Rigid-Flex with flex top / bottom layer? - Page 1
Forum section: electronics / general

author: StefanHamminga
author_rank: Contributor
timestamp: August 05, 2020, 10:25:17 am
content: How common is it to have an 8 layer stackup for a rigid-flex PCB, where the outer layers are (air-gapped) flex?This is for a board of moderate complexity (QFN ICs / USB 3, DisplayPort signalling), not very compact. The flex parts carry USB3 / Thunderbolt. The engineer involved is convinced the stackup should be as above, but the few manufacturers I spoke to say it's impossible to make. Anyone have experience with such a stackup and is it a good / terrible idea (in contrast to a traditional rigid-flex with the flex as middle layers)?

author: SerieZ
author_rank: Regular Contributor
timestamp: August 05, 2020, 10:37:59 am
content: I do not understand how he gets the Idea of having the Flex in the outerlayers? Ask him for his sources maybe.Does he imagine the flex-material pressed on top of the Regular FR-4?Never heard of such a thing...

author: StefanHamminga
author_rank: Contributor
timestamp: August 05, 2020, 10:43:29 am
content: Apparently. I asked him for sample pictures of a ready-made PCB like that and he produced an image with just enough detail to show what indeed looks like two completely black PCBs with two flex bits between, coming from the top and bottom side.

author: SerieZ
author_rank: Regular Contributor
timestamp: August 05, 2020, 11:44:03 am
content: It is already an Issue when you bend Starflex with several flexible Layers that the "inner" layer suffers more strain than the outer when bending. I would Imagine that such a stuckup would make this Issue much worse. I believe it would be probably possible to produce but I would ask your Engineer if there is any reason for such exotic stackup other than it being cool.Edit: Have you also asked we direkt or LeitOn directly via mail if this is possible by them?

author: GerardG
author_rank: Regular Contributor
timestamp: August 05, 2020, 01:08:19 pm
content: Hello Stefan,If it will be a prototype then it can be made in this way. Pictures online will show very beautiful designs, without the price paid to make it.For a prototype I would say it is possible. You make 10 and scrap 8. (and no warranty on the 2)When you want to make a reliable board, you need to work the other way around. Start at the manufacturer. Let them make a stackup.Then make your pcb design and discuss with the series manufacturer if changes are needed.The result will be a reliable board with a good price.Best regards,Gerard

author: StefanHamminga
author_rank: Contributor
timestamp: August 06, 2020, 08:23:34 am
content: QuoteIt's a static assembly and space has been made for recommended bending radiuses (at the generous side), so I'm not too worried about that. Though two flex cables do make assembly more difficult (one is easier to glue in-place).QuoteI did. So far it's been hard to get an answer to that question (besides vague "trust my experience" statements).QuoteThanks for the referral. I asked others (and got one "maybe" and "impossible!" from all others), but not these two. I'll ask them.QuoteHi Gerard! Yes, this is exactly what I normally do for my own designs: Stick to technology available from lots of manufacturers to be sure to have purchasing leverage and supply line failover. But in this case I was brought in to help sort out a process with an existing designer.

author: Day101
author_rank: Contributor
timestamp: August 07, 2020, 11:54:55 am
content: https://www.leiton.de/pcb-technology-overview.html#starr-2Near the bottom there is "Bookbinder technique"Buchbinder-TechnikDie Buchbinder-Technik erlaubt das Einbringen von mehr als 2 flexiblen Innenlagen bei Wahrung einer höchstmöglichen Flexibilität. Die flexiblen Lagen sind hierbei übereinander, aber nicht verklebt/verbunden. Würde man die innenliegenden flexiblen Lagen alle in einem Verbund verpressen, so wäre die Flexibilität deutlich geringer.