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For nearly all applications, customers continue to demand smaller, lighter, and more cost-effective products. Competitive pressures also force designers to bring these new products to market at an ever-increasing rate. Designers can deploy flexible PCB materials (flex/rigid-flex) to meet challenging form-factor requirements, eliminate connectors, and improve performance.
Fabricators are ready for your design’s advanced requirements that have driven components onto the flex substrate as well as required multi-layer flex to shrink in size and improve in terms of high-speed performance.
To minimize wasted time and money, it is imperative to collaborate early with your fabricators to establish a mutual understanding of capabilities, materials, and documentation expectations for your rigid-flex PCB. Design standard IPC-2223C, “Sectional Design Standard for Flexible Printed Boards,” provides information on adhesive material selection and adhesive placement relative to plated through-holes and vias. (See the link at the end of this blog post for more information.)
To improve first-pass success and minimize iterations, designers should include many more design rules to ensure a “correct-by-construction” manufacturing hand-off and final execution. These design rules include inter-layer checks between conductive and non-conductive layers on flex and rigid-flex designs. New tools are available now to make these tasks more automatic and encourage adoption earlier in the process.
As always, close coordination with mechanical design (MCAD) will minimize the unexpected problems during the packaging phase of the project. The team can confidently enjoy the flexibility of rigid-flex designs (pun intended) when new rules are included to bridge the MCAD-ECAD domains. Since reliability is key, design rules are typically focused on the degradation of the system in the transition zone and on the flexible substrate. Rules include: minimum bend radius, avoid placing vias in bend areas or transition zones, avoid placing component pads too close to the bend area, and, finally, avoid placing stiffeners that can interfere with the bend radius and are too close to vias and pins.
We redesigned the Allegro cross-section editor to accommodate many new rigid-flex features with different stack-ups for different technologies. You can now define the complete stack-up inclusive of conductor and non-conductor layers such as soldermask, coverlay, stiffener, and adhesives. You can create, edit, and manage physical zones and assign any stack-up to any zone, including constraint regions and rooms (unbending parts of the flex where vias are allowed). As your design evolves, you can move a part from the rigid area to the flex area and Allegro dynamic zone-aware placement automatically transitions the components to the internal database layer representing the flex outer layer. Previously, this was done with workarounds involving pad edits or use of embedded component technology.
In typical rigid-flex designs, the creation of various masks, bend areas, and stiffeners require special clearances or overlaps of materials and spacing. For this purpose, we have introduced a new inter-layer checks spreadsheet with a configurable matrix of custom DRC rules to ensure you meet the requirements for rigid-flex designs. This spreadsheet provides an actual view of what is being built and allows designers to perform more accurate DRC checks, receive better feedback, and provide better data to the CAD CAM tools for fabrication. Since there are many different materials and rules a PCB designer needs to deal with, it should be easy to enable and specify rules for the combination of layers. With our simple process, you can select two layers, define the DRC type and value, and assign a special DRC error code for easy identification on the canvas.
Flexible circuits frequently have complex routing paths to match the unique capabilities of this technology. With Allegro arc-aware routing, designers can easily route a bus while contouring the complex board outline as well as push and shove traces to match changing requirements.
Rigid-flex designs are unique when transferring the design data to the fabrication process. The various build-up of materials that make up the final product must be clearly defined, especially for impedance control or complex flex/rigid-flex designs. To confidently communicate build intent, PCB designers now use IPC-2581 to exchange stack-up data electronically. IPC-2581 is an open, intelligent, neutral design data exchange format well supported by PCB design and fabricators worldwide. IPC-2581 revision B now supports bi-directional exchange of stack-up data to eliminate discovery of problems late in the design hand-off cycle.
As always your comments are welcome!
How have you used rigid-flex in your designs? What are your top three tips to share? Please sign-in above and start contributing to the community discussion.
We will be posting new reasons to "Move Up to Allegro 17.2-2016" shortly! Please remember to use the "Subscriptions" box at the top of this page to subscribe to this "PCB Design" blog to get updates.
Check out this Allegro rigid-flex design demo video:
10 Top Reasons to Move Up to Allegro 17.2-2016 Release
Cadence Allegro Rigid-Flex Overview on Cadence.com
IPC-2223C, Sectional Design Standard for Flexible Printed Boards (ISBN 978-1-61193-026-9)
IDX News from Printed Circuit Design & Fab Magazine
IPC-2581 Consortium Website