Designing Flex and Rigid-Flex PCBs to Prevent Failure
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Flex and rigid-flex printed circuit boards (PCBs) can be considered at the basic level some of the most complex PCBs in the industry. With that in mind, it’s incredibly easy to make a mistake, to leave something out, or to create a design that was doomed from the start. Such design failures can end up leading to an eventual failure by delamination, short circuits, damage to the flex portions, and many other things. The easiest way to circumvent these is to start at the beginning, to design with preventing failure in mind rather than trying to fix existing designs to accommodate for problems. In this webinar, we cover how to design flex and rigid-flex PCBs with failure prevention in mind to save time, money, and headaches, and what failure can look like. For more information on our flex and rigid-flex PCB solutions, visit https://www.epectec.com/flex.
Designing Flex and Rigid-Flex PCBs to Prevent Failure
- 1. Manufacturing That Eliminates Risk & Improves Reliability Designing Flex and Rigid-Flex PCBs to Prevent Failure 02.27.2025
- 2. Manufacturing That Eliminates Risk & Improves Reliability 2 Agenda Best design practices for layouts – Preventative measures for failure in drill placements, trace routing, copper fill, etc. Best mechanical practices – Considerations such as bend radius, creasing, and vias in flex – Arrays and handling without causing defects
- 3. Manufacturing That Eliminates Risk & Improves Reliability 3 Agenda Modes of failure and what they look like – Delamination • What does it look like? • What causes it? • How is it prevented? – Shorts and opens • Mechanically caused vs. layout caused • What can prevent them? – Mechanical failure Solutions to mitigate failure-likely issues in existing designs
- 4. Manufacturing That Eliminates Risk & Improves Reliability 4 Introduction What constitutes a failure? – A design is partially or completely inoperable, either by mechanical or functional means such as: • Delamination • Cracked vias • Cracked traces • Unintended creases Does it cost more to prevent failure? – Sometimes. But savings are recouped in time and money by preventing late rebuilds, redesigns, or repairs.
- 5. Manufacturing That Eliminates Risk & Improves Reliability 5 Best Design Practices for Layouts
- 6. Manufacturing That Eliminates Risk & Improves Reliability 6 Best Design Practices for Layouts Avoid sharp turns/curves in traces where a flex or bend is to take place. Drills should be placed at a minimum 0.040” from a rigid-flex interface, but preferably 0.050” or higher. – Dictated by IPC-2223 guidelines, but distance varies by manufacturer. – If drills are placed within this zone, there is a chance that the drill will go through an adhesive layer, which may cause delamination during thermal excursions.
- 7. Manufacturing That Eliminates Risk & Improves Reliability 7 Best Design Practices for Layouts Copper fill should be done when there are large areas devoid of circuitry or features. – Copper fill exists to keep consistent thickness throughout the board during lamination but does not have to be electrically functional. – Without copper fill, some portions of the board may be thinner or thicker, which can lead to delamination in extreme cases, or at least topography issues. – Fill is not limited to a solid pour and can be done as a crosshatch design.
- 8. Manufacturing That Eliminates Risk & Improves Reliability 8 Best Design Practices for Layouts Contrary to popular belief, unused/non-functional pads should be left in designs. – Unused pads add support in drill areas and prevent hole collapse during production. – Many programs default to rigid production requirements, which may remove the unused pads. Following minimum line and space is good, going above it is better. – For 0.5 oz, recommended above 4 mil line/4 mil space.1 oz is 5 mil line/5 mil space, etc.
- 9. Manufacturing That Eliminates Risk & Improves Reliability 9 Best Mechanical Practices
- 10. Manufacturing That Eliminates Risk & Improves Reliability 10 Best Mechanical Practices Avoid creasing or heavily bending flexible boards unless the design requires it. – Creasing is a permanent bend. Once done, it cannot be undone without damaging the circuit. – Going beyond the minimum bend radius can have a similar, albeit not as drastic/immediate effect.
- 11. Manufacturing That Eliminates Risk & Improves Reliability 11 Best Mechanical Practices Avoid bends against transition regions when possible. – Transition regions sometimes contain prepreg squeeze out or rougher cuts, which may damage the flex region. – If needed, use strain relief to allow for a bend away from the region.
- 12. Manufacturing That Eliminates Risk & Improves Reliability 12 Best Mechanical Practices Avoid routing critical vias through bend regions. – While redundant vias or stitching vias can be done for shielding in flex regions, any non-redundant vias risk connection being broken during bending.
