What is Rigid-Flex PCB?
Rigid Flex PCB are printed circuit boards highlighted by both rigid and flexible areas that make them ideally suited for a wide range of applications. The typical rigid-flex PCB circuit includes two or more conductive layers that comprise either flexible or rigid insulation material between each one - the outer layers may have either exposed pads or covers. Conductors are found on the rigid layers, while plated through-holes are found in both the rigid and flexible layers.
Rigid-flex PCB is a complicated product that demands a lot of interaction between the PCB provider and the customers. Like other complex products, early discussions between RayMing Group and the designer is necessary to optimize the design for manufacturability and to optimize costs.
Rigid flex circuits have been used in the military and aerospace industries for more than 20 years. In most rigid flex circuit boards, the circuitry consists of multiple flexible circuit inner layers selectively attached together using an epoxy pre-preg bonding film, similar to a multilayer flexible circuit. However, a Multilayer rigid flex circuit incorporates a board externally, internally or both as needed to accomplish the design.
Flexible circuits are normally divided into 2 usage classes: static flexible circuits, and dynamic flexible circuits. Static flexible circuits (also referred to as use A) are those that undergo minimal flexing, typically during assembly and service. Dynamic flexible circuits ( also referred to as use B) are those that are designed for frequent flexing; such as a disk drive head, a printer head, or as part of the hinge in a laptop screen. This distinction is important as it affects both the material selection and the construction methodology. There is a number of layer stack-up configurations that can be fabricated as rigid-flex, each with their own electrical, physical and cost advantages.
About Rigid-Flex PCB Design
Rigid flex circuits combine the best of both rigid boards and flexible circuits integrated together into one circuit. The two-in-one circuit is interconnected through plated thru holes. Rigid flex circuits provide higher component density and better quality control. Designs are rigid where extra support is needed and flexible around corners and areas requiring extra space.
Designing a flex or rigid-flex circuit is very much an electromechanical process. Designing any PCB is a 3 dimensional design process, but for a flex or rigid-flex design the 3 dimensional requirements are much more important. Why, because the rigid-flex PCB may attach to multiple surfaces within the product enclosure, and this attachment will probably happen as part of the product assembly process. To ensure that all sections of the finished board fit in their folded location within the enclosure, it is strongly recommended that a mechanical mock up (also known as a paper doll cut out) is created. This process must be as accurate and realistic as possible with all possible mechanical and hardware elements included, and both the assembly-time phase and the finished assembly must be carefully analyzed.
The rigid-flex PCB manufacturing process is time consuming and laborious when compared to traditional rigid board fabrication. It involves several steps that must be carried out with extreme accuracy. Mishandling or misplacing any of the flexible components in the board affects the efficiency and durability of the final assembly substantially.
What are the steps involved in rigid-flex manufacturing?
Rigid-Flex Manufacturing – Steps
Rigid-flex circuit board manufacturers assemble the boards by following the steps listed below.
1. Preparing the Base Material – The first step involved in the board fabrication is preparing/cleaning the laminate. The laminate, which contains copper layer – with adhesive or adhesiveless coating – must be cleaned thoroughly before processing with other fabrication processes. This pre-cleaning is important because, copper coils are normally offered by vendors with anti-tarnish features to provide oxidation protection. However, this coating poses a hindrance to rigid-flex PCB manufacturing, hence must be removed.
To remove the coating, PCB manufacturers commonly perform the following steps.
I) First, the copper coil is completely immersed in an acid solution or exposed to an acid spray.
II) The copper coil is then micro-etched by treating with sodium persulphate.
III) Finally, the coil is coated comprehensively using appropriate types of oxidation agents to prevent adhesion and oxidation.
2. Circuit Pattern Generation – Generating circuit patterns is the next step followed by the laminate preparation. Nowadays, this circuit pattern exposure is done using two main techniques, such as:
• Screen Printing – This technique is popular as it can generate the required circuit patterns/deposits directly onto the surface of the laminate. The total thickness is not more than 4–50 microns.
• Photo Imaging – Photo imaging is the oldest, but still the most popularly used technique for depicting the circuit traces on the laminate. In this method, a dry photoresist film consisting of the desired circuitry is placed in close contact with the laminate. This assembly is then exposed to UV light, which helps transfer the pattern from the photomask to the laminate. The film is then chemically removed, leaving behind the laminate with the desired circuit pattern.
3. Etch the Circuit Pattern – Following the circuit pattern generation, next is etching the copper laminate containing the circuit pattern. Rigid-flex manufacturers either dip the laminate in an etch bath, or it is sprayed with an etchant solution. Both sides of the lamination are etched simultaneously to achieve the desired results.
4. Drilling Processes – Now, the time is for drilling required number of holes, pads, and vias. High speed drilling tools are used to make precision holes. To create ultra-small holes, rigid-flex circuit boards manufacturers use laser drilling techniques. Usually, Excimer YAG, and CO2 lasers are used to drill small and medium holes in the substrate.
5. Through-hole Plating – This is one of the crucial steps in rigid-flex PCB manufacturing process that must be carried out with extreme precision and care. After holes with required specifications are drilled in, they are deposited with copper, and chemically plated. This is done to form layer to layer electrical interconnection.
6. Apply Cover lay or Covercoat – It is crucial to protect the top and bottom side of the flex circuit by applying a cover lay. This is done to provide comprehensive protection to the circuit from aggressive weather conditions, harsh chemicals, and solvents. In most cases, manufacturers use a polyimide film with adhesive as a cover lay material. Cover lay material is imprinted onto the surface using screen printing, which is then cured with UV exposure. In order to ensure proper adhesion of the cover lay material on to the substrate, cover lays are laminated under specified limits of heat and pressure. Unlike the cover lay material, which is a laminated film, covercoat is a material that is literally applied onto the surface of the substrate. The decision regarding the type of coating must be made after considering the manufacturing methods, materials used, and the application areas. Both cover lay and covercoat augment the electrical integrity of the entire assembly.
7. Cutting out the Flex – Blanking or cutting the individual flex board from the production panel is yet another important step that must be executed with caution. When producing rigid-flex PCBs in high volume, manufacturers usually choose the hydraulic punching method. However, the same is not chosen for prototyping or small production runs due to the high tooling cost involved. When creating prototype rigid-flex PCBs in small production runs, a specialized blanking knife is used.
8. Electrical Testing and Verification – The last and final stage in rigid-flex circuit boards manufacturing is testing and verification. The boards undergo stringent electrical testing for continuity, isolation, circuit performance, and quality against the design specifications. Several kinds of testing methods are used, including grid and flying probe test methods.
Rigid-flex PCBs are vastly replacing traditional rigid PCB in several applications. Since the quality of the PCBs determines the integrity of the entire electrical assembly, it is imperative that they must be manufactured maintaining a high level of quality. A small design or manufacturing flaw drastically affects the performance, functionality, and durability of the final product. Hence, PCB manufacturers must be extremely careful right from the beginning of the planning, and designing to material selection, manufacturing, and testing. This helps in producing a board that is superior in performance and unmatched in reliability.