The Role of Adhesive in the Construction of Flex Circuits

Construction of Flex Circuits

In the construction of flex circuits, adhesive plays a key role. Almost every flex circuit stack-up uses adhesives in some way or another; either as the main material laminate and copper adhesive, for coverlay bonding, for stiffener adhesion, and more. In fact, in the case of the popular FR-4 rigidized flex constructions, most of the adhesive is used in the stiffeners. Adhesives in the form of acrylic and epoxy adhesive films, are used to attach the stiffeners to the flex circuit boards.

A flex PCB is a thin substrate that conductor traces are etched on, with as many layers as the design calls for. Then a coverlay is placed on the surface to protect the traces from moisture and damage, and to facilitate soldering and assembly processes. The coverlay can be a dry film, liquid photo-imaged, or polyimide material. The film is usually either polyester or polyimide with an appropriate thermosetting or pressure-sensitive adhesive.

The adhesives for a flex circuit are chosen to match the material of the base film. Adhesives are available in a variety of thicknesses, but for the best performance they should be as thin as possible. The choice of thickness is based on the needs of the application; thinner materials will offer better flexibility, while thicker materials will provide higher temperature resistance.

The Role of Adhesive in the Construction of Flex Circuits

When designing a flex circuit, it’s important to consider the stress on the copper layer as the circuit bends. Repeated bending can cause work hardening and fatigue fractures in the copper layer. This is especially true in tight radii where the copper layer will experience significant compression and tension. To reduce this stress, a good practice is to limit the number of flexes and to use single-layer flex circuits.

Stiffeners are commonly attached to a flex PCB using an adhesive such as FR-4 fiberglass or a pressure-sensitive adhesive (PSA). The PSA is applied directly to the flex circuit’s surface, and then additional heat and pressure is applied to the system in order to create a permanent bond. The stiffeners can be placed at the corners of a double-layer flex circuit to stabilize them, or they can be used in the center to add support for zero insertion force (ZIF) connectors.

If the flex circuit will require vias, it’s important to keep in mind that they are more likely to be damaged during handling and assembly than other surface components. Vias are exposed to a great deal of physical stress during the assembling process, and this can lead to cracking in the copper plating inside the via hole.

To help prevent this, it’s a good idea to use “rooms” in the layer stack manager to confine flex via placement to areas where there will be no bending. This will minimize stress on the vias during assembly and operation, as well as reducing the likelihood of a failure caused by an electrical short in the via. Also, if the flex will require a ZIF connector, it’s essential that the connector be designed with a large enough opening for proper attachment.

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