Automatic assembly of a large Flex PCB - Page 1
Forum section: electronics / general

author: Electr0nicus
author_rank: Regular Contributor
timestamp: February 22, 2020, 09:45:42 am
content: HiFor a current project I have designed a flat flex PCB with the size of 436x599mm. On the PCB there are 256 temperature sensors (I2C) in a 2x3 DFN-8 package with thermal pad. After the challenge of finding a PCB manufacturer, who can deliver this PCB in only 10 work days and a PCB Assembly house not far from our company, which can handle this large format, i have a little remainig problem.Maybe one of you has a answer.To automatically pick and place those temperature sensor ICs on the flex PCB, the PCB assembly house needs the flex to be glued on a rigid carrier board made out of 1,55mm FR-4 or even thicker material. Although the PCB manufacturer on the other hand, can supply the carrier boards in the required size and the flex PCB, he says he can't glue the flex PCB onto the carrier board. The PCB assembler says he can't do that either, because that should be the PCB manufacturers businessSo as the whole project is time critical, i could not afford to invest further time in surcing different suppliers, and going trough the lengthy process to negotiate all the process parameters which I need with them.So the idea is to glue the flex PCB myself onto the rigid carrier board at our company. As we do mainly carbon ink printing, we have the necessary machines to laminate materials with a width of up to 1m together. Unfortunately I have no idea what glue or adhesive film is normally used for this application. The adhesive should not tack to much to the flex and the carrier board, as the 2 items should be easily separable after going trough the reflow soldering process.I have thought of using double sided Kapton adhesive tape, as it would easily handle reflow soldering temperatures without degrading. But I have concerns that the kapton tape will stick to much and it then would be impossible to separate the flex from the carrier without causing damage to the assembled flex PCB.So my question is, does anyone know of a adhesive which can be used in this case. Maybe you have used it yourself before.Kind regards,Gregor

author: SMTech
author_rank: Frequent Contributor
timestamp: February 22, 2020, 12:04:45 pm
content: This thread has some pointershttps://smtnet.com/Forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=22277&#Message83611Different story in volume of course.

author: fcb
author_rank: Super Contributor
timestamp: February 22, 2020, 12:15:55 pm
content: PVA glue dissolves in water. No idea if it will survive reflow.

author: GerardG
author_rank: Regular Contributor
timestamp: February 24, 2020, 01:12:49 pm
content: Hello,Without knowing the shape of the board, this is how we did it.Make a border around the flex and laminate only this border with a 3M glue sheet to the FR4.If there are extra waste pieces within the contour it is possible to glue these down as well.When separating you only have to cut the tabs that are connecting the border to the Flex.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: February 24, 2020, 10:19:20 pm
content: Thanks for your replies so far.The board has a simple rectangular shape as log as it isn't completly assembled. As a last step, the assembled board will be put on our laser cutter and the final really complex shape is cut. As you know there are 256 sensors in a 16x16 arragement on this flex, and the material in between those sensors will be cut away, only leaving a grid like pattern, where the pcb- tracks join the sensors. And this has a really complex shape, as the final cut flex pcb, should be even more flexible. Imagine glueing the cut and finished foil onto a pillow, without the foil developing any wrinkles. Literally that will also be the final application, only it is not really a pillow, but smth similar.That it does this, this complex cutout shape had to be developed, and verified to work. Tested it with bare PET- film with similar thickness as the final flex PCB, which was cut with the laser to the flex-pcb's final shape. I would really like to show you pictures how good it works, but unfortunately this is a company project and so i can't disclose more info about it.But i'm drifting away from the topic anyway.I thought about only glueing the foil around the border. But wouldn't that lead to the foil sagging in the middle. Maybe I'm too overcautious about that topic, as the adhesive film would be 200µm in thickness anyway.My idea, which i came up so far, and which is similar, is to laminate the double sided kapton tape onto the carrier plate first. Then the carrier plate will be put into the laser cutter. The laser cutter will be configured that it will only cut as deep as the kapton foil thickness. Then the kapton will be cut in such way, that there is material around the border of the flex-pcb and a circle of kapton approx. 10mm in diameter under each sensor location. So only approx. 15% of the complete flex-pcb area will be glued down, and only those areas where components will be placed. This seems to me as the most professional approach, although also the most time consuming and complex one.Quote from: GerardG on February 24, 2020, 01:12:49 pmI would've used double sided kapton tape, as I've used kapton often and know it's properties. That 3M glue sheet you referring to: Do you have a more exact product name for it. Maybe I give it a try.Thanks,Gregor

author: ejeffrey
author_rank: Super Contributor
timestamp: February 25, 2020, 04:00:30 am
content: I think double sided kapton tape is a pretty good bet. The standard technique to reduce the adhesion is to touch the adhesive with your finger a but before sticking it down. The oils and dirt from your fingers will reduce the stickiness and make it easier to remove after assembly.Putting the adhesive only in waste/sacrificial areas should also work as long as you can load it into the laser cutter with the backing still in place. Then you don't actually have to remove the adhesive.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: February 25, 2020, 06:08:08 am
content: Silly question, do you have enough time to reach out to a PCB carrier manufacturer?This is how we handled some rigid flex assemblies back at my old work

author: GerardG
author_rank: Regular Contributor
timestamp: February 25, 2020, 08:21:36 am
content: Hi,We did not use only a frame of Fr4.We used a sheet the size of the Flex PCB and only glued the edges. This way the middle will rest on the FR4.