Cruced Flex Circuits
A flex circuit is one or more layers of electrical conductors that are mounted on a flexible substrate. Polyamide and polyester are the common substrate materials for flex circuits. Electrical components and circuits are attached to the conductors using an epoxy resin that encapsulates them all. The encapsulation protects the conductors and allows them to be folded and creased without losing their functionality.
A flex circuit can be used to establish electrical connections between different electronic devices and components. Flex circuits are usually single-sided, however multilayer flex boards can be fabricated as well, with plated through holes. Regardless of the type of flex circuit board, it is important to verify its layout with simulation software before sending it for fabrication. Once the gerber files are sent for manufacturing, the flex circuit can be fabricated and the components assembled to it.
The general rule for a flex circuit is that its minimum bend radius should be at least three times the maximum bending diameter of the circuit. However, this is just a rough rule of thumb and the actual bend radius should be determined by stress testing.
Folded and Cruced Flex Circuits
Depending on the design of your flex circuit, you may need to add support in areas where there is likely to be significant bending or folding such as near connectors and other critical components. A stiffener is a good option for this and can be made out of a wide variety of materials such as polyimide, FR-4, or even stainless steel.
The flex circuit will also have to deal with stress on the copper as it is bent and compressed. The best way to reduce this stress is to use a flex circuit design that uses radiused traces rather than straight lines. This will reduce the copper stresses along the fold axis and is recommended for all flex designs.
It is also recommended to avoid any sharp angles on the flex circuit. This is to reduce the risk of tearing the flex substrate material at those corners. If you must have a corner that is less than a 90deg angle IPC recommends terminating it with a circular section having a radius greater than 1.5 mm.
Another key aspect of a good flex circuit is that the bending regions should be sparse in terms of traces, vias, and annular rings. These can be a little difficult to do in some cases, but it is better to have more reliable circuits than ones that are not working. It is also important to keep in mind that the PCBWay process does not yield as tight of a feature set on flex circuits as it would for a rigid board. This is why it is best to use “rooms” to define non-flexing sections of the flex circuit and then limit your via placement to these areas.