- 13. Manufacturing That Eliminates Risk & Improves Reliability 13 Best Mechanical Practices Some issues stem just from array handling alone. – Unintentional bends can occur during assembly or handling, which may damage the boards. – Boards should be handled carefully to avoid extraneous or additional bends.
- 14. Manufacturing That Eliminates Risk & Improves Reliability 14 Best Mechanical Practices Place stiffeners under component regions to avoid solder joints breaking or stress on the circuit. Design with bend radius in mind from the start.
- 15. Manufacturing That Eliminates Risk & Improves Reliability 15 Best Mechanical Practices Static bends – 1-2 layers = 8-10x board thickness – 3+ layers = 12x or more for board thickness Dynamic Bends – Limited to 1- to 2-layer designs per IPC 2223 • 1 layer preferred – Minimum bend radius is 100x the board thickness But always allow more than the minimum to account for error
- 16. Manufacturing That Eliminates Risk & Improves Reliability 16 Modes of Failure
- 17. Manufacturing That Eliminates Risk & Improves Reliability 17 Modes of Failure - Delamination Delamination, or “delam”, commonly defined as a separation between layers that were laminated together, in flex, rigid-flex, or rigid boards. The cause can range among many things: – Insufficient baking prior to assembly – ‘Resin-starvation’ – Insufficient preparation of copper prior to lamination – Mechanical stresses
- 18. Manufacturing That Eliminates Risk & Improves Reliability 18 Modes of Failure - Delamination
- 19. Manufacturing That Eliminates Risk & Improves Reliability 19 Modes of Failure – Delamination Prevention Prevention can be simple: – Always pre-bake immediately before any thermal excursion to remove any traces of moisture. – Ensure proper copper fill throughout a design. – Follow PCB manufacturers’ guidelines or recommendations to avoid resin starvation, such as by increasing prepreg or changing thicknesses.
- 20. Manufacturing That Eliminates Risk & Improves Reliability 20 Modes of Failure – Shorts and Opens Shorts and opens are unfortunately common in the industry due to the numerous causes. Shorts can be caused by many things (too many to list), but most importantly, it’s best to follow minimum spacing requirements to avoid them. Opens operate similarly but can also occur mechanically. – Best practice is to follow minimum bend requirements, trace widths, and to ensure flexible portions of boards are handled properly.
- 21. Manufacturing That Eliminates Risk & Improves Reliability 21 Modes of Failure – Shorts and Opens
- 22. Manufacturing That Eliminates Risk & Improves Reliability 22 Modes of Failure - Mechanical Mechanical failures are by far the most diverse failure mechanism, but can usually fall into these categories: – Creasing – Broken traces – Tearing – Cuts Because these failures are so diverse, they can be hard to plan for, which goes back to proper handling and mechanical application design.
- 23. Manufacturing That Eliminates Risk & Improves Reliability 23 Will Planning for Failure Cost More?
- 24. Manufacturing That Eliminates Risk & Improves Reliability 24 Will Planning for Failure Cost More? Sometimes, but not every time. Some changes are as small as moving vias or curving traces gradually, while others can be more intensive such as redesigning housings to prevent flex circuits from going beyond their minimum bend. The earlier in the process the manufacturer can be involved, the higher the likelihood that the production and assembly goes without issues.
- 25. Manufacturing That Eliminates Risk & Improves Reliability 25 Summary Problems are inevitable in flex and rigid-flex designs, given that they can be more complex than a normal rigid board. While failure is costly, preventing failure is cheap comparatively. Less time is lost, and usually only at a fraction of the cost (if that). Given the nature of mechanical and layout changes, it’s always best to include manufacturing teams in the design from the start. A redesign early is always easier than a remake later.
- 26. Manufacturing That Eliminates Risk & Improves Reliability 26 Our Products Battery Packs Flex & Rigid-Flex PCBs Cable Assemblies Printed Circuit Boards CNC Machining User Interfaces Flexible Heaters EC Fans & Motors
- 27. Manufacturing That Eliminates Risk & Improves Reliability 27 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
- 28. Manufacturing That Eliminates Risk & Improves Reliability 28 Thank You Check out our website at www.epectec.com. For more information email sales@epectec.com. Stay Connected with Epec Engineered Technologies Follow us on our social media sites for continuous technical updates and information: