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Fresh PCB Concepts: Designing for Success at the Rigid-flex Transition Area 

Rigid-flex PCBs come in all shapes and sizes. Manufacturers typically use fire-retardant, grade 4 (FR-4) materials in the rigid section and flexible polyimide materials in the flex region. Because of the small size, some rigid-flex PCBs, like those for hearing aid devices, are among the most challenging to manufacture. However, regardless of its size, we should not neglect the transition area between the rigid and flexible material. The transition area will experience both mechanical and thermal stresses throughout the life of the product. The stress comes from manufacturing and assembly processes, and during normal field use. If we encroach on this area, we should be aware of the potential risks. IPC-6013 defines the transition area of a rigid-flex PCB as a 3mm area, centered about the axis of the rigid edge of the PCB. (See Figure 1). 

When manufacturing rigid-flex PCBs, the trip around the factory floor is slightly different. After laminating the flexible layers, all materials then undergo a final press cycle to bond the rigid and flexible materials together. Before the second press cycle, spacers are placed into the stack to fill the void and prevent resin flow into the flexible area. When the boards are routed the flex area is opened by a milling process that will allow the spacers to be removed, and the flexible area is exposed. 

Figure 1 also shows a list of acceptable imperfections permissible in the transition area. Placing any functional features within the transition area could be detrimental to the function of the rigid-flex because of these exceptions.

• Crazing and haloing occur when there is mechanical, thermal, or chemical stress causing separation in the glass fibers of FR-4 materials. As a result, this weakens the materials and could allow moisture to ingress. The dielectric strength may also decline. However, only mechanical stress causes haloing.
• Lamination voids in the transition area are acceptable. As previously mentioned, a typical rigid-flex PCB is made from FR-4 materials and polyimide. The pre-impregnated (pre-preg) bonding material for a typical rigid board is designed so the resin will flow and fill in voids between copper features. This provides the best bonding because it prevents air from becoming trapped between the inner layers and dielectrics. Materials for a rigid-flex PCB are slightly different. The FR-4 pre-preg has “low-flow” or “no-flow” properties that prevent the resin from flowing into the flexible region of the rigid-flex PCB. As a result, lamination voids can occur in the transition area.
• Squeezout. Although PCB factories use no-flow pre-preg to manufacture rigid-flex PCBs, sometimes the bonding material resin flows beyond the rigid edge in the transition zone. For a one-time flex-to-install, this may not be a problem. However, for a dynamic-flex application, damage could occur to the flexible polyimide material from the sharp edge of the cured resin. 
• Protruding rigid dielectric. As mentioned, a milling operation opens the flexible area. Sometimes, lamination voids and resin protruding from the edge of the rigid section can cause the rigid dielectric to protrude slightly when the flex area pocket is milled open. This does not cause performance problems, so the imperfection is considered acceptable in adherence with IPC-6013.
• Copper deformation. Copper features may easily become deformed because of the material instability in this area and are similarly at risk. More specifically, cracks in the copper may develop, or delamination may occur. This is due to the resin starvation that copper could cause by blocking the area. Additionally, the layer-to-layer registration could suffer.
• Coverlay protruding into the transition zone: Placing coverlay material in the transition area can also cause problems. Coverlay was designed to not adhere to FR-4 materials. When coverlay extends into the transition area, the poor adhesion may cause delamination. 

How far could features extend into the transition area before the product performs poorly, or the material becomes overstressed? The short answer is not far at all. As Figure 2 depicts, it is possible to design and manufacture features in the transition area, as well as into the flex region. This is not traditional practice, which is why consulting with your PCB supplier is critical. As I have advised in previous articles, your PCB supplier’s engineering team will know the exact minimum values that can be applied to the technology. Therefore, they will be prepared to share how much of the transition area is usable without causing problems later in the product life cycle. The values given will also be based on the PCB supplier’s capabilities and specifications.

Consider a rigid-flex PCB with intentionally extended copper into the transition area. Who will accept the responsibility for the reduced transition area and potential field failures down the line? Some PCB suppliers with advanced manufacturing capabilities may readily accept the risk. Otherwise, the liability for this decision will rest with the customer. 

In many circumstances, it is challenging for PCB suppliers to produce all the latest technology that a rigid-flex board may demand. However, some can readily produce this technology with a reduced transition area. We always recommend an abundance of caution and strict quality tests in these cases. In addition, we have seen more factories working to advance capabilities to produce smaller rigid-flex PCBs and expect to observe an uptick in prioritization of this topic soon.

In Figure 2, the recommended values in the left column are the measurements that should be used when little to no risk is acceptable. If the values in the advanced column are used, the utmost care should be taken. 

With the phasing out of old circuit technology, new solutions and production methods emerge to meet the growing needs of today’s applications. Miniaturization forces rigid-flex PCB engineers to push manufacturing capabilities to their limits and beyond. 

Although many things can go wrong in the rigid-flex transition area, it does not mean this area cannot be encroached. However, pushing the limits of this area will not result in a revolutionary design, either. I always encourage revising a PCB design to be as innovative and bleeding-edge as possible, but only to fit the application’s needs, and only if all parties involved are fully aware of the potential risks involved.

Following best practices at the rigid-flex transition zone and working closely with your PCB supplier will help avoid costly prototype scrapping or ultimate field failure.

Ryan Miller is a field applications engineer with NCAB Group.