How to solder resistive touch panel flexible pcb on mcufriend 3.5" TFT

The resistive touch panel flexible pcb on mcufriend 3.5" TFT has detached from the PCB. Is there a way to solder the flexible PCB back to the board? The location is a bit difficult without detaching the LCD from the PCB.
Thanks a lot.

First off. Purchase some new double-sided sticky foam pads from your local stationers. Choose 2mm or 3mm thick pads.

Then carefully separate the display module from the pcb. The existing foam pads will probably tear. Alternatively cut through the foam with a sharp blade. e.g. Xacto or Stanley knife.
This will be safer than just pulling apart.

Solder the flexi-ribbon. Clean off the remains of the adhesive pads. Replace with your new pads / tape.
Press the module back down onto the new pads.

Vow to never do this again.

When the Display was new it is composed of:

  1. TFT Panel
  2. Glass Touch Panel
  3. Soft Polythene film with a coloured tab.

You should always unpeel the Polythene film. It is not very transparent and seems to have a Polarising effect that ruins your view of the TFT.

David.

It is not conclusive from the photo if the flexi has separated from the PCB, if the touch screen is not working then it might be that there is an incompatibility with the sketch or library you are using and that actually the connections are OK. The connectivity can be checked with a multimeter from the Arduino connection pins.

If you are totally convinced that the flexi has seperated from the PCB then the compexity of the repair process is dependant on wither the solder pads have been pulled off then PCB, in which case you will need to make a more complex repair to the PCB connections. You will probably need to dismantle the screen from the PCB to assess the damage.

Taking the screen off risks damaging the working TFT so maybe it is better to live with it as a non touch screen and buy another one. Since you have asked how to do this, it suggests you have never attempted a repair like this before, thus your chances of success are reduced significantly.

+1.

A working TFT is better than a broken one.

Your first step is to identify the XP, XM, YP, YM pins.
Use the Diagnose_Touchpins.ino sketch.

It will tell you if the Panel is broken / missing / good.

David.

Thank you David and Bodmer. I actually started with Diagnose_Touchpins.ino sketch but could not get any result. Then I changed the order of XP XM YP YM pin order and also tried other pins but nothing happened.

Until yesterday I came across David's dedicated thread about these mcufriend tft displays about measuring the the resistance between A1/7, A2/6. I measured but they were opens circuits. I have another working 3.5 inch and did get ~300 and ~600 ohm resistance. This convinced me the problem is on the hardware.

I looked carefully where the 4-wire touch ribbon was connected and found it was soldered near where I took the picture. The PCB pads are very close to the module edge. I tried to take a clearer picture but the phone could not focus on that point and the magnification is not enough. But I saw clearly using bare eyes the separation of the flexible ribbon and pads on the PCB.

I understand the risk of damaging the TFT LCD completely during the repair. Since I have another working one, I will keep this one as a non-touch screen for now.

Thanks again for the quick reply.

One more question, in case if I decide to repair it later, will hot air gun blown from the backside of module help to separate the foam pads? Thanks.

I don’t know. If you buy a length of new adhesive foam tape / pads first, you can experiment with heat or scalpel blade. See which works best.

Just remember that the Touch glass will crack if you pull too hard.
Most 3.5" displays arrive from China ready-cracked.

One single crack on the Touch glass makes it unworkable. So be gentle.

David.

Thanks David. Good suggestion. I will experiment with adhesive foam tape first.

I have a display that has separated from the backlight reflector so I had a look at how it is made.

The touch flexi on mine solders to the display flexi, as does the flexi from the backlight LEDs. This aids assembly as only a single strip of pads needs to be soldered to the PCB. Here you can see the solder joints of the touch flexi and the Kapton sticky tape covering it (this tape stops the joints shorting to any pads underneath it on the PCB):

The display flexi is then soldered to the circuit board, you can just see the row of joints in the picture.

Of course your display may be constructed differently.

Thanks a lot Bodmer for the nice pictures.

I took a picture again today with flash turned on and place the lcd near the edge of table so that the camera can focus better on the area of interest.

It looks like there are 4 strips of soldering pads on the PCB.