How do those ribbon cables that connect LCDs work? Sure, if you’re having trouble with missing pixels on a character or numeric style LCD, you know the ribbon cable to the circuit board is connected with hot glue, so it’s merely a matter of reheating with a hot air gun and pressing down with a pencil eraser (be careful with the abrasive!) to seal it back down to a nice connection. But what exactly are all the materials?
The Wikipedia articles are only vaugely informative, they lack any photos that look consistent with what I’ve been looking at.
20200625/https://en.wikipedia.org/wiki/Liquid-crystal_display
20200625/https://en.wikipedia.org/wiki/Tape-automated_bonding
20200625/https://en.wikipedia.org/wiki/Elastomeric_connector
20200625/https://en.wikipedia.org/wiki/Anisotropic_conductive_film
After searching on Digi-Key, I got some hints. What I’m looking for is called a “heat seal.” Going back to the Wikipedia articles, indeed, I found a word-drop of “heat seal connector,” so I search for that, and tada! I found lots of great technical information. Unfortunately, Digi-Key appears not to sell the bare LCD modules where you must attach the heat seal connectors yourself.
20200625/https://www.digikey.com/product-detail/en/lumex-opto-components-inc/LCM-S01602DTR-M/67-1781-ND/469805
20200625/https://www.lumex.com/spec/LCM-S01602DTR-M.pdf
20200625/DuckDuckGo heat seal connectors
20200625/http://elform.com/
20200625/http://elform.com/bond_parameters.html
20200625/http://elform.com/peel_strength_spