The Flexperts
Flex in the Stackup
Does it make a difference where the flex on a rigid-flex board resides?
I am working on a rigid-flex design. Does it matter where the flex layers are in the stackup?
Where flex layers are in a stackup does matter. Rigid-flex circuits come in all configurations. Fabricators can make rigid-flex boards with the flex at all different locations in the stackup; each has their reasons and constraints.

As a general rule, we recommend putting the flex in the center of the stackup. This permits the design to have an asymmetric stack. (Symmetry is very important when it comes to managing bow and twist.)

That said, there are a number of reasons why the flex may not reside in the middle of the stackup.

First, the board may not have an even number of total layers, or the flex itself may have an odd number of layers. This can throw the flex off the centerline. This is common and is usually not a concern.

Sometimes the flex may be shifted toward the top or bottom for the purpose of managing high-speed signals. This can permit a short z-axis transition before the signal is transported across a flex region. This may also be coupled with back-drilling to manage stub lengths.

In other cases, the flex may be biased to create separation between certain noisy signals and other sensitive layers.

Occasionally, designers will move the flex toward the top or bottom of the stackup to increase the bend radius. This can be done but may only add a few mils to the total bend radius.

Sometimes there are multiple flexes, and the designer will create extra separation between them to try to make bending easier. Some say this helps; others say it makes no difference.

In each of these situations, rigid laminate remains on both sides of the flex. Having rigid material on both sides of the flex means the flex will not get copper-plated, maintaining the original foil thickness. However, there is some risk that bow and twist may be impacted. The amount of bias of dielectric and copper on each side of the centerline of the stackup drives bow and twist.

If the desire is the flex as an external layer (top or bottom), keep in mind these considerations.

First, do you want that external flex layer to be copper-plated? This means adding electrodeposited copper onto rolled annealed copper. This impacts the flex endurance of the copper due to the grain structure of the plated copper. This can be minimized by performing selective or button plating where the plating buildup exists only at the through-holes.

Next, will you use coverlay or flexible solder mask on the external flex layer? Coverlay is more robust for tight bends and dynamic flexing. However, if you have dense SMT patterns on the part, then solder mask may be the better choice. We can’t create square or rectangular openings in coverlay. In some cases, a designer may take a hybrid approach. This involves placing coverlay on the external flex layer in the flex region and applying solder mask in rigid zones. This provides the best of both worlds: robustness in the flex zone and standard SMT openings at the components. This adds cost but can be worth it.

In some cases, flex may be used as the top two layers to aid with HDI. Layer 1 copper may exist only in the rigid sections of the circuit, while layer 2 spans the rigid and flex zones. Microvias can drop signals from layer 1 to 2 with very shallow vias and no stubs. As a big extra benefit, the flex core is completely resistant to CAF (conductive anodic filament) because there is no fiber in the flex laminate.

One challenge when the flex is on the outside is that at the rigid-to-flex transition, the flex substrate will not be perfectly smooth. This can impact etch quality at the transition. There can be a tendency to have irregular edges on the traces here, which may limit the density of the pattern that spans the transition. We recommend lines and spaces be at least 0.008″ (0.2mm) wide to minimize these risks.

Flex as an external layer creates another constraint. If designing with flex as an external layer, nonconductive epoxy via fill is not an option. Reason: the very high risk of damage to the substrate when the fill is planarized. Any filled vias need to terminate on a rigid layer to avoid risk of material damage to the flex.

To conclude, for best uniformity and processing, keep the flex in the middle of the stackup. When needed, however, flex can be anywhere in the stackup, provided the designer takes into consideration the via structures, bow and twist needs, and features on the external layers.
Nick Koop
Nick Koop
is senior field applications engineer at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee; nick.koop@ttmtech.com. He and co-“Flexpert”
Mark Finstad
(mark.finstad@flexiblecircuit.com) welcome your suggestions.