Stiffeners are a common requirement in a lot of flex designs, by definition, a circuit board stiffener provides a mechanical support function and is not part of the electrical schematic of a design.
For the majority of applications, stiffener requirements fall in one or both of the following categories:
- Rigidize localized areas to support components and or connectors.
- Increase the flex circuit thickness in specific areas to either meet ZIF connector specifications or to constrain bend areas to predefined locations.
Rigidizing Stiffeners Help Support Components
In many instances a stiffener is used to create a localized rigid area in the flex circuit where components and or connectors are attached. This also prevents the circuit from being bent in or adjacent to the component area(s) which can potentially compromise the part’s solder joint integrity.
FR4 Stiffeners for Flexible PCBs
The most common material used for stiffeners is FR4 When utilizing a stiffener for plated-through hole (PTH) components, the stiffener(s) need to be located on the same side of the flex from which the component is inserted to allow access to solder pads on the flex circuit. FR4 stiffeners may also have copper features, e.g.; pads or plated holes used for component mounting purposes etc.
Additional Stiffener Materials
In some flexible circuit designs, alternate materials, such as stainless steel or aluminum are available. These are typically used for applications requiring heat sinking or added rigidity but will significantly increase the cost of the parts and should only be used when required.
Using Stiffeners To Increase Thickness
Polyimide stiffeners are the most common method to achieve the thickness requirement, at the contact fingers, as specified by the ZIF connector that the flex circuit plugs into. Some common thickness requirements are 0.3mm or 0.2mm. This thickness is be achieved by adding a thin Polyimide layer in the finger area only per the connector specifications.
It is not recommended to design a “thicker” flex PCB in an attempt to eliminate the need for a ZIF stiffener. This will result in an excessively thick part that will not have the required flexibility or bend reliability and will be much more costly. ZIF connectors have tight dimensional tolerance in the finger area width, so only a thermally bonded Polyimide stiffener can be used for this application.
Stiffeners for ZIF connectors can be attached to one or both sides of a flex circuit design. It is recommended to include the ZIF connector part number in the part data/drawing to allow your supplier to review and ensure the flex design meets the appropriate connector specs.
The purpose of a flex circuit stiffener is to add a significant amount of functionality to your design and can be configured in an extremely wide range of combinations. All stiffener types can be combined in any flex PCB design. The mechanical nature of a stiffener’s function is one area of flex design that differs significantly from that of rigid printed circuit boards. This is a significant area in which our customers require design support to ensure that the finished parts meet their requirements.