FFC stripping single or both sides?

[Buildingthings]

I am repairing some marine electronics. The original flat cable was stripped back from one end about 4mm (exposing the conductors on both sides), then a plastic reinforcing strip glued on.

Is it an acceptable practice to just strip one side of the FFC, and re-insert the cable? I don't understand the need to strip both sides of the plastic coating off, and I do not have the tools for that process. (beginner)

[Fraser]

Stripping an FPC cable is not standard practice. Normally one side of the FPC cable has exposed conductors that make contact with the ZIF socket. Yen other side of the FPC is normally fitted with a reinforcement tape that can be of at least two different thicknesses. Some cable and ZIF socket combinations use a plastic reinforcement on the rear of the FPC cable, simplifying insertion into the ZIF socket.
The ZIF socket grips the FPC cable insulation so the thickness is very important.

There are complications with FCC cables that you should be aware of…….

1. The thickness of the part that inserts into the ZIF socket can vary and some FPC cables will not fit a ZIF socket because they are too thick. A thinner FPC cable should be sourced.

2. An FPC cable comes in different exposed contact versions….. same side and opposite side exposed. You need the correct type to match the original used in the equipment.

I suspect that someone has tried to repair the equipment that you are working on and they did not have the correct type of FPC available so they butchered a similar cable to fit the ZIF socket ! It was either the incorrect exposed contact orientation or too thick to sit the socket.

I advise you to purchase a new FPC ribbon cable. Select one with the correct length (or greater) correct pitch to match the ZIF Socket, correct thickness to match the ZIF socket and correct exposed contact orientation. Repairing a damaged FPC cable is possible but I would not do it in a commercial environment. Simple straight FPC cables are cheap and available. Note that the cable you are currently working with may not be the original….. hence why it has been butchered.

Fraser

[voltsandjolts]

Just buy a new cable, they are super cheap. Get the correct length, pin pitch and top or bottom exposed contacts at each end, as per original.

[Buildingthings]

The ZIF socket grips the FPC cable insulation so the thickness is very important.

It seems that I could select a reinforcement strip that adds up to the correct total thickness? (measure with calipers to verify)


2. An FPC cable comes in different exposed contact versions….. same side and opposite side exposed. You need the correct type to match the original used in the equipment.

The connectors have a "U-shaped" female receptacle in the plastic housing, so it appears as if there is no difference which side of the cable is used to carry the signals. Maybe I am not understanding the engineering used in the connector design?

I would love to swap cables, but the other end is potted into an unusual SD Card reader. It looks to be very difficult to source.

[Buildingthings]

The other end of my cable is potted into this. Forced to re-work the cable it seems.

[Fraser]

Yes that is a challenge !

OK, how to proceed…..

You have a choice…..

1. Rework the cable to create something that will work and be secure in theZIF socket
2. Rework the other end of the cable bu finding the landing points of the cable on the PCB before fitting a ZIF socket modification to the PCB. There are only a few conductors.
3. Strip the FPC insulation and solder a ZIF socket to them. Then use a new FPC between the new ZIF socket and destination ZIF socket

Option 1 will be worth a try but options 2 and 3 will still be possible if your efforts fail.

About stripping the FPC…..

1. It is not an easy task !
2. Remove only only one side of the insulation ! The FPC is two layers with adhesive holding them together.
3. Use a very sharp scalpel to carefully cut away the insulation and peel it away from the other side of insulation.
4. Adhesive residue may remain after cutting away the insulation. Very carefully clean the adhesive off of the copper conductors with white spirit (terpentine) and allow to dry. Do not remove the adhesive that is holding the copper conductors to the other insulation layer ! this is challenging !
5. Attach layers of Kapton tape or similar shiny, non stretch, ‘hard’ tape to the insulated side of the FPC to create the correct thickness for an easy, but not too loose, fit in the ZIF socket
6. Very carefully insert the FPC into the ZIF socket. Be aware that the copper conductors can peel away from the insulation layer and ‘crumple up’ as the cable is inserted. Care and patience is needed.
7. Apply any locking bar that the ZIF socket uses to hold the cable in place.
8. Apply some acid free RTV to the FPC and ZIF socket to reinforce the mechanical fixing between the two. Hot melt glue is not recommended.

If you are working with a non ZIF socket that requires the FPC to be forced into sprung contacts, the chances of success are slim to nil as the copper conductors are pushed away by the sprung contacts as you try to insert the FPC Sometimes a removable thin polyester sheet may be used to protect the copper conductors during insertion and then slid out carefully afterwards. Overhead projector plastic (acetate) sheets work well in this roll as they are thin, hard and shiny.

Fraser

[Fraser]

More thoughts from me…….

You mention that you are working on Marine electronics……

I am an Ex Merchant marine Radio Officer and understand the implications of equipment failure at sea. If you are working on any equipment that is in any way safety related (SOLAS) or “Mission critical” you should not modify it in any way that could create a vulnerability to failure at a later date. In some cases, any modifications, no matter how minor, are prohibited. You could be held liable if the equipment fails leading to loss of money or worse still, life. Tread very carefully. I consider ships electronics similar to aircraft electronics when it comes to liability and regulations.

If modification is permissible, there is also Z Axis double sided tape that may assist you. This tape  conducts in the Z Axis only and may be used to join two FPC cables. You would still need to prepare the end of the damaged FPC but you could then attach a suitable new FPC ribbon to it using Z axis tape. My problem with that tape is it’s unknown longevity in terms of the adhesive. I would  want to create a clamp around that jointing interface to physically hold the two FPC cables together. Remember to select a new FPC that maintains the correct orientation of contacts at the destination connector. This idea is not my favourite but might help if you are dealing with a forced fit sprung loaded connector as you will have a nice new FPC end to insert into it. Do be aware that Z axis tape is relatively new and I have no idea how much current it can cope with !

https://thepihut.com/products/3m-z-axis-conductive-tape-9703-2x6-50mm-x-150mm-strip?variant=27739532241&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=Cj0KCQjw166aBhDEARIsAMEyZh6WCiM7gucVWQncRpwBSG8a5LXl-DUSseF6PEBnBhLI73UiW6MF2tcaAu0dEALw_wcB

Is the complete PCB and FPC very expensive or not available ? It may be better to fit a new complete assembly if such is available.

Fraser

[Fraser]

ZIF FPC sockets may look ‘double sided’ in terms of contact shape but they are normally a single sided connector with a clamp that holds the FPC against the contacts. The same side/opposite side configuration of a double ended FPC is important to maintain correct pin 1 to pin 1 connectivity that matches the contact location within the ZIF socket. Some ZIF sockets have the contacts nearest to the PCB surface but others have then furthest from the PCB surface. At the end of the day you need correct Pin 1 to Pin 1, Pin 2 to Pin 2 etc connectivity.

[Buildingthings]

That drawing really helped Fraser thank you.  I get it now, the wedge (or lever) presses from the top and the conductors mate with the bottom section of the receptacle mandating that the bare side face down.

I first made a mark across the cable and taped it flat to an aluminum plate. Than a soldering iron screwdriver style tip at just enough temp to melt the plastic, then a very sharp stanley blade held at 90 degrees to the cable, and as flat as possible and removed the plastic very carefully. Pretty good result, only I did the wrong side The belt sander technique mentioned elsewhere would not be very difficult in this one, but looked great.

I measured the thickness of the conductors at .1mm, and an undamaged reinforcement as .22mm so total of .32mm. I did a test fit with a scrap of plastic the same thickness and the clamping force was firm. Looks like a success so far.

The trick of slipping a thin shim in with the cable is brilliant, will be my plan for installation.

Fortunately this piece of gear was given to me free, and will go on my own sailboat (uninspected, no passengers for hire) so all good for liability. It is a quite expensive chartplotter, so if it works quite a nice win.

That z-axis tape is something to order asap and practice with. I have scrapped  expensive gear for the lack of a new flat cable after it failed. 

[Buildingthings]

I found a reference for "standard" ZIF cable dimensions that is close to my measurements. Maybe confirmation bias, but taking it.

I did confirm that this is the way the Simrad factory made the cables, crap adhesive on the backing caused it to fall apart after ten years.

So for a standard ZIF cable= .003" copper (.08mm), backing .009" (.23mm), total .012" (.30mm)

[KrudyZ]

I would still buy a new cable of the correct type and use it to make an extension.
That way you have proper mating with the connector and plenty of space to connect the two flex cables with the ability to pot it once you verified your work.

[Buildingthings]

I would still buy a new cable of the correct type and use it to make an extension.
That way you have proper mating with the connector and plenty of space to connect the two flex cables with the ability to pot it once you verified your work.

That seems reasonable, thanks. I looks like a FFC to FFC splice with a solder join should be a lasting fix when done with high skill. I will practice that. Perhaps the "zero axis" tape previously mentioned by Fraser.

[Fraser]

Well done on your investigation into options. I hope you manage to repair the unit. If it is fir yourself the. There are options for replacing a pin SD card receptacle with an alternative product. There are SD to TF card adapters that might be adapted to fulfil the roll in your chart plotter.

https://cablematic.com/en/products/microsd-tf-adapter-to-sdhc-sdxc-sd-card-slot-with-48-mm-flat-cable-SL00100/

In your position, I would attach the TF card end to the PCB with an epoxy glue and run jumper wires to the PCB connections of the original ZIF connector.

Another option would be to carefully disassemble the original SD card PCB and investigate how to attach a new FPC to it. Z axis tape may be an option plus a clamp to hold it firmly in place. Another option might be soldering the FPC conductors to the PCB, if that was the original technique used.

There are many ways to adapt this design to save the chart plotter from the scrap heap.

Fraser

[Buildingthings]

A brilliant find and suggestion for that cable mounted SD card reader!

That I will be sharing. These chart plotters have a door on the front that of course will leak sprayed salt water over time, and down goes the card reader. The usual situation is that the repair cost is so expensive that the customer just decides to upgrade. Another 4,000 bucks for the latest model. I'm sure that card reader can be epoxied in place, 10 bucks and done! Bob's your uncle.