Design Considerations with Atypical Rigid Flex Circuit Boards

Asymmetrical Rigid Flex Material Layups:

Asymmetrical rigid-flex board designs are surprisingly common despite the challenges associated with manufacturing these types of boards. They are very difficult to manufacture and may even be impossible to build in some circumstances.

Maintaining balance and symmetry is crucial with rigid-flex printed circuit boards (see IPC 2223 A.8.1), so avoiding asymmetrical designs altogether is best practice. Building out-of-balance rigid-flex circuit boards creates a higher potential for manufacturing issues and points of failure you don’t want to create when you don’t need to. The manufacturing panels and boards themselves will be prone to warp, making both manufacturing and assembly difficult.

Common Examples of Asymmetrical Rigid Flex Circuit Boards

We commonly see two types of asymmetrical designs. The first is where the flex layers aren’t centered with the neutral axis. When manufacturing designs of this nature, the printed wiring boards are prone to warp due to dissimilar dimensional X/Y movement . This defect creates more issues down the road in the manufacturing process, such as accurate drilling or plating. Overall, you can expect to see sharply reduced yield compared to the material used. The best way to prevent these issues is not to design asymmetrical boards outright.

Another common asymmetrical rigid-flex circuit board is when the design features flexible layers on the external layers of the board. Using flex material on exterior layers instead of glass-reinforced componentry presents the same problems as off-center flex layers in that they come with a much higher risk of warp and reduced fabrication and assembly yields as a result. These designs also present more issues regarding the photoimaging and plating processes. The driving force as to why this is an issue is that flexible materials can deform during dry film photoresist lamination. The flexible layers cannot withstand the direct pressure the outer layers are exposed to during manufacturing. The deformation of the flex during the dry film application process assures that the circuits along the hardboard edge will not image or etch faithfully, resulting in dramatically reduced yields, if manufacturable at all.

Depending on the severity of the issue, the design may be entirely non-manufacturable. Regardless, rigid-flex printed circuit boards with flexible outer layers will experience some level of yield loss, which can be as high as 50% or more.

As flex circuit board manufacturers, we understand the value added from accurate design with tolerable yields and manufacturing costs. Give us a call to request a quote or a design consultation: https://www.allflexinc.com/contact-us/

We can help you prevent design flaws from costing you during the manufacturing process and recommend alternative engineering solutions for your rigid-flex circuit boards.

Or, if you need a good starting place for your design, check out our Valu Builds Brochure. The Valu Builds Brochure has very stable, very high yielding, material layup suggestions for four layer up to 10 layer rigid flex designs. They were developed to provide you with the lowest possible cost layups at the highest potential manufacturing yields. You can get your copy here: Flex Circuit Design Guide | All Flex Solutions | Minnesota, USA (allflexinc.com)

Rigid Flex with Flex Layers on External Layers of PCB

A typical rigid-flex board design includes flexible portions that terminate into hardboards. However, some rarer circumstances lead to designs with flexible portions terminating in a flexible board instead. Some designs are this way to accommodate ZIF connectors and through-hole assemblies. We can manufacture around the design in cases like these, where a flex circuit must terminate at a flexible board. However, the extra time and process required reduces yield and increases manufacturing time, so it should only be used when necessary.

Understanding Rigid Flex Board with Flex Arm Manufacturing

Our traditional rigid-flex printed circuit boards (with all flex portions terminating into hardboard circuitry) have high yields and can be manufactured with few issues. When manufacturing flex arms terminating in flexible circuitry, extra steps must be taken to preserve the delicate, flexible arm.

Despite not being included in initial designs, flex arms that don’t terminate into rigid circuitry must be first manufactured with a rigid board in a process called “pouching.” Pouching protects the flexible circuits while the outer layers are processed. If we instead left the flexible portion exposed, the manufacturing process would attack the circuits and pads, causing failure and reduced yield of the internal flexible layer features.

The pouching process involves using extra layers of core material around any exposed flexible arms. The extra material isn’t removed until the entire manufacturing process is complete and just before electrical testing.

The pouching process is necessary for any rigid-flex circuit boards with flex arms. In these circumstances, the drawbacks are less related to manufacturing failure and instead are tied to the additional manufacturing time. Pouches are usually removed by hand, so depending on the order quantity and the number of exposed flex arms per finished board, the time to fulfill orders can increase exponentially. The cost of producing rigid-flex circuit boards with flex arms is not prohibitive, but customers on a time crunch may not be able to receive their orders in time because of the additional hands-on steps.

Further steps are taken to protect the connection points on flex arms. Pouching leaves an edge along the transition points between rigid and flexible components. These edges must be beaded to prevent abrasion of the flex arm by that exposed edge. Without this extra step, it creates an abrasion risk between the rigid board and the flexible arm. Like the pouch removal process, the beading process is also completed by hand. There are some circumstances that require these extra steps, but the pouching and beading process should be avoided in your designs when possible.