Stiffeners for Flex Circuits

Estimated reading time: 1 minute

One of the most recent topics in the flex world has been the evaluation of materials required to build a rigid-flex or flex circuit. Discussions around flex and materials include variations of polyimide and copper. Here I will discuss another material that plays a very important role in the world of flex circuits: stiffeners. These are critical components to a flex circuit because they broaden its scope and potential applications.

A “stiffener” in the flex circuit world is a generic term used to identify a material that prevents a flex circuit from bending and flexing. In all these instances, we will not be talking about rigid-flex—just flex circuits only. Stiffeners are, in most cases, a non-electrical component/addition to a flex circuit and function as a mechanical support.

Stiffeners are attached in a secondary operation by either thermal bonding, pressure sensitive adhesives (PSAs), or epoxy. Stiffeners require a secondary operation in the manufacturing process, so it is a cost adder.

Materials we have used to add stiffeners include FR-4, polyimide, aluminum, alumina, steel, and ceramic. As technology progresses and flex is used in more and different applications, a variety of materials are being used to support and create flex stiffeners.

Reasons to include stiffeners into your flex circuit design include:

• The SMT process and component support
• Through-hole component mounting
• Flatness requirements
• Mechanical infrastructure and design support
• Increased material thickness requirements
• ZIF connector interface
• Heat dissipation
• Wire bond pads 

Figure 1: Mechanical support for SMT connector.

FR-4 and polyimide stiffeners can be placed in panel form on top of the flex panel. The material is drilled, routed, and pressed onto the flex panel. The flex with the stiffeners, now attached and in place, get routed or lasered and removed from the panel.

Manufacturers will always try to attach stiffeners in panel form; it is the most cost-effective solution to adding stiffeners to flex. In some instances, there is a need to hand place the stiffeners, requiring a separate lamination process. This adds cost. Your manufacturer should help you to design the lowest cost solution.

To read this entire article, which appeared in the July 2023 issue of Design007 Magazine, click here.