Electrical vs. Mechanical Requirements in Flex Rigid and Flex PCB Designs
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A standard requirement for all flexible and rigid-flex circuit board designs is that they all contain both electrical and mechanical requirements. The demands of each of these two distinct areas, when combined in a specific design, can create conflicts that may compromise it. A complete review of all the requirements, and how they interact, is part of the design process that ensures all performance requirements are met reliability. In this webinar, we identify the specific technical items, for both electrical and mechanical portions, that can create design challenges or limitations and the methods used to resolve them. For more information on our flex and rigid-flex PCB solutions, visit https://www.epectec.com/flex.
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Electrical vs. Mechanical Requirements in Flex Rigid and Flex PCB Designs
- 1. Manufacturing That Eliminates Risk & Improves Reliability Electrical vs Mechanical Requirements in Flex & Rigid-Flex PCB Designs 11.18.2022
- 2. Manufacturing That Eliminates Risk & Improves Reliability 2 Agenda Electrical Requirements – Controlled impedance – Controlled impedance configuration: • Surface microstrip & stripline – EMI & RF shielding – Current carrying – Gerber layout/layer count
- 3. Manufacturing That Eliminates Risk & Improves Reliability 3 Agenda Mechanical Requirements – Minimum bend radius requirements – Application: • Static: “bend once to fit” • Dynamic bend motion • Folded flex bends Design Options/Solutions – Integrated vs. multiple-part design – Gerber layout – Material stack-up – Material selection
- 4. Manufacturing That Eliminates Risk & Improves Reliability 4 Introduction The Challenge – Rigid-Flex & Flex create a unique combination of electrical and mechanical requirements in one design. Mechanical & Electrical Requirements – Independent of each other – Often create design conflicts: • Electrical tends to increase flex thickness • Mechanical bends require thin flex constructions Minimum Bend Radius is a Function of Flex Thickness – Common customer statements: • Static: “Only has to bend once” • Dynamic: “Rules are different?”
- 5. Manufacturing That Eliminates Risk & Improves Reliability 5 Electrical Requirements
- 6. Manufacturing That Eliminates Risk & Improves Reliability 6 Necessary for high-speed signal performance Easily accomplished in most designs Requires thicker than standard core layers to achieve impedance values. 120 Ohm diff & 60 Ohm single signals further increases required core thickness 1-layer coplanar available – Requires additional adjacent ground lines on signal layer – Often prohibitive due to added space requirement Controlled Impedance
- 7. Manufacturing That Eliminates Risk & Improves Reliability 7 Controlled Impedance Configuration: Surface Microstrip & Stripline Most Common Configurations – Surface Microstrip: • 2 Layers – Signal layer + 1 reference plane – Stripline: • 3 Layers – Signal layer + 2 reference planes Stripline 67% Thicker Due To: – Added 2nd reference plane – Physics necessitate thicker cores on each side of sandwiched signal layer
- 8. Manufacturing That Eliminates Risk & Improves Reliability 8 Controlled Impedance Configuration: Surface Microstrip & Stripline
- 9. Manufacturing That Eliminates Risk & Improves Reliability 9 Controlled Impedance Configuration: Surface Microstrip & Stripline
- 10. Manufacturing That Eliminates Risk & Improves Reliability 10 EMI & RF Shielding Added “external” shield/ground layers required Increased flex thickness Options – EMI shielding films • Preferred if impedance not required – Copper layers • Required if impedance controlled • 50% thicker than shielding films – Silver ink • Old technology, not recommended
- 11. Manufacturing That Eliminates Risk & Improves Reliability 11 EMI & RF Shielding
- 12. Manufacturing That Eliminates Risk & Improves Reliability 12 EMI & RF Shielding
- 13. Manufacturing That Eliminates Risk & Improves Reliability 13 Current Carrying Requirements Higher Current Levels – Thicker copper required if design does not allow for wider circuits – Copper layers – Thicker coverlay adhesive required • 1 mil. adhesive per OZ of copper thickness required to encapsulate – High voltage • Not a concern in most designs • Polyimide = >6KV per 0.001”
- 14. Manufacturing That Eliminates Risk & Improves Reliability 14 Gerber Layout/Layer Count Design complexity + limited real estate = increased layer count to achieve routing requirements Primary cause of increased flex thickness 3+ layers require additional adhesive layers to bond multiple layers together Example: – 3-layer flex = 40% thicker
- 15. Manufacturing That Eliminates Risk & Improves Reliability 15 Mechanical Requirements
- 16. Manufacturing That Eliminates Risk & Improves Reliability 16 Application: Static or Dynamic Static “Bend to Fit” – Up to 5 layers max. recommended – Limited number of bends: • Up to 10-20X • Enough for install and servicing – Controlled impedance capable – Higher current capable – EMI shielding available
- 17. Manufacturing That Eliminates Risk & Improves Reliability 17 Application: Static or Dynamic Dynamic – Infinite bend cycles – 1-layer construction recommended – 2 layers max. with thin materials – Rolled annealed copper required – Not suitable for: • Controlled impedance • High current – IPC does not define “less than” infinite bend cycle applications • In-house testing required
- 18. Manufacturing That Eliminates Risk & Improves Reliability 18 IPC 2223 Estimation Formula – Sec. 5.2.3.3.1 & .2 – Based on • Max. allowable copper elongation • Material thicknesses Rules of Thumb – Static: • 1 Layer = 6X flex thickness • 2 Layers = 10 -12X • 3+ Layers = 15X+ Minimum Bend Radius Requirements
- 19. Manufacturing That Eliminates Risk & Improves Reliability 19 Minimum Bend Radius Requirements Rules of Thumb (continued) – Dynamic: • 1 layer (preferred): Min. bend = 100x flex thickness • 2 layers (max): Min. bend = 150X flex thickness – Folded Flex (0” bend radius) • Allows direction change in straight design • 1 layer preferred • 2 layers max with thin flex core • Coplanar controlled impedance only • 1 time only bend only • PSA at bend location for fold retention
- 20. Manufacturing That Eliminates Risk & Improves Reliability 20 Design Solutions
- 21. Manufacturing That Eliminates Risk & Improves Reliability 21 Design Solutions: Integrated vs. Multiple Part Designs Excessive Integration Challenges – Increased layer counts – Impedance + current combinations Multiple Part Configuration – Separate “add-on” flex for specific requirements • Impedance, current, etc. – Reduced flex thickness for improved minimum bend capability – Simplified individual parts
- 22. Manufacturing That Eliminates Risk & Improves Reliability 22 Gerber Layout Techniques Thinner Copper – Allows for reduced line widths/spacings & higher density circuit routing • ½ OZ copper: 0.0035” line/space • 1 OZ copper: 0.005” line/space Offset Circuits – Layer-to-layer circuit offset utilizes flexible properties of polyimide Reduced Layer Count – Odd layer counts allowed
- 23. Manufacturing That Eliminates Risk & Improves Reliability 23 Material Stack-Up Options Multi-Layer Flex Areas – Rigid-Flex: • Air gap construction • Flex area configured as 2 “independent” pairs of 2 layers • Min. bend capability based on individual layer set thickness 8-Layer Rigid – 4-Layer Flex (Air Gap)
- 24. Manufacturing That Eliminates Risk & Improves Reliability 24 Material Selection Adhesiveless Flex Cores – Eliminates added thickness of adhesive layers in standard flex materials Thinner Materials – ½ mil. polyimide & coverlays available Copper Type – Rolled annealed – Elongated grain structure – Improved bend capabilities over standard electro deposited copper
- 25. Manufacturing That Eliminates Risk & Improves Reliability 25 Summary Combination of the electrical & mechanical requirements creates most challenges Application will define which solutions are available to resolve conflicts Individual requirements often interact/compound with others Supplier support required at early design stage to: – Identify areas of conflict/challenge – Provide potential solutions – Ensure design is manufacturable – Ensure design will meet design requirements
- 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: