Introduction: Fixing LEGO NXT Brick Dead LCD

Forgotten by everyone, the LEGO NXT brick sat on a shelf for one and a half year. It was built into a balancing robot, but after the initial fever was gone, it just stood there waiting better days. The batteries were pulled out so nothing bad would happen to it, but we know this is a false presumption. When I wanted to reanimate the robot after many months and inserted new batteries in the brick, I only heard the "tadadadadam" but no image got displayed on the LCD. Worried, I began to search the web and found many complains about the same phenomenon. As it turns out, this is a common problem, presumably caused by a fabrication issue. There are TWO commonly adopted theories:

    • The first theory presumes that bad soldering of the three SMT capacitors from the small, detachable PCB (external components of the LCD controller booster circuit) may be the root cause of this malfunction. Many users soldered again the capacitors and the image was regained.
    • The second theory suspects the ribbon cable between the LCD and the small PCB being damaged, the recommendation being its replacement, if ever possible.

    Because re-soldering the capacitors didn't help me in getting back the image on the LCD screen, I concluded my case should be the damage of the ribbon cable. I found two methods on the web offering detailed solutions for ribbon cable replacement:

    • The first method describes how to peel off the ribbon cable from the small PCB and LCD pads, then how to glue some wrap wires to the LCD contact pads and how to fix them to the LCD glass using melted plastic and how to solder them after to the small PCB contact pads.
    • The second method applies mechanical forces by pressing the ribbon cable over the PCB or the LCD's contact area against the NXT brick upper cover wall, or by pressing some kind of delicate metal contacts (extracted from a PC MB's extension connector) over the LCD's contacts.

    Both methods seem too difficult to be applied mostly because they both look a bit imprecise and rely more on luck than on reproducibility. So I hibernated the NXT brick repair project for another year. But the moment of its reborn came closer and closer, and now it's finally here.

    Step 1: Trying Some Fundamental Methods... Resulting More in Garbage

    Before concluding on the final solution, a couple of alternative methods were tried. All of them were missing good results or were almost useless, however registering and detailing them could help understanding the key constraints. The LCD was lifted off carefully from the white plastic fixture (using a not very sharp, wide blade kitchen knife applying slow motions when pushing through the adhesive tape between the LCD's back and plastic) and the battle for the life of my NXT brick began.

    • The first attempt was to heat up and firmly press the ribbon cable over the LCD and/or PCB contact areas. This attempt didn't gave me any good results.
    • The second attempt was to cut down the ribbon cable from the PCB, clean up the mess from the contact pads, peeling off the black cover membrane of the ribbon cable, then try to glue tiny wire chunks with conductive glue to the ribbon cable pads. Sadly, at the first bending attempt of the ribbon cable all the precisely glued wire chunks piled off. Again, no results, but the main problems were identified: the method lacks mechanical stability and the conductive glue is too "weak" in terms of adherence and mechanical strength to keep the wires in place.
    • The third attempt was to find and solder a 0.5mm pitch SMD ZIF connector to the small PCB's pads, then gluing a hardener over the ribbon cable back side and then try to match the 1.2mm pitch of the ribbon cable with some ZIF contact poles. The method had some results (a few, horizontal black lines appears on the LCD) but still lacks stable and full image on the LCD. After taking out and pushing back the ribbon cable in the ZIF a couple of times, the ribbon's traces (some sort of silver-ink printing) were damaged and mechanically destroyed by the ZIF's contacts.
    • The fourth attempt was the closest to the final version: a small piece of PCB, with scored pads was glued to the ribbon cable chunk, then wires were soldered to the PCB's pads then the end of the wires were glued to the ribbon cable traces. Again, some horizontal black lines appeared, but nothing else.
    • Finally, I ended up with the ribbon cable peeled completely from the LCD getting a "naked" LCD only with its contact pads.

    Conclusion: in my particular case the damaged section of the ribbon cable was too close to the LCD contact pads, so there was no real chance to get it repaired by any methods described before. The concluded key constraint was: how to stabilize the wires against the LCD pads? The picture shows the garbage left behind the battle.

    Step 2: The Concept. Tools and Manufacturing. CNC or Needle Files?

    I hope this step will be simple enough for everyone who will try to reproduce the fixture. The fixture will be glued firmly to the LCD glass from behind, then thin wires will be soldered to the fixture pads and finally the end of the wires will be glued to the LCD contact pads with conductive glue. Feel free to use the attached mechanical drawing to understand the idea better. It shows the fixture dimensions (mm scale). Please note the LCD pitch is very close to 1.2mm.

    Because I had the opportunity, I decided to use a hobby grade CNC to manufacture the fixture, but - as described before - this fixture can be manufactured manually using a jigsaw, some needle files and a cutter. The base material can be any small piece of single sided, 1.6mm thick PCB. For those who are interested in CNC machining there are two basic g-code files attached to this step. One is for the fixture itself, the second is for scoring the pads. Machining time is 5 minutes for the fixture and one and a half minute for scoring (without counting the tool change and leveling). The first g-code program presumes a 2mm PCB end-mill (can be found on e-bay searching for "Carbide End Mill Engraving Bits for CNC PCB Rotary Burrs 0.5 to 3.175mm" or similar) the second program uses a 0.2mm engraving bit (can be found on e-bay searching for "2x Titanium Coated Carbide PCB Engraving CNC Bit Router Tool 30 Degree 0.2mm" or similar). The scoring depth is set to 0.2mm. Please note that the g-code files are more like amateur approaches than professional ones, but they do the job well. The result is shown in the picture.

    Step 3: Applying the Fixture

    Once the fixture was machined, I made a basic alignment test to see how it fits to the LCD. The result was satisfying enough to go forward and glue the fixture to the LCD. For this I used a very common superglue (Picatura) and a needle.

    • Take the small bottle and unscrew the dosing plug.
    • Carefully immerse the needle in the glue and spread glue on the inner surface of the fixture. Two or three drops of glue are enough to cover the fixture inner surface. Too much glue can waste the whole procedure.
    • Carefully align the fixture pads with the LCD's pads and press down the LCD gently over the fixture. Even if some glue comes out between the LCD's glass and PCB edge do not touch or wipe the small drops!
    • Two clips can be used to squeeze the fixture to the LCD.
    • Let the assembled parts rest for at least one hour (even if it's superglue, be patient).

    Step 4: Preparing and Soldering the Wires

    Until the glued parts are drying we can prepare the wires. Any 0.3 or 0.4mm diameter copper wire will be good for this operation. The wire should be "naked", without isolation or any other cover material on it.

    • Prepare a 30cm long wire.
    • Use a soldering iron to apply a thin solder coat over the wire by melting a small amount of solder on a paper sheet, then gentle pulling the wire trough of the solder spot. You can add a small amount of resin to more get a more satisfactory result.
    • The wire must be cut in small pieces of approximately 20-25mm length. These pieces need to be straightened by pressing and rolling them under a hard surface. This can be a small USB stick with plane, rectangular surface.
    • The fixture pads must also be covered with a thin solder coating. I recommend everyone to use some 0.5mm solder wire for this purpose.
    • Take a wire segment with a clip, align it with the center-line of the LCD and PCB pads then solder it down to the PCB. Make sure that each wire end is aligned with the top edge of each LCD pad too.

    • Continuously check the alignment between the wire, the LCD and the PCB pad.

    Step 5: Gluing the Wires

    After the soldering work is done, you can go carry on with the PCB pad - LCD pad bonding. For this we will need a special glue, named Good Helper (this can be found on e-bay searching for "Conductive Wire Glue Surface Mount Flex Boards Solar Cell Leds without soldering" or similar). Be careful, this glue dries in a couple of minutes even at room temperature! Once opened, I always keep the glue in the fridge and I try to decrease the room temperature to 15-17 degrees of Celsius before using it. I know it is a mess and madness, but consider that this is probably a forged version of the original glue, thus these preventive actions are needed. Being a forgery has its advantages, the low price is attractive, you can purchase two bottles for a couple of dollars (I had luck with having two bottles because one of it dried out - the one I kept in my office desk drawer. The other one I kept in the fridge is still undamaged and usable).

    • Take an unnecessary wire segment with a clip, gently immerse it in the glue.
    • Try to carefully coat the soldered wires ends and the corresponding LCD pads so the wires will adhere to the LCD pads and will make electrical contact after the glue dries.
    • Continuously check against pads that weren't coated and bonded together by the glue. If you observe any evidence of bonding between adjacent pads, remove it immediately with a very sharp cutter or blade and try to separate the contact pads by gently removing the excess glue. Do not wait until the glue becomes hard!
    • Once the "surgery " finished, let it rest and let the glue cure.

    Step 6: The LCD Is Back to Life

    After a couple of hours you can power the brick up and make a try. Carefully solder the wires one by one to the NXT PCB pads (previously coated with a thin solder layer). The LCD should come back to life, at least mine does. I was very happy to see it working after two and a half years, since I observed it broke. Please note this step can be skipped if you are confident that it will work at the first attempt. I was a bit unsure because of the conductive glue (which I have never used before).

    Since this was only a functional test, next we need to bend the wires and pack everything together, after that we can close the NXT lid back in place.

    Step 7: Bending the Wires. Soldering to the PCB Pads

    There is a very narrow space left between the LCD and fixture lower edge and the NXT rubber buttons. Presumably this was the cause the ribbon cable failed over time (even with the brick resting on a shelf, the ribbon cable was bent too tight in that area and it finally gave up).

    • For bending use a needle aligned with the fixture edge. The needle will prevent the wires to be bent too sharp in that area (over the fixture's edge).
    • Bend all wires separately, or in groups of few wires together by 180 degrees to the back.
    • After this you need to bend the wires up by 90 degrees as the closeups shows. Leave around 2mm from the edge of the fixture then use a clip or press the needle firmly over the wires then bend them back up. These bends will form a little step thus the rubber buttons can fit in that area without touching the wires.
    • Be careful not to touch or press the glued end of the wires.
    • Once you finished bending each wire, solder the wires on the sides first to the corresponding external PCB pads.
    • Finally the remaining wires must be soldered one by one to the corresponding PCB pads.
    • Once soldered down, the excess of the wires must be cut at the near edge of the PCB.

    We are almost done now.

    Step 8: Final Assembly

    After bending and soldering the wires it's time to re-assembly the whole thing. Put the rubber buttons in their place (have two rubber pins, insert them in the PCB's small holes). The closeup shows that the wires do not interfere with the rubber buttons. Before closing the lid it worth a try to see if the LCD is still working. If everything is fine you will be able to see the LCD displaying some graphic signs and hear the glorious "tadadadadam". Job finished. Good luck for everyone coming through.