Stiffeners are a common requirement in many flex circuit designs. By definition they are to provide a mechanical function only and are not part of the electrical schematic of a design.
Rigidizing stiffeners create a localized rigid area in the design where components and or connectors are attached.
They prevent the circuit from being bent in or adjacent to the component area(s) which can potentially compromise the solder joint integrity. The material is typically FR4 with thicknesses varying from 0.008” up to 0.059”. The most common thicknesses are 0.020”, 0.031”, 0.039 or 0.059”. A general rule is to use the thickest stiffener that the design will allow up to 0.059” which replicates a traditional rigid PCB.
Stiffeners for Plated Through Hole (PTH) components need to be located on the side of the flex from which the component is inserted to allow access to solder pads on the flex circuit. FR4 stiffeners can have copper features, e.g.; pads or plated holes, for component mounting purposes etc. Alternate materials, stainless steel or aluminum, are available for applications requiring heat sinking or added rigidity but will significant increase the cost of the parts and should only be used when required.
A preferred stiffener attachment method is thermally bonding to the circuit (heat and pressure) utilizing the same flexible adhesive used to attach the Coverlays. Results in a very strong permanent bond.
An alternative attachment method is with a PSA, if the design prevents the use of a flex adhesive. Specific PSA will depend on whether the flex will be subjected to an automated reflow cycle.