the flexperts
Mark
Finstad

Flexpert Bending with Attached Structure

Dynamic flex applications require careful consideration of bend geometry, material selection and stackup design to minimize mechanical stress and improve long-term reliability.

When posed with the question of bonding a flex circuit to a thin piece of plastic in an application where the bonded area will flex dynamically, much like a diving board secured at one end, the answer is that there are many variables that affect long-term reliability.

Below, I’ll examine the factors with the greatest impact on performance and discuss how they interact in the final assembly.

Bend radius and bend angle. These are the first factors to evaluate. How tight is the bend? Is the dynamic flexing from zero to X degrees, or from zero to the same angle in both directions? These two variables, and how they interact, will have the single biggest impact on how the flex performs during service.

Right from the start, violating the IPC-2223 bend ratio guidelines for the bare flex is not a good sign. While exceeding the IPC-2223 bend ratio guidelines does not, by itself, guarantee failure, it is rare for the situation to improve as additional materials are added.

Flex circuit construction. The number of flex circuit layers and the overall thickness are both critical to the evaluation, both independently and in conjunction with bend radius and bend angle. The analysis becomes even more complex once the flex is bonded to a flexible plastic support.

Mechanical support type and thickness. The type and thickness of the mechanical support material are also important considerations. A softer-durometer thermoplastic with some elasticity will behave differently than a more rigid material such as ABS.

Mechanical support bonding method. The bonding method also plays a role. For example, a pressure-sensitive adhesive will affect the assembly differently than a thermosetting adhesive.

Interactions between variables. Whenever a material is bonded to a flex circuit, whether a thick FR-4 stiffener, a thin piece of plastic or a piece of stainless steel, the bonded area behaves differently when flexed. The change extends beyond the obvious increase in stiffness caused by the additional thickness. The same principle applies to areas with overmolded plastic on one or both sides of the flex.

Once additional materials are incorporated, flex circuit reliability can no longer be evaluated solely on the bend ratio of the bare flex. Instead, the entire stackup must be considered to predict how the assembly will perform during dynamic flexing.

The first illustration below shows a cross-section of a single-layer flex and the approximate location of the neutral bend axis. Depending on the construction, the neutral axis generally falls within the middle third of the cross-sectional area. The second illustration shows the flex bonded to a mechanical support. In this configuration, the neutral axis shifts completely out of the flex circuit and into the mechanical support.

The amount of this shift depends on the thickness and material properties of the mechanical support. A support made from a softer, more elastic material, such as rubber or a soft thermoplastic, has less effect on the neutral axis than a rigid material such as FR-4. The location of the shifted neutral axis significantly influences the forces applied to the flex circuit as the assembly bends.

Figure 1. Bonding a flex circuit to a mechanical support shifts the neutral bend axis into the support material, increasing mechanical stress on the flex during dynamic bending.

The illustrations below show how shifting the neutral bend axis (blue line) into the mechanical support exposes the entire flex circuit to either tensile or compressive forces during bending. On a bare flex circuit, the outer portion of the bend experiences tension, while the inner portion experiences compression. The middle region of the flex experiences relatively little of either force.

Once additional material is bonded to the flex, these forces can increase rapidly, leading to failure. Excessive compression can create wrinkles that lead to delamination, while excessive tension can stretch the materials, resulting in tears and conductor cracks.

Figure 2. When the assembly bends in one direction, the shifted neutral axis subjects the flex circuit to increased tensile forces that can lead to stretching and conductor cracking.
Figure 3. Bending in the opposite direction places the flex circuit under compression, increasing the risk of wrinkling and delamination.

Design recommendations. Revisiting the original design considerations, bend radius and bend angle have the greatest influence on long-term reliability. As bend radius decreases and bend angle increases, the forces acting on the flex rise dramatically. Any design changes that increase the bend radius or reduce the bend angle will improve reliability.

Using a thinner or softer mechanical support material can reduce the shift in the neutral axis. Another option is to use two mechanical supports, each half the required thickness, with one placed on each side of the flex. This configuration positions the flex closer to the center of the stackup and shifts the neutral axis back toward the flex circuit.

All these variables interact and can influence the performance of a dynamic flex circuit application in ways that are not always predictable. For that reason, it is important to have an in-depth discussion with the flex circuit supplier early in the design process. Experienced suppliers have seen thousands of applications and can provide valuable guidance on what works and, perhaps more importantly, what does not.End of article content

Mark Finstad is director of engineering at Flexible Circuit Technologies (flexiblecircuit.com); mark.finstad@flexiblecircuit.com. He and co-“Flexpert” Nick Koop (nick.koop@ttmtech.com) welcome your suggestions.