Defining Stack-Ups for Flex & Rigid-Flex Circuit Boards

The question of complexity with rigid-flex and flexible circuits is never an easy one. Just in the concept alone, rigid-flex and flexible PCB designs pose challenges, whether it’s due to the thin nature of the flexible areas, the larger process to manufacture, or the different materials. But to add to this complexity, the process and difficulty can change due to a variety of factors.

Factors such as layer counts, air gaps, asymmetrical constructions, ZIF connectors, shielding, blind and buried vias, and many other things can factor into a stack-up, turning what was already a difficult construction into something many board manufacturers might not even consider building. While these may all seem overwhelming, tackling all of these one at a time makes them that much more manageable, allowing us to build what was a complex board one piece at a time.

In this webinar, we cover stack-ups, ranging from the most basic 1-layer flexible PCB design, all the way up to the more daunting 14-layer rigid-flex PCB stack-up with airgaps, showcasing the various items that can go into a stack-up, and what can be made possible within the world of flexible circuits.

For more information on our flex and rigid-flex PCB solutions, visit https://www.epectec.com/flex.

1. Defining Stack-Ups for Flex & Rigid-Flex Circuit Boards 08.15.2024

2. Agenda Overview of Stack-Ups The Different Materials Used in Stack-Ups and Their Importance What Materials are Standard and Non-Standard Examples of Stack-Ups Flex Stack-Ups When is it too many layers to flex? Overview of bend radii Rigid-Flex Stack-Ups Flex to rigid transition zones

3. Agenda Specialized Constructions: What Can Be Done? Airgap constructions Asymmetric constructions ZIF connectors Blind/buried vias What Can’t Be Done? Bookbinding Different rigid section thicknesses How Your Stack-Up Affects the Overall Cost

4. Introduction Why is Defining a Stack-Up Important? Defines Requirements for the Design Helps to Demonstrate the Purpose of the Design Shielding would indicate signal integrity being a concern Stiffeners can indicate ZIF connectors or component areas They Can Help or Hinder a Design

5. Overview of Stack-Ups

6. Overview of Stack-Ups Stack-ups are typically shown as a side-viewed cross-section, a shrunk-down two-dimensional representation of the entire board Typically done in drawings, but can also be done via Gerber files, Excel spreadsheets, etc.

7. Materials in Stack-Ups Standard Materials Typically Consist of FR4 or Polyimide (PI) for Rigid Materials Specialized materials are on a case-by-case basis and require order lead times Only Polyimide is Supported for Flexible Cores and Coverlay Available in adhesive-based and adhesive-less Coverlay is available in black, white, and amber (standard) Epoxy and Acrylic are the Only Materials for Adhesives Some manufacturers support polyimide-based adhesives, but require specialized equipment, and come at a higher premium

8. Materials in Stack-Ups More Specialized Materials are Available, Depending on Use Cases: PSAs (Pressure-Sensitive Adhesives) Available in standard and assembly temperature-resistant varieties Shielding Films Stiffeners Beyond the Standard PI and FR4 Stainless steel Aluminum

9. Flex and Rigid-Flex Stack-Ups

10. Flex PCB Stack-Ups Flex PCB Stack-Ups Typically Range from 1 Layer All the Way up to 6 Layers While Flex Boards Tend to Remain Flexible, This Flexibility Can Greatly Diminish Due to Various Reasons: Copper thickness increases Dielectric thickness increases Layer count increases

11. Flex Stack-Ups Bend Radius The maximum amount a board can be bent, either once or multiple times, before permanent deformation occurs, defined typically by thickness of the board and the layer count Defined by either static bends or dynamic bends Bend Radii are Defined by IPC-2223 as Guidelines

12. Flex Stack-Ups Static Bends 1-2 layers = 8-10x board thickness 3+ layers = 12x or more for board thickness Example: For the below 3-layer design, total thickness is approximately 254um or ~10 mils (0.010”). For a static bend, this means the minimum bend radius is approximately 120 mils (0.120”)

13. Flex Stack-Ups Dynamic Bends Limited to 1- to 2-Layer Designs Per IPC 2223 1 layer preferred Minimum Bend Radius is 100x the Board Thickness Example: A 1-layer design shown below, with total thickness of 93um or ~3.6 mils (0.0036”). This would mean a minimum bend radius of 360mils (0.360”)

14. Rigid-Flex Stack-Ups Rigid-flex stack-ups can have a wide range of layer counts, from 2 layers up to 14 layers and beyond The only limiting factor is the flex layers themselves The minimum bend radius follows similar rules for the flex sections However, the rigid-flex transition zone needs to be considered when laying out the design, ensuring that no vias are within that 0.050” distance to the rigid-flex transition zone

15. Rigid-Flex Stack-Ups

16. Specialized Constructions

17. Specialized Constructions Airgap constructions, while technically a specialized construction, are among the easiest of the constructions Stack-ups are typically defined by showing a space within the flex section indicating where the airgap lies. Some constructions will label the space “airgap” or “air” for clarity

18. Specialized Constructions Asymmetric Constructions Typically defined as any construction with an odd layer count in the rigid or flex sections of the stack-up Available for both flex and rigid-flex, with rigid-flex having it available for the flexible region or rigid region

19. Specialized Constructions

20. Specialized Constructions ZIF Connectors – Connectors Require “Zero Insertion Force” (ZIF) Typically controlled to tight tolerances for thickness, trace widths, etc. Available for both rigid-flex and flex designs Most ZIF connectors are controlled to 300um thickness, with select others at 200um

21. Specialized Constructions Multiple Rigid Section Thicknesses A very complicated form of construction Pricing is equivalent to multiple boards since construction is essentially building two rigid-flex boards Not available with many manufacturers

22. Specialized Constructions Bookbinding constructions, also known as constructions with different lengths of flex sections Used in scenarios where a board is meant to bend in one direction and aids in that bend The process of creating the board is highly specialized and complex Not available with many manufacturers

23. Specialized Constructions Blind and Buried Vias An incredibly common form of vias in rigid-flex and flex designs Can be done in flex sections only in a rigid-flex, or purely in rigid sections in a rigid-flex The main limitation is the aspect ratio, or the size of the via in relation to the depth it travels Another common limitation is the number of sequential laminations is limited

24. Pricing and the Effects of the Technology

25. Flex Pricing Cheapest Would be the Simplest, or a 1-Layer Flex Design Price goes up from there Dual access is roughly equivalent to 2-layer designs Non-Standard Options Increase the Price White or black coverlay Shielding Multiple stiffener thicknesses or stiffeners that aren’t PI or FR4 Higher copper weights

26. Rigid-Flex Pricing Pricing is typically greater than that of a flex combined with a rigid board that it would be replacing Pricing goes up with more technology like blind/buried vias, different coverlay colors, higher layer counts, air-gap constructions, etc. The best way to decrease price is the same as rigid, flex, and any other board: reducing board size and layer count or increasing lead times

27. Summary

28. Summary Stack-ups are the best way to define requirements for a build and can tell the story of what the board is being used for Bend radius is easy to define but can be difficult to follow. It becomes a balancing act of layer count and thickness to the required bend radius The more complicated the build, the higher the price, and the longer the lead time

29. Our Products Battery Packs Flex & Rigid-Flex PCBs Cable Assemblies Printed Circuit Boards CNC Machining User Interfaces Flexible Heaters EC Fans & Motors

30. Q&A Questions? Enter any questions you may have in the control panel If we don’t have time to get to it, we will reply via email

31. Thank You Check out our website at www.epectec.com . For more information email [email protected] . Stay Connected with Epec Engineered Technologies Follow us on our social media sites for continuous technical updates and information: