends to propagate. A sharp inside corner on the circuit outline is a prime location for a tear to start. It can act as a stress concentration point if the circuit flexes in that area.

For that reason, radius all inside corners a minimum of 0.030 in. on the circuit outline. If space allows and the inside radius can be relaxed to a value of 0.060 to 0.075 in., do it. The larger the radius, the lower the possibility of a tear starting in that location.

It’s well known that heat softens a flexible circuit and makes it easier to bend. While heat makes forming easier, one must ensure that the heat source does not overstress the flexible circuit.

A heat gun is capable of extreme temperatures that are well above the maximum temperatures a flex circuit tolerates. It is virtually impossible to regulate the circuit temperature. It is a function of the heat gun setting and the distance the flex circuit is from the nozzle.

Even at a low setting, a heat gun can produce temperatures high enough to blister and delaminate a circuit if it is placed too close to the nozzle. For this reason, one should not use a heat gun to heat form flex circuits.

The recommended method for heat forming is to first cold form the circuits using a customized forming tool and then load the circuits into an oven while still constrained in the tool. Once the tooling and circuits have warmed sufficiently, remove them from the oven and let them cool while still constrained in the tooling.

Use extreme care when removing heated circuits from the oven because they are quite soft and easily damaged. The best temperature to heat-form circuits is the lowest temperature that yields good results. For most circuits, this temperature ranges from 200 to 275°F. Keep in mind that if the circuits see elevated temperatures after they are formed but no longer constrained, they will return to a flat state.

Don’t use rigid PCB temperature profiles to reflow flex or rigid-flex PCBs. A reflow oven temperature profile for a rigid PCB will most likely be far beyond what a flex circuit can tolerate. The most obvious signs of excessive temperature exposure will be blistering and delamination. It is advisable to contact a flex-circuit manufacturer that also does assembly work for guidance on proper reflow temperature profiles.

Wide conductors, those greater than 0.010 in., are more robust and tolerate bending better than small conductors. If a bend is pushing the minimum bend ratio limits, it is a good idea to widen small conductors in the bend area.

Because forces from a bend can radiate out beyond the bend zone, the widening should be gradual and the conductor should reach its maximum width at least 0.10 in. before entering and after exiting the bend area.

Second, any hole represents a mechanical discontinuity in the circuit that is prone to cause cracking of the outer cover material. If cracks form in the outer cover, they will almost surely propagate with time and may cause the plated hole to crack and fail.

Flex circuits with multiple layers are thicker and the plated barrels are deeper. The combination of deep-plated barrels with higher stretching and compressing forces from the added thickness aggravates the problem. It is best to place vias in areas that see limited or no bending during installation or service. When the design does not allow sufficient space in nonflexing areas for all plated holes, try to place the holes in areas that experience the least amount of bending.

It is a good practice to place fillets on all termination and via pads. While not every design needs fillets to function reliably, they almost never cause a detrimental effect on the circuit. Fillets used around termination pads eliminate stress concentration points that could otherwise cause cracks where the conductor enters the pad.

As a final word of advice, consult your flex-circuit manufacturer early and often. Every year flexible-circuit manufacturers see hundreds, even thousands, of design options. Start a dialog with a flex-circuit maker during the design process, before the design and drawing are locked in and signed off. Their experience can dramatically improve the odds of finding the most efficient solution to even the most unique design challenges. 

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