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96 PCB007 MAGAZINE I SEPTEMBER 2019 they are flexible, of course, and can resist vibration more effec- tively. One of the most popular features of flexible circuits is that they can be designed into three- dimensional configurations. Rigid-flex circuits combine the best features of flexible and rigid circuits to meet a variety of needs. The rigid areas make it possible to mount sta- tionary components while the flexible areas can be custom configured and serve as protec- tion against vibration. Despite the fact that flex boards can be ex- tremely thin, they are remarkably durable. These circuits are capable of repeating the same bends through millions of cycles with- out interruption. This is a critical point when it comes to applications that face intense vibra- tion and/or acceleration. Challenges With Flex and Rigid-flex Circuits The conductors on flex boards are covered with polyimide. This solution offers more com- plete protection for the circuit than a solder mask. One of the first challenges you are likely to face is the fact that polyimide films are dif- ficult to activate, as they are inert materials. This creates seeding issues with the palladium catalyst. Though they are extremely reliable, getting metallization to adhere and cover poly- imide is an issue that must be overcome. An excellent way to overcome these issues is to either employ a low deposition rate elec- troless copper or one of the direct metaliza- tion systems that are commercially available. Carbon-based systems have been user-friendly and production-proven for difficult to metalize substrates, including flexible circuitry. In par- ticular, graphite-based metalization will easily coat and adhere to a variety of substrate mate- rials. This enables the direct electrodeposition of copper to the conductive pathway provid- ed the very conductive graphite layer. Further, this eliminates adhesion barriers created by electroless copper catalyst (palladium/tin) and the electroless copper deposit itself. Another challenge you may face is the need to adjust your chemical practices if you are re- lying on an adhesive base flex. A schematic of a double-sided flex board is shown in Figure 1. It is common for adhesives—particularly acrylic adhesives—to falter or completely suc- cumb to attacks when exposed to highly alka- line solutions. If your adhesive flex application is impacted by this issue, you may choose to rely on plasma alone instead of alkaline per- manganate. A good example of an attack of the adhesive by strongly alkaline solutions is shown in Figure 2. In a future column, I will present additional process modifications to ensure the optimum metalization for flexible circuitry. Don't let the challenges of flex and rigid-flex circuits pre- vent you from enjoying the benefits of this technology. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 1: Schematic of a double-sided flexible circuit with adhesive. Figure 2: Highly alkaline conditions, causing swelling of the acrylic adhesive.