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Material and Methods for High Quality Rigid-Flex PCBs

Material and Methods for High Quality Rigid-Flex PCBs

Contrary to the construction of standard PCBs with a metal or fiberglass base, flex PCBs consist of a flexible polymer core and a Polyimide film as a substrate. The advantage of Polyimide is it does not soften when heated, and stays flexible after the initial thermosetting. Unlike several types of thermosetting resins that become rigid after being heated, Polyimide remains flexible, and that makes Polyimide a superior material in flex and rigid-flex PCB construction. RUSH PCB UK Ltd uses upgraded Polyimide film that has good resistance to humidity and tearing.

Rigid-flex PCBs basically connect flex PCB materials to rigid PCB materials. This allows the PCB to bend in certain areas, and to stay rigid in others. Therefore, the board remains strong, but flexible at the same time. When designers want to transfer signals between the rigid and flex parts, they need to design the rigid-flex PCB. While the flexible part of the board resembles a regular flex circuit, the rigid sections may use materials that standard rigid PCBs use, such as fiberglass.

According to RUSH PCB UK Ltd, both the manufacturer and the OEM benefit when they involve in the conceptual design of Printed Circuit Boards (PCBs). Specifically, for flex and rigid-flex PCBs, it is necessary for both to understand the full project requirements and implications up front.

The Concept Design Phase

The project usually starts with discussions between the two engineering teams to develop a broad understanding on several factors such as:

  • Functionality of the rigid-flex PCB
  • Objectives of the project related to the rigid-flex PCB
  • Will the PCB have active components?
  • Number of interconnects on the PCB
  • Requirement of special signal capabilities such as impedance control, high current carrying traces, and other factors such as RF design and or shielding and protection requirements, operating temperature requirements, and similar
  • Shape and size of the PCB

Answers to above questions help to clarify the requirements, based on which, the manufacturer can then give their inputs to the OEM regarding rigid-flex technology best suited to their needs. This mostly relates to the material and methods for high quality rigid-flex PCBs. The first consideration is to decide whether the application really needs a flex-based solution.

While identifying the additional levels of functionality, it is also necessary to look into higher levels of integration between parts. By simplifying complex levels of integrations, it may be possible to reduce the cost of the PCB project.

Completing the design concept phase and determining that the application does require a flex and rigid-flex PCB, the designers must delve into deeper specific details of the design.

Specific Detail Requirements

Although there may be several detail requirements specific to a particular project, those most common to all designs are:

  • Minimum and maximum bend requirements
  • Will the bend be permanent or will it flex periodically?
  • Length of flex between bends
  • Requirement of stiffeners and type of stiffeners necessary

Based on the requirements in the concept design and specific details the manufacturer can suggest various suitable materials and methods for the high-quality flex PCB capable of meeting the quality and life requirements of the OEM.

 

Also Read: Five Reasons Why RushPCB is the Leading LED Board Manufacturer in UK

 

Materials and Methods of High-Quality Flex PCBs

In the last decade or so, RUSH PCB UK Ltd has taken the rigid-flex circuit design and fabrication to significant levels of evolvement. For instance, the rigid areas of our rigid-flex designs are capable of the same complexity and density as that of our HDI boards. For instance, just as for our HDI boards, the rigid areas of our rigid-flex designs can have the same fine lines/spacing, high operating temperatures, high layer counts, and compliance to RoHS standards.

In earlier methods of fabrication, manufacturers used several layers of adhesives while fabricating the rigid areas. However, the high coefficient of thermal expansion of adhesives led to a significant amount of stress on vias during thermal cycles that the boards undergo during the assembly cycles, and during operation. As a result, vias placed within the rigid areas would develop cracks in the copper plating.

The adhesives in the rigid-flex system may come from the copper clad flex laminate, the coverlay, and the material that bonds the rigid and flex layers. For solving the issue of reliability of vias, RUSH PCB UK Ltd has made necessary changes in the materials and methods of construction to eliminate or minimize the use of adhesives.

Adhesive-less Construction

Where earlier manufacturers used layers of copper bonded to the polyimide core with some acrylic type of adhesive, RUSH PCB UK Ltd uses adhesive-less laminates where the copper bonds directly to the polyimide core, eliminating the adhesive bond layers. Not only does this technique allow thinner PCB construction, it also allows for higher flexibility and vastly superior reliability.

Adhesive-less copper clad laminates have further advantages. They can operate at higher temperatures, their copper peel strength is higher, and they exert much reduced stress on vias due to lower Z-axis thermal expansion coefficients.

Similarly, earlier coverlay construction in rigid-flex design used full coverage types that covered the entire rigid area of the PCB. As the coverlay adhesive expanded, it would put vias and other PTH to severe expansion stress. RUSH PCB UK Ltd uses selective coverlay constructions that remain restricted to the exposed flex areas only, while extending to a maximum of 0.05 inches (1.27 mm) into the rigid areas. No via or PTH is placed in this interface area.

RUSH PCB UK Ltd also uses high temperature no-flow FR4 prepregs to laminate the rigid and flex layers together rather than layers of flex adhesive. The structure this technology achieves is dimensionally highly stable. In fact, this stability matches that of standard rigid PCBs.

High quality flex and rigid-flex circuits from RUSH PCB UK Ltd conform to IPC 2223C, the Sectional Design Standard for Flexible Printed Boards. This standard defines the minimization/elimination of the use of adhesives within rigid areas, use of adhesive-less substrates, and the use of partial/selective coverlay construction.

Conclusion

As the demand for portable electronic equipment grows, engineers are increasingly taxed with improving the capabilities of combining functionality with flexibility. This flexibility also takes the form of flexible circuits that fit where no other design solution can help. Along with a significant reduction of interconnects, and a substantially greater freedom of packaging geometry, the integrated hybrid of the rigid PCB and flex circuits allows designers to retain the precision, density, repeatability, and reliability of regular PCBs.

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Productive Approaches to Creating Prototype PCBs

So far, for printed circuit board or PCB prototype, designers limited their choice of PCB design tools to high-end, enterprise-level solutions, as these tools were expensive. The early design tools also added the cost of an extended setup time and learning curve, were limited in their capability, and most often, error-prone. Thankfully, modern tools are now affordable and come packed with all the features designers need for complex designs. Moreover, they are easy to use and focus more on ease of adoption.

Typically, a designer takes about two to three iterations for developing a custom PCB prototype for a working product with a high-speed computer-based design tool. However, with decreasing product life cycles, the time-to-market is steadily gaining in importance. Depending on overheads, board iterations can be expensive, because delaying the product’s market launch and the missed opportunity could cost the company several thousand dollars, or even the total loss of market share.

The above is prompting designers to employ simulation in the design cycle before they order PCB prototypes, as this dramatically reduces the cost of development. As the cost of change increases with development time, design changes occurring early in the design process cost substantially lower compared to those introduced during full-scale PCB fabrication. Using virtual prototyping has the advantage of identifying issues early on in the design process, and rectifying them is cheaper and simpler before they become a major problem.

Virtual Prototyping as a Productive Approach

Although entry-level tools did allow quick designs and prototype building, most designers relied on reference designs provided by chip vendors. Increasingly, designers are finding they cannot rely on reference designs to make their products work in operating environments—they need design for reliability and manufacturability for the real world.

Implementation of each new technology introduces multiple fast rise-time signals propagating at increasingly faster speeds. That does not allow the luxury of building prototypes, testing, reviewing, and revising the design approach with each build.

With virtual prototyping, designers can do with fewer PCB prototypes and improve their design efficiency. Virtual prototyping includes simulation of signal and power integrity, design for manufacturability, thermal analysis, and 3-D interference validation.

Earlier, skills of PCB designers and engineers were necessary for the entry-level tools to detect possible issues as they came up during the design process. High efficiency, complex designs, on the other hand, require a more constraint-driven approach, with correct-by-construction methodology. Once the engineers establish the rules, designers with downstream tools will follow them and use various design rule checks (DRCs) to conform their validation.

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Designing for Profitability

Modern PCB tools are able to handle several tasks other than simply laying traces. It is possible to use these design tools at every phase of the project from initial concept to the final assembly documentation. The addition of 3-D rendering engines in most of these design tools and the complete integration of 3-D component bodies in the footprint libraries makes this possible.

This capability allows designers to give shape to concepts very quickly. They can use virtual prototyping along with vendor-supplied 3-D step models to make a preliminary PCB layout. This allows a quick look at the finished product, such as the position of the I/O connectors, without the detailed board design.

This method helps designers handle request for change early in the design, as the visualization tools allows an upfront view of the design process, and everyone involved has a good idea of the direction the design is headed, and they can spot the misconception or conceptual errors early.

Virtual prototyping has an added side benefit. Every step of the process can confirm the mechanical fit of the product. Moreover, designers can now put together the fab drawing first and get quotes on any proposed design, material, and so on. They can pass the mocked-up fabrication drawing to the assembler for a review to uncover any issues. This not only saves time, but also the unnecessary expense of multiple prototypes. Silk-screeners, pick-and-place machines, and reflow ovens require tabs, holes, and other modifications that are not part of the original circuit design, but part of manufacturability. Assemblers reviewing the preliminary fab drawing often make recommendations based on their experience with materials and others.

Keeping a Dynamic Supply Chain Visibility

A major reason for design iterations is the supply chain information not being available to the designers in real time. Many a time designers have to manufacture printed circuit board prototypes only to have to change their design because of the non-availability or a certain component or components, which they have to replace with a suitable alternative and now requires a change in the PCB design. The cost of having to change the design at the prototype stage is much lower compared to that required once the product is in full production. Therefore, it is necessary to have a real-time view of the supply chain from the product management or procurement team.

Additive Manufacturing Rather than Subtractive

Regular PCB prototype making is a subtractive process. The fabricator starts with more material than needed, and removes the unnecessary parts. However, in an additive process, the fabricator starts with a thin substrate and adds the required copper traces with conductive ink.

Although still in its infancy, 3-D printing and additive manufacturing for electronics is a great way to generate less waste than traditional subtractive methods for PCB fabrication, especially for custom PCB prototypes. With additive manufacturing machinery deployed in-house, engineers can make necessary changes in a design without the traditional penalties in cost and time. The designer does not have to wait for the fab house to send the order PCB prototypes back, as he can create the prototype on his or her desktop.

Designers can select from 3-D printing and 2-D printing processes for additive manufacturing. The 3-D printing prints PCBs from scratch, using a variety of gels, inks, and substrates, layer by layer, manufacturing them at the nanoparticle level. This is a very new process and involves material complexity and extrusion requirements.

The 2-D inkjet-style printing matches more readily with the Gerber files designers generate to communicate designs to manufacturing. These machines only have print the conductive traces on a flat horizontal substrate. This is ideal for printing PCB prototypes for quick prototyping.

Conclusion

There are several ways to reduce the iterations involved with designing, PCB prototyping, and cutting down on the time to market factor. While virtual prototyping can actually save on the time required for ordering, testing, and reviewing prototypes, additive manufacturing can cut down the actual time for fabricating prototypes. Additionally, keeping a clear dynamic supply chain visibility precludes the necessity for redesigning the board at a later stage.