Repairing traces on PET film from calculator keyboard

[Buriedcode]

Hi,

I have an 90's TI calculator with about 7mm of a trace missing from its keyboard matrix - which is PET film - leading to several non functional buttons.  It's a nice calculator, not worth the time to repair, but I'm using it as practice in case I need to do it on something more important/expensive.

There also, a few mm missing from one of the carbon fingers used for one of the buttons - this is less important as it looks like the other fingers will sitll make conact, but as it is one of the affected buttons it can't be tested yet.  Would be nice if I had a way to repair these as well.

So, my thoughts are:

1) copper tape with conductive adhesive.  I had some, tested it on OHP film with two strips and it worked (R = ~ 20ohm), but every time I try and lay at few overlapped mm-wide strips to make a trace, one of the connections between the strips never connects.  I've tried pressure, and replacing strips, can't get it to work - I'm unsure why, but its unreliable. The original traces are about 6mil and way less than 35um thickness, it seems getting a good conection to these is difficult.

2) I've seen conductive glues/inks, and have a very old vial of "silver ink" (Labelled: Leitsilber L100) that I've tested on OHP film - waiting for it to dry, but its so thin it will not form thin tracks an a plastic surface, plus I'm sceptical about its usefulness - given its 15 years old I dont' know if its just age or this is just the wrong application.

3) I have seen "wire glue" the carbon/black stuff.  This seems to set hard, similar to the carbon fingers used on keyboard.  This seems more promising, and if its thick I can at least use to to repair the carbon fingers, and join copper tape strips to form traces whilst still keeping it thin.  Has anyone used this?

Given its on PET film soldering is out of the question.  I've googled it and theres a lot of either very old websites for restoring vintage equipment (that don't have flex/PET film) or questionable amateurs.

[jpanhalt]

A researcher named Jennifer Lewis published a modified Tollens reagent in 2012 that deposits pure silver on virtually any substrate.  It is supposed to be solderable.  Here's the original citation: https://pubs.acs.org/doi/abs/10.1021/ja209267c

I have seen it sold under various names.  It is important to distinguish "conductive ink," silver ink" (i.e., silver colored ink) and this nanoparticle pure silver reagent, which is also called "silver ink."  An alternative would be the repair inks used to repair glass window heaters in automobiles. One of the press releases showed it on plastic film, but I don't recall the substrate.

PS: Note, the ink is virtually clear until it dries and is warmed.

[WattsThat]

Caig Labs makes a silver based ink product that works, I’ve repaired HP 48’s with the stuff. Pricey but it works well when fresh. Gets thick and lumpy as it ages. IIRC, you can thin it with ether.

Carbon based inks would probably be okay as well.

https://caig.com/product/circuitwritertm-pen-cw100p/

[Buriedcode]

Ahh I've seen teh "circuitwriter" pen.  A quick browse on youtbe shows it could work - although I have only seen it tested on paper.

My brief test with the stuff I have "Leitsilber 100" which seems to be similar to the stuff above was a disaster.  On paper/card, it leaves a conductive path thats more than conductive enough for keyboards, but on the PET film I tested it on, nada.  It spread out far too much and was pretty much see-though = no conductive path.

I think I'm going to have to try a combination of the carbon/black conductive paste - so I can build up layers for repairing the carbon pads - and copper tape.  With dots of the paste to ensure a good connection between foils trips.

After another test several buttons were also non functional, unrelated to the above missing trace.  These all connect to the PCB via one long meandering trace - with a resistance of about 500k.  So, looks like I'm going to have to cut it in half several times to narrow down the high resistance part, and repair with the paste   Still, hopefully it'll at least force me to try the stuff to see if its of any use - I've ignored conductive inks/pastes before because I haven't found any practical use in electronics, aside from a novelty.

[edpalmer42]

Here in Canada, our cars have a conductive grid on the rear window to melt snow and ice.  There are repair kits that consist of a thick paint that deposits a conductive copper layer on the glass.  I have used this to replace a trace on a standard PC keyboard where the trace just disappeared.

Check at your local auto parts store or search amazon for <rear window defogger repair kit>.

Ed