Flex Circuits
A flex circuit is a PCB that has both rigid and flexible sections. This combination allows the engineer to build a stackup that meets the needs of a specific application while saving on material cost by using a minimum amount of rigid board stock in the rigid section and a smaller area of flex for the flexible portion. It also allows for different materials to be used for each part of the stackedup, giving a designer a wider range of options for their circuit. The benefits of a flex circuit include lower weight, smaller size, increased reliability and more design freedom.
The etching process for the copper on a flex circuit is a bit different from that on a rigid PCB. The seed conductors are typically deposited directly onto the flex circuit’s base laminate and then electroplated to their final thickness. This is done by using techniques such as sputtering or electroless plating. The resulting web of copper is then etched back to expose the pads and conductors that are required for the circuit.
After the etching is complete, a coverlay is applied to protect the circuit from moisture and contamination and for ease of handling during the assembly process. The coverlay can be made from a variety of different films including polyester and polyimide, and can have a pressure sensitive or thermosetting adhesive on one side.
Can Flex Circuits Be Recycled?
Another layer that is added to the flex circuit is a stiffener. This helps to reduce the stress on the copper conductors during bending. This is a very important step, as a weak stiffener can cause the copper to detach from the flex circuit. In order to reduce the risk of this, the stiffener can be made from the same material as the flex circuit, or even from the same material as the copper (known as an adhesion layer).
Finally, the flex circuit is laminated to a rigid board in a sandwich format. This gives the designer the option of mounting rigid components on the flex circuit in place of those that would be mounted on the rigid board (this is known as a hybrid). The rigid-flex PCB may have a bead of epoxy, acrylic or hot-melt glue where the flex and the rigid meet which is designed to improve longevity in harsh environments.
However, this adds steps and expense to the production process. In addition, the dispensing of these liquids is not an easy task and requires close collaboration with the assembly engineers to ensure that there are no globs of glue on the flex when it is being routed.
As with all electronics, there are a few quirks to consider when designing a flex circuit. The main ones are the stresses within the materials during bending, especially tight radii. Copper, being a non-ferrous metal, is prone to work hardening and fatigue fractures when subjected to repeated bending with tight radii. This can lead to a failure mode called “flaking” which is difficult to detect during testing.