Flexible Circuit Boards
In general, PCBs can only withstand a onetime fold or crease. Repeated flexing will crack the copper traces, so careful design is required to make sure that this doesn’t happen. This includes determining the type of flex circuit you require – rigid flex or flexible hybrid – and taking into account the requirements for your flex PCB in its operating environment, including the temperature, humidity, mechanical stress, chemicals and shock & vibration.
The conductive layer of a flexible circuit board is made of pure copper combined with a dielectric film which can be glass epoxy, polyimide or other material. The material is bonded together with an adhesive or vapour deposition to produce the flexible PCB. For rigid flex, the base material is typically glass epoxy FR4 to give it good electrical properties and mechanical strength. For flexible hybrid, the base is often polyimide to provide flexibility and resist tearing.
A key factor to consider in designing a flex circuit is its intended application. If it will be used in a static application, then you are free to increase the number of layers and use a thicker copper thickness. If it will be subjected to frequent flexing and repeated folding, then you should reduce the number of layers, use a thinner copper and choose a more flexible dielectric material.
Can Flexible Circuit Boards Be Folded Or Creased?
Another important consideration is the fabrication process. Fabricators of flex PCBs employ different methods for the various stages of production. For example, they may use a lithography process for the copper traces and an uncured adhesive to attach them to the underlying substrate. They can also apply hot-melt glue to the traces and substrates to ensure a solid connection. They might also use a laser to etch the conductors from the back of the board and then use a drilling machine to create the metal pads on the top of the circuit.
In terms of quality control, a fab company will visually examine the finished product to look for contaminants and scratches. They will also test the flex for proper functioning and ensure that all of the components are in place before shipping it to its final destination.
To reduce stress on the flex circuit during bending, it is a good idea to route traces perpendicular to the bending direction. Wider traces will also help to distribute stress over a larger area. Avoid overlapping traces and use alignment features like notches, registration marks and holes to help assemblers accurately align the board during assembly.
To reduce the chance of the flex breaking under pressure, it is also a good idea to anchor pads with stubs and limit the number of coverlay access openings on the extreme ends. This will allow the pad to remain flat during flexing and also reduce stress on the pads. In addition, SMT component pads should be arranged with more space between them than would normally be the case for rigid boards to prevent them from rubbing together under a bend.
