What is this black adhesive connecting a foil ribbon to PCB, and how to repair? - Page 1
Forum section: electronics / repair

author: bobtiki
author_rank: Newbie
timestamp: February 16, 2020, 08:12:10 pm
content: A little context: I am trying to restore a Casio PT-7 keyboard, and the biggest issue is that the keyboard has stopped working. As you can see in the photo, the keyboard is a smooth flat surface, there are foil layers inside, and pressure on the keyboard triggers a key press.Opening the very thin keyboard up, the culprit appears to be that the adhesive holding the connector from the foil keyboard to the PCB has gotten brittle with age, and the ribbon of foil contacts are no longer connected to the PCB surface.As you can see in the photos, the contacts on the PCB are not bare, but are covered in black, what I am currently assuming was the adhesive. The interesting bit is that if I hold the foil connector back in position with, say, a piece of weatherstripping foam and a clamp to provide even pressure, the keyboard does work again. I suppose I could try to repair the keyboard by making that foam/clamp situation more permanent (and in fact, I know one other person that has repaired their PT-7 keyboard in exactly this way), but I would love to better understand how this connection works, and repair it in a more permanent way.I have considered stripping the black off of the contacts on the PCB, and soldering the foil connector directly to them, but I fear the plastic/foil connector may not be able to withstand the heat.Could anyone please enlighten me as to what this black adhesive might have been? Might it possibly be carbon or graphite impregnated in some way? I find it odd that the black extends across the entire pin area, and isn't just on each individual pin, so I expect it can't be that conductive, or it would short all of the pins together. I'm as a bit of a loss as to how this connection works, as it's something I haven't come across before.Any recommendations on the best method to repair this connection would also be very welcome. Thanks!

author: Fraser
author_rank: Super Contributor
timestamp: February 16, 2020, 10:07:07 pm
content: Not black adhesive. More like a carbon coating. The flexible PCB is attached to the PCB with an adhesive between, and around the contacts but that commonly fails with time. Repair requires either a physical clamp arrangement that employs a pressure bar with rubber or hard foam pad and securing screws at each end, or the use of Z Tape that conducts only through the Z axis.https://www.ebay.co.uk/itm/Adafruit-3M-Z-Axis-Conductive-Tape-9703-2-x6-50mm-x-150mm-Strip-ADA1656/263111585574?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649https://thepihut.com/products/adafruit-3m-z-axis-conductive-tape-9703-2x6-50mm-x-150mm-strip?variant=27739532241&currency=GBP&gclid=EAIaIQobChMIjruMt4rX5wIVRLTtCh1nzAFbEAQYASABEgKRI_D_BwEThe Z axis tape is likely the quickest and simplest solution and it is intended for this type of deployment.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: February 16, 2020, 10:13:31 pm
content: 3M conductive tape data sheet. This details various conductive tape types available.https://thepihut.com/products/adafruit-3m-z-axis-conductive-tape-9703-2x6-50mm-x-150mm-strip?variant=27739532241&currency=GBP&gclid=EAIaIQobChMIjruMt4rX5wIVRLTtCh1nzAFbEAQYASABEgKRI_D_BwEFraser

author: bobtiki
author_rank: Newbie
timestamp: February 16, 2020, 10:28:54 pm
content: Quote from: Fraser on February 16, 2020, 10:07:07 pmThank you! I actually just stumbled across the 3M 9703 tape in a different post, and was wondering if that would make for a good fix here. Thanks for letting me know that it should work! I might still put a strip of foam across the case to provide pressure in that spot so hopefully the fix will last a bit longer.One question: do you think it will be necessary to remove the old adhesive before the repair, or might it work with just adding the new adhesive on top. (I'm guessing I'll need to strip it, but curious what your thoughts are.)Thanks! MB

author: Fraser
author_rank: Super Contributor
timestamp: February 17, 2020, 01:54:15 am
content: You can apply the Z axis conductive tape without removing anything from the flex cable or PCB. Removing the black carbon coating is undesirable.Fraser