Hey,
I’ve been doing a little bit of work trying to find a more permanent fix to the display cable problem, and I’ve got some new methods, and some new info/ideas. First of all, as some have noticed, the oven method is easy, but isn’t necessarily a permanent fix. This has been my assumption since if it failed after being pressed at a factory, re-pressing it by hand will likely not last as long since the adhesive is over 10 years old, and I’m sure it was pressed with the proper method originally, and I assume the oven method isn’t the optimal method.
So, all along I’ve been trying to come up with a good way to add a layer of solder across the back to solder the cable to the display, which would make a permanent connection between the cable and the display. Then the only point of failure should be the cable itself, which in this application should last for a very long time since there are no kinks in the cable and because it’s rarely moved. Kinks will eventually weaken the copper, which is what typically causes failure on this type of cable, especially when moved… and since the cable is only moved when the IPD adjustment is changed, movement doesn’t happen often, and the amount of movement is very small.
First, let me explain the cable a little bit. The cable itself is a type called Flat Flex Cable (FFC), also sometimes called Flat Printed Circuit (FPC). It’s a thin flexible cable with a clear coating and copper for the conductor. There are many types of FFC, and I’m not 100% sure of the specifics of the type used on the VB, but I have made some assumptions which seem correct from my observations. The coating is a Polyimide, which is strong, high heat tolerant, and chemical resistant. The copper is standard copper (rolled thin), attached to the polyimide with an adhesive (acrylic and epoxy adhesives seem common, not sure which one is used, but it doesn’t really matter).
All of the information is AS-IS for informational use only. I’m not a chemist or an expert in this field, I am just reporting from my own observations and research I have done, so if you try any of this, you’re doing it at your own risk… I’m not responsible if you try any of this and something goes wrong. Do your own research, and make sure you’re comfortable with what you’re doing before doing anything, and remember SAFETY FIRST.
Anyway, the challenge was to take the coating off the top of the cable to expose bare copper, so it could be soldered down to the copper on the display PCB. The problem is that the coating is very chemical resistant, and also that the copper is very thin, so physically removing the coating with a knife or soldering iron easily damages the copper cable. I read that Polyimides are very resistant to chemicals except hot Sodium Hydroxide (NaOH, AKA Lye, typically used in drain cleaners), and possibly hot Sulfuric Acid. Sulfuric Acid would damage the copper though, and I’m not sure of a good place to buy it (besides removing it from my car battery), and it’s pretty dangerous to work with. NaOH is easy to find, I was able to pick up a large container of it at the hardware store (drain cleaner section) for about $8, and it doesn’t damage the copper. You still need to be careful working with it, but chemical resistant gloves and goggles, along with adequate ventilation should be fine.
Here’s an overview of the methods I tried:
I tried many chemicals I had laying around the house to dissolve the cables, and nothing worked well (I tried Goof Off, Paint Stripper, Hydrochloric Acid, Brake Fluid, etc.). I left the chemicals on the cable for about 30 mins (at room temp) with no visible damage.
I also tried scraping the coating off with a knife. This worked okay, although the copper scraped off in several places, but I did just a small amount at a time, so I was able to expose enough copper to solder it down. In the end this method was successful, although not really easy. I tried a similar thing using a hot soldering iron instead of the knife, figuring it’d melt the coating, but this damaged the copper really easily by breaking the copper at the bend in the cable.
Then I tried the NaOH, first by dipping the end of a cable in the NaOH solution. At room temperature, it didn’t dissolve the cable very much, but after heating the solution, it dissolved through the cable very quickly:
. Then I tinned the copper on the display where I’d be soldering the cable to:
, and soldered it:
. This method turned out good, but wasn’t very easy since the cable had to be removed, and the copper is very thin and difficult to get perfectly lined up with the display after the coating is dissolved.
Another method I tried was leaving the cable attached to the display, and to dissolve the cable, I took a small strip of paper towel soaked in NaOH, placed it on the edge of the cable, then heated the paper towel with an iron. This worked well, after heating it at probably 150F-200F and pressing on it for a minute or two, the coating began to peel underneath the paper towel. I peeled the coating away, which exposed the bare copper, and I was able to easily solder the cable, since the cable was still lined up and attached to the display PCB. Unfortunately, I didn’t take any pictures of this method, but it’s pretty simple, and turned out much like the next method. The NaOH did seem to react with the bottom of my iron (which I bought from a thrift store just for this kind of stuff), so I’d recommend not using your good iron for this.
The last method I tried was using a toaster oven to heat a strip of paper towel soaked in NaOH. This method worked really well. Basically, I placed the display in a toaster oven at about 200F, placed a thin strip of paper towel soaked in NaOH over the edge of the display cable, then let it sit in the oven for a few minutes. After pulling it out, a lot of the coating was dissolved, and just a small amount of what appeared to be the adhesive was left. So I used some Goof Off to remove the remaining adhesive (the adhesive can be removed with Goof Off, the Polyimide coating won’t be affected), then I used a fiberglass pencil to clean the copper contacts and bare copper cable:
Then, I soldered it like usual:
. I really liked this method since it worked pretty quickly and easily, and produced good results. The one problem I had was the first time I tried this, the cable got ruined because I put the display in the oven while heating, which had much hotter radiant heat than convective heat (the air was only about 120F, but the radiant heat from the burners produced a much hotter temperature, and the cable curled and deformed in just a few seconds). So, the next one I did I preheated the oven to 200F, then turned the burners off and let it sit in the hot air.
Some notes on all of these methods:
Since the copper in the cable is very thin, it can be very easily damaged by too much heat. Make sure to turn your soldering iron down to a low temp. On my temperature controlled iron, I had good results at 450F-500F, and damaged several pins at 650F.
Also, when using NaOH, make sure you neutralize it after you’re done dissolving the cable (in whichever method you try). If you don’t, it’s possible that it’ll further damage the cable if left there for too long, and it could damage your soldering iron tip, or give off harmful gases when heated by your soldering iron, and really, you just don’t want NaOH residue on your display. I’m not sure what the best way to neutralize it is though. I’ve been just running it under water for a long time, which eventually rinses it off/neutralizes it (after a minute or so). It could probably be neutralized better using a mild acid, like diluted vinegar, although you’d need to be careful not to cause a reaction (like the baking soda and vinegar volcano 😉 ). Although I don’t recommend touching it, if you do touch it and it feels slippery, you need to rinse/neutralize it some more.
The other thing you need to be careful of is the clear plastic piece that covers the LED bar on the front of the display. It gets fogged up easily from chemicals, and when rinsing, if you’re not careful it’s easy to get water (and rinsed chemicals) underneath the cover. If you do get moisture underneath there, if you push on one of the 4 clear tabs popping through the back of the PCB with a pointy object, you can get a small gap between the PCB and the cover, and then use a hair dryer or other heater to warm up the display, which will cause the moisture to evaporate and escape out the gap. If you got chemicals in there, I’d probably rinse purposefully underneath the cover, and then drain/evaporate the moisture. Of course make sure it’s completely dry before hooking it up.
Because of the problem of getting moisture under the cover, one thing that might work well is neutralizing the NaOH with a Q-tip dipped in a mild acid and rubbing just the spots that had contact with NaOH. Then use another Q-tip to rub wipe with water, and then dry. This should prevent getting water/chemicals underneath the cover when neutralizing, although you still need to be cautious when working with the chemicals in every step to keep them from making contact with the cover.
So… there’s my long winded post… I think I posted everything I tried, and all the important steps/parts, although there may be a few things missing. Every method done correctly produced working results, and I have no reason to believe that they will fail any time soon. As long as the cable isn’t physically damaged, either before or after you work on them, the rest of the hardware seems to be very durable. I’ve had a 100% success rate after properly reattaching the cables, although I also destroyed a few cables in the process. Anyway, I hope it’s interesting, and good luck if you decide to try any of the methods, and feel free to post any comments, and let me know if you’ve got any questions.
KR155E: feel free to post a link to this thread from the other thread, or merge it if that’s appropriate.
DogP
Cool, this looks interesting and impressive, although it also seems almost more difficult than the bypass method 😉
In that last method, do you get any solder in underneath the cable (between the cable and the pcb), or is the soldered connection restricted to the very edge of the cable? (and the whole top of it I guess, which would be enough…)
