advice on replacing ribbon cable fro PCB to LCD?

[Keith Ostertag]

I found a vintage calculator, cheap Chinese but I'd like to see if I can repair it. The ribbon cable connecting the LCD to the PCB has detached from both sides. And it appears that the cable (a very thin mostly transparent 28-conductor flat film like type) lost most of the conductive material at the connection, so I will need to replace it.

Anyone with experience doing this? Where to source a cable like this?

Also, I found a reference to Adafruit 3M Z-Axis Conductive Tape 9703, any experience using it for these type repairs? Evidently it's pressure sensitive and provides electrical conductivity vertically, through the thickness of the tape, but not along its length or width.

Thanks,
Keith

[blackjetrock]

These connectors are called 'heat seal' connectors and are used to connect PCBs to LCDs in a lot of electronic equipment. They have a thin plastic substrate that has conductors printed on it, sometimes in carbon, sometimes some other material. I think the ends that attach to the PCB and LCD have a heat cured conductive glue that is similar to the 3M tape that you mentioned. In manufacture the connector is aligned with the conductors on the LCD and the PCB and heat and pressure is applied, the glue melts and the connection is made.

This connector is a cheap and quickly applied way to connect the display and electronics, but unfortunately they all seem to eventually detach from the items they were glued to. The word 'eventually' is used here in the way a collector uses it, so I'm talking years, probably around two decades.

There are a number of failures of the strip that I've seen. Firstly, the glue detaches from whatever it was glued to but there's no other damage. This can sometimes (almost most times, but not always) be re-affixed by applying heat and pressure to the connector, 'squishing' it back onto the pads it was glued to. This sometimes works, and is the repair that you see a lot on YouTube.

The next type of failure I've seen is a break in the conductor, usually near the ends, probably due to rough handling or repeated flexing. This causes a break in signals. This is maybe fixable with conductive paint, I have tried metalic paint and carbon based paint. Sometimes this works, but I don't think it will hold up to much in the way of mechanical stress. It is best for LCD signals, but not for power signals.

The third type of failure is a break in the plastic substrate. This is similar to the break of a conductor and the two faults quite often appear together. Conductive paint sometimes helps.

My first attempt at fixing this type of connector was by applying heat, and was fairly successful If you apply too much then you melt the substrate and destroy the connector. I have managed to fix the odd device this way, but they sometimes fail again. It looks like the heat application technique only works a few times, I'd say twice at the most, really, as the glue gradually gets squashed flat and doesnt want to adhere any more.

I then spotted the 3M tape and tried that. It's very expensive in reels but you can sometimes buy offcuts. It does what it says on the tin, and I did get an LCD segment to light up now and again but there are a few drawbacks:
1. You are only fixing the pads and the glue, so you need a ribbon to attach to, and if that is broken the 3M tape won't provide a full fix.
2. The tape itself has a 'cell size' of the conductive material, this means the tape can't work with connector pitches below a certain size. This is a problem with high density connectors. The cell size is about 0.1mm I think, and you need to have at least that distance between conductors.
3. The datasheet says you need a minimum gap of 0.4mm which causes problems with a lot of connectors. the datasheet also says you need heat, pressure and time to attach the tap, and this in't always easy to do.
4. If you have a stiff connector then the ribbon can pull the tape off the pads, as the adhesion isn't as strong as the heat seal bond.

I didn't find that the 3M tape worked very well for the repair of these connectors.

https://www.youtube.com/watch?v=b_ETp0afNds

I gave up trying to fix these connectors for a while, then I spotted that the heat seal problem was common in cars, and found listings for heat seal connectors that can be used to repair dashboards and radios. If you search for 'dashboard repair' then you get this sort of thing:

https://www.ebay.co.uk/itm/315695406431?...BMmtKR7oBl

This is an unused heat seal connector that I assume is either a manufacturer part or someone has had it remade. I bought some of these and they should work, if you can apply them correctly (time and pressure is needed). You need to buy the correct cable, in terms of length and width and also conductor pitch. Unfortunately for me, I couldn't find the right pitch. I wasn't too good at the heat and pressure thing at the time, either, but that was sorted later.

I think buying one of these replacement cables should work, if you can find one that fits your device. You can't cut them down, either, as the glue is on the ends of the ribbon.

Having seen these cables I realised that I could maybe have some made, so I found a company in China that would deal with me, the main problem was getting money to them, in the end they used a personal account of a friend and I sent them enough money and a drawing of a replacement connector for the PC-G850VS that I was trying to repair. They aren't cheap, not in the quantity I was buying, but there is an up-front tooling cost. I think the cost per ribbon was about $25 (this was a few years back). The company also supplied a tool for applying the heat and pressure needed for attaching the connector. This was a tool a bit like this:

https://www.ebay.co.uk/itm/285778014782?...R6q_hO-AZQ

It's a wide tip for a soldering iron and a strip of silicone that protects the ribbon and also spreads the heat a bit. You need a temperature controlled iron for this. Using the tool and the ribbons I did manage to get most of an 850VS display working again. I had a few (single digit quantities) of pixels that still didn't work. I think with enough practice and these remade ribbons that repair is possible. After my first, almost successful repair, I removed the ribbon and tried again and that was much worse. I think that was down to me, and probably alignment of the ribbon, The 805VS connector is high density and alignment is tricky. I think the gap between pads is 0.1mm or so (hence the failure of the 3M tape), and getting the ribbon aligned while holding it in place and applying the pressure and heat isn't easy. A jig may help.

There is an invasive way to get some equipment working and that is to solder wires between the two components. This only works if you have a solderable pad at both ends and that isn't the case with most LCDs. You can replace the LCD with something else if you just want a working display again:


https://trochilidae.blogspot.com/2020/02...y-for.html

https://youtu.be/PFEQpZJrdY4

3M tape datasheet here:

https://cdn-shop.adafruit.com/datasheets/3m%209703.pdf

To sum up, I really don't like these connectors and am very disappointed when I open a case and see one there. I think they are fixable, but only with a replacement heat seal cable in most cases. Some connectors with wider pitches may be fixable with the dashboard repair ribbons but it's a matter of luck finding one that fits. 3M tape may also work for those displays.