It’s true rigid boards are quicker to make – up to a point.
Q: I just ordered two boards, one a 12-layer rigid PCB and the other a 12-layer rigid-flex. The lead time on the rigid-flex was much longer. Why is that?
A: As always, once the design is done, the program is always anxious to get parts. Often, customers are surprised when the rigid-flex lead time is longer than expected. There are a number of reasons rigid-flex circuits have a longer lead time. We will consider a “standard” rigid-flex (if there is such a thing). For this example, the rigid-flex will have drilled holes only in the rigid zone and no exposed etched features on the flexible zone. While these other options are possible and common, they add additional process time.
First, there is the tooling. While a rigid PCB uses standard tooling holes and panel layouts, rigid-flex typically has a custom tooling scheme. Rigid-flex also requires extra tooling to create the flex portion of the circuit. Prepreg windows need to be sized and tooled, as do the flexible coverlayers and bond plies. Extra attention is paid to placement of breakoff tabs if parts are to be delivered in arrays, in order to properly support the parts during assembly while also enabling removal of the parts from an array without damage. This means extra time to prepare the tooling for manufacturing a rigid-flex.
Once manufacturing begins, the number of process steps involved can be up to double that of a rigid board. In addition, certain process steps take additional time when handling flex materials.
Rigid-flex is sequentially laminated. Once all the innerlayers are etched and ready, coverlayers must be laminated to the flex layers first. Then, depending on the design, the various flex layers may be laminated together as one fully bonded item, or laminated with air gaps between certain layers to manage flexibility and degrees of freedom for the various flexible zones. It is only after this that the rigid and flex layers are joined.
Behind the scenes, a number of prep steps take place to make this circuit have both rigid and flexible zones. Flexible coverlayer and bond ply materials are not recommended to extend into the rigid plated through-hole area. This is because the flexible adhesives are soft and expand rapidly during high-temperature operations such as assembly soldering. To avoid this, the materials are drilled and then routed or cut in pieces that can be laid up to the panel so the material overlaps the rigid areas by 0.020" to 0.100". After preparing the materials, they must then be applied to the panel. This is a time-consuming, labor-intensive process.
A mirror image process of sorts also needs to take place with the prepreg that will be used to bond everything together in the rigid zones. The prepreg must be removed from the flexible zones so the board is not bonded together in these regions during lamination. After tooling holes are drilled in the adhesive, windows get routed in the prepreg at each flexible region.
Now that the flex layers have been processed and the prepreg is ready, we can lay up and laminate everything together. Extra time is required at layup to properly align the prepreg, and to make sure no prepreg debris gets in the flex zone.
After lamination, operations from drilling through etch are parallel to that of a standard rigid board. However, extra care is taken to ensure there is no encroachment of process chemistry in the flex region of the panel, which can damage the panel, as well as the manufacturer’s wet process.
Once the final outline of the part is routed, additional work is needed to remove any rigid or excess materials over the flex regions. Those materials were in place to protect the flex regions during the previous processes, but now must be removed to create the rigid-flex function as designed. Again, this is a manual operation that takes time to complete.
Finally, many rigid-flex designs will incorporate a strain relief fillet along the rigid-to-flex transition. This is used to manage stress and strain at the flex when the part is bent or folded. This material is applied as a liquid bead, which must be cured. It is similar to applying caulk around a sink or bathtub. The cure cycle is long, usually at least one or two production shifts.
What can be done to manage this lead-time challenge? Work with the supplier to lock down circuit construction strategy and materials while artwork is being finalized. Once the stackup and materials are defined, some flex materials may have lead times longer than those of rigid laminates. Offset this by having an agreement or material purchase order with the supplier that permits it to order material while the design is finalized.
Working in parallel with the supplier also helps shorten the learning curve and reduce startup questions. The CAM department can be aware of the build plan and be ready to tool once the design is done, rather than having to interpret the procurement package and understand the design intent prior to beginning the tooling process.
In summary, rigid-flex designs take longer because they require extra operations and longer cycle times for certain operations. However, rigid-flex designs will usually help shorten assembly cycle time, which can be a significant offset. Budgeting for extra manufacturing lead time, and working in parallel with the supplier when incorporating a rigid-flex, will keep the project on schedule.
is senior field applications engineer at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee;
Ed.: PCD&F welcomes Nick to as an contributing editor. His column will run regularly this year.