If you've ever taken apart any modern electronics, you've seen those flat yellow cables. They are used to connect circuit boards with cameras, displays and other boards. Designers use them because they are less bulky than wire cables and are relatively durable. Because of their flexibility, a display can connect to the main board and then flopped on top. The flex circuit Cable will form a sideways "U" ( or "C").
Flex circuit cable can bend but they have a limit, they cannot make a very tight bend. At least not over the long term. And this is where I had an issue.
I had a cheap tablet that almost immediately after the warranty expired started having display problems. It started with the display occasionally flickering, then proceeded to the screen disappearing entirely, but would return then tablet was knocked. Then one day nothing; no display. Hoping that the display flex circuit cable had worked loose from its connector I opened the tablet to investigate. I found the connector and carefully removed the flex cable and reseated it. Turned it on and a big Nothing. I looked further and found that the flex circuit cable was not a smooth U but more like a V. I suspected there was a break and that is how it stayed for about a year.
I looked around for a fix, but the closest tip I could find was to bypass the break by soldering a wire from connector to connector (or flex circuit test points). This would not work as one end of the flex disappeared into the back of the display. After considering and rejecting a couple of ideas I thought I could bridge the break by patching on a similar flex over the break.
Here are my steps.
0. When problems first appear; investigate and repair. In this case, I could have reinforced the flex at the bend.
1. Gather equipment: stereo microscope, X-acto knife, tweezers, soldering iron, fine solder and Kapton tape.
2. Find a donor flex cable. Trace width is somewhat important, but the trace pitch is critical for alignment to traces.
3. With microscope and sharp knife remove the yellow plastic covering one side of donor cable. Do the donor cable first for practice using the knife. You will see shiny copper.
4. Do the same with broken cable.
5. Cut the donor cable so it will work as a patch.It should overlap the broken cable and exposed copper traces.
6. With soldering iron tin all the exposed copper with solder on broken and donor cables.
7. Clean both flex cables with alcohol and dry.
8. Position patch over break and align properly.
9. Hold donor flex down with tweezers and heat the donor flex with soldering iron. Previously added solder should melt and connect the two traces.
If this worked then you're better at this then I am. Keep reading to find out how this didn't work for me.
10. At this time the plastic of the flex cable started to de-laminate from the copper trace. Say a little curse.
11. Tack the copper traces to the broken flex cable traces. Should be easier since the flex plastic has de-laminated. All donor flex traces from one side should be soldered down before proceeding.
12. Try to solder through flex plastic, it will probably de-laminate the flex plastic. Solder the copper traces of the donor flex to the broken flex cable.
13. Check everything under the microscope. I gently pulled the copper traces to verify that it was connected.
14. Add Kapton tape below and above the patch. I didn't do any cleaning because the repair seemed very delicate.
15. Turn on and test. The tablet display worked immediately.
16. Reinforce the bend by adding several layers of Kapton tape ( I added some foam on each side of repaired flex cable for support).
DISCLAIMER: This worked for me and there is no guarantee that this will work for you. Displays use high-speed serial signals and patch like this could degrade the signal potentially causing the eye to close. I either had extra signal margin or I was working on power traces. I'm surprised it worked.
Follow-on note: Perhaps low temp solder would allow soldering the traces withoug de-laminating the flex plastic.
