Rigid-Flex Bonding methods - Page 1
Forum section: electronics / projects

author: Teun
author_rank: Regular Contributor
timestamp: March 27, 2025, 08:19:41 am
content: Hi,I'm oriënting on Rigid-Flex pcb's and I was wondering on methods of how Flex pcb's are bonded to Rigid pcb's. I found this link:https://electronics.stackexchange.com/questions/503630/how-to-bond-flex-to-rigid-pcbBut is something like that possible in a mass production process?In most of the documentation on the subject it looks like the flex pcb is always sandwiched between some layers of the Rigid pcb. Is that the main method for mass fabrication?Are there other methods for bonding the two?

author: David Hess
author_rank: Super Contributor
timestamp: March 27, 2025, 05:55:35 pm
content: Solder is mechanically weak so I would not rely on it alone.There are connectors which sandwich the edge of the flexible circuit making a connection.

author: incf
author_rank: Frequent Contributor
timestamp: March 28, 2025, 12:36:29 am
content: Yes, hot bar soldering FFC type assemblies is common.Often the flexible circuit needs a stiffener epoxied onto it.FFC type connectors are other other common way of doing it.While I don't have direct experience, some PCB manufacturers can have the flexible PCB go "inside" of a rigid PCB as an independent copper layer. (sounds expensive!)

author: Kasper
author_rank: Frequent Contributor
timestamp: March 28, 2025, 05:26:02 am
content: I did rigid-flex once at my client's request and it was so expensive that they decided to make room for FPC connectors for the next build.Including connectors on the flex part added complication.I used 2 middle layers of rigid to be top and bottom of flex. Saw that recommended in a few places. The assembler didn't like it though because it needed a fine pitch connector soldered on the end of the flex and since it was a different height, they had to use a stepped paste stencil. They recommended using rigid layer 1 and 2 for flex top and bottom and soldering connector on flex top.From design perspective it was easy. There are a few other parts to consider but mostly you just have 1 layer for rigid outline, 1 layer for flex outline and 1 layer to show stiffener outline.Probably not required but may be helpful to incluse silkscreen print to indicate where stiffener goes. Assuming you want a stiffener.

author: eugene
author_rank: Frequent Contributor
timestamp: March 28, 2025, 04:01:08 pm
content: I've had plenty of rigid-flex boards fabricated, but never asked about the process. I had always assumed (!!) that it was done pretty much the same way as any multilayered PCB or FPC using plated through holes to connect the different layers as needed.FWIW, the fab houses preferred to make boards with the flex sandwiched between two rigid sections.Perhaps apropos, flex circuits are bonded to LCD modules using something called anisotropic conductive film (commonly called ACF.) ACF is a film made of green adhesive (often epoxy) impregnated with many little conductive balls. The film is cut to the correct size and placed between the flex and the LCD module. It is then heated to soften the film and placed under pressure. This brings the flex and LCD close together such that the conductive balls make contact with both. It is anisotropic because it conducts in only one direction: through the film. It does not conduct along the length of the film which would short the multitude of closely space smd pads.The resulting strength of the cured film can be "very good" depending on the adhesive used. The downside is that it is not as easy to implement as it might sound. Each of temperature, pressure, and time need to be controlled well enough that doing it by hand on a hot plate (for example) is likely to result in a lot of scrap.

author: Teun
author_rank: Regular Contributor
timestamp: March 31, 2025, 07:35:44 am
content: Thanks for the info guys.I designed a small rigid-flex pcb for one of our products. The idea is to separate a small USB-C board from the rest of the components to relieve mechanical stress on the solder joints of the USB-C connector.(very rough 3D model)It doesn't have very strict specifications otherwise. So the cheapest solution to join those 2 boards is probably fine. Any thoughts on the best/cheapest way to do this?

author: Kasper
author_rank: Frequent Contributor
timestamp: March 31, 2025, 02:46:51 pm
content: Quote from: Teun on March 31, 2025, 07:35:44 amCheapest is put it all on 1 rigid PCB. USB-C with thru hole shield pins are fairly tough. Make a thread about USB-C mechanical failures if you are concerned. Or build some and do destructive testing.Next cheapest is probably 1 FPC with stiffeners where you want 'rigid'.Which nets are you using from USB? If only VBUS and GND need to connect, could use 2 rigid PCBs and solder wires between them. Or 2 rigid PCBs with connectors for FFC.Rigid-flex is probably the most expensive option.

author: Teun
author_rank: Regular Contributor
timestamp: April 01, 2025, 07:05:23 am
content: We use VBUS, GND, D+, D-, CC1 and VCONN. The USB-C connector needs to be assembled in a plastic part which does not have much room. Meaning that the upper rigid board will be only as big as the white crossed section. Soldering on wires would mean the plastic cover will not sit flat. So that is not an option.

author: Kasper
author_rank: Frequent Contributor
timestamp: April 01, 2025, 03:02:32 pm
content: Are you doing the mechanical design or is it someone else?Often mech designers will push for non-feasible electronics design until they hear the costs of their wild dreams and then they become more reasonable. I find it very helpful to do the mech design myself, then I can chose fair compromise between mech and elec.Depending what D+/- are used for, I would probably not want to hand wire them. Are you running high speed data through them? Does impedance matter?I'm leaning towards FPC with stiffeners where you want rigid.

author: thm_w
author_rank: Super Contributor
timestamp: April 03, 2025, 01:08:01 am
content: You can see how others have done it:https://www.aliexpress.com/item/1005004571866213.htmlhttps://www.aliexpress.com/item/1005003433623794.htmlhttps://www.aliexpress.com/item/1005006998996362.htmlhttps://www.aliexpress.com/item/4000318644284.htmlYou have to be careful about rigidly mounting the USBC connector, usually they give some tolerance to allow it to move around.