The most obvious benefit to a Flexible Printed Circuit (FPC) is the ability to bend and flex in order to allow for three dimensional packaging. However, the flexibility of an FPC is not always inherent in the design. When looking at a Flex circuit in a 2-D fashion, that is flat on a surface, one can easily see the ability to bend the circuit (up or down) in this plane. A good example of this is shown in Figure 3-1 of the Sectional Design Standard for Flexible Printed Boards, IPC-2223a.
In this model, the X and Y dimensions are controlled by the geometry of the circuit outline, and any flexibility or bending takes place in the Z direction. A flexible circuit really does not bend in the X or the Y direction on a 2D model. A change in the X or Y direction is handled by folding over by 180 degrees which technically happens in the Z-Direction.
The outline geometry can be a cost driver in Flexible Printed Circuits, in that the larger the design, the more real estate is used on the process panel. It can often be economical to design a more streamlined flex outline and use folding or bending to control your X and Y endpoints.
To really consider 3-D flexibility, you literally need to think outside the box (pun intended!). A flexible circuit is made from a plastic film (typically polyimide or polyester), however, it will not stretch or bend laterally on the X-Y plane. Trying to make a lateral bend in the flex on this plane will cause the material to “buckle” or “twist”, which can cause undue stress on the outline corners, resulting in a tear condition.
By manipulating the outline geometry, the designer can control how and where the material will “buckle”, and effectively prevent tearing. An application where this might become necessary is one where a flexible circuit interconnect is used as a jumper between two PCB’s or devices or some combination thereof. One end of the flex may be fixed in the assembly, and the other end will mate to another device in the package. Manufacturing tolerances of the Flex as well as the other devices could make it challenging to mate.
In order to accommodate this type of lateral movement, a relief slot or slots can be made in the material. The slots will require a hole at each end as a tear stop, and the slot width will be determined by copper design as well as tool capability, but generally not less than 0.020” width.
By creating a slight bump in the tail (Z axis), the slots will give the flex material a place to “buckle” and the space for the material to shift without creating a high stress point. This technique is not designed to provide a large amount of movement in the tail, but can be effective in taking up a tolerance mismatch where needed.