Multiple Flex Sub-Stacks
While it’s possible to build just about any stack-up with rigid and flex sections, it can get ridiculously expensive if you’re not careful to consider the production steps and the material properties involved. One important aspect of flex circuits to remember is the stresses within the materials occurring as the circuit bends. Copper, being a non-ferrous metal, is known to suffer work-hardening, and fatigue fractures will occur eventually with repeated flex cycling and tight radii. One way to mitigate this is to only use single-layer flex circuits, in which case the copper resides at the center of the median bend radius and therefore the film substrate and coverlay are in the greatest compression and tension, as shown in Figure 4.1. Since the Polyimide is very elastic this is not a problem, and will last much longer under repeated movement than multiple copper layers will.
Along the same lines, having multiple separate flex circuits is often necessary, but it’s best to avoid having bends at overlapping sections where the length of the flex sections limits the bend radius.
There are times when you need to consider using strengtheners where the flex circuit exits the rigid board. Adding a bead of epoxy, acrylic or hot-melt will help improve the longevity of the assembly. But dispensing these liquids and curing them can add laborious steps to the production process, increasing cost. As always with PCB design, there are trade-offs.
Automated fluid dispensing can be used, but you need to be really careful to collaborate with the assembly engineers to make sure you don’t end up with globs of glue dripping under the assembly. In some instances the glue must be applied by hand which adds time and cost. Either way, you need to provide clear documentation for the fabrication and assembly folks.
Stiffeners & Terminations
Extreme ends of flex circuits typically terminate to a connector if not to the main rigid board assembly. In these cases, the termination can have a stiffener applied (more thick Polyimide with adhesive, or FR-4). Generally then, it’s convenient to leave the ends of the flex embedded within the rigid-flex sections as well.
The rigid flex circuit stays together in it’s panel for the assembly process, so components can be placed and soldered on to the rigid terminations. Some products require components to be mounted also on flex in some areas, in which case the panel has to be put together with additional rigid areas to support the flex during assembly. These areas are not adhered to the flex and are routed out with a controlled-depth router bit (with “mouse-bites”) and finally punched out by hand after assembly.