Issue link: https://iconnect007.uberflip.com/i/1503357
62 DESIGN007 MAGAZINE I JULY 2023 FR-4 and polyimide stiffeners can be placed in panel form on top of the flex panel. e material is drilled, routed, and pressed onto the flex panel. e flex with the stiffeners, now attached and in place, get routed or lasered and removed from the panel. Manufacturers will always try to attach stiff- eners 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 pro- cess. is adds cost. Your manufacturer should help you to design the lowest cost solution. SMT Component Support e most common use of a stiffener tends to be SMT component support. For SMT sup- port, a stiffener—usually FR-4 material—is attached underneath an area where SMT com- ponents will be placed. e addition of the stiff- ener serves a few purposes during assembly: e stiffener creates a flat surface for precise component mounting, provides a "hard sur- face" for the actual placement—preventing the flex from bouncing during the placement pro- cess—and will support the assembly through the reflow oven. Aer assembly is complete, the stiffener protects the solder joints by not allowing the flex to bend in the assembled area. Through-hole Components For through-hole assembly, FR-4 is the most common use, but it depends on the applica- tion. e use of the stiffener supports the com- ponent as it is attached to the flex, protecting the solder joints and the flex from bending awkwardly and cracking traces. Flatness In most cases, it works to use FR-4 to create a flat, stiff surface. In some instances, however, there is a need to create a perfectly flat sur- face (minimal tolerances across an area). FR-4 material and the adhesive process can provide tolerances that may not be acceptable. As an example, we once needed to create a perfectly flat surface to mount a large BGA component: 360+ balls on a five-layer flex. Our assembly and inspection group shuddered, as the need to keep both the flex and component flat to make perfect contact across all the balls Figure 1: Mechanical support for SMT connector. Figure 2: Polyimide stiffener supporting SMT. Figure 3: Through-hole stiffener.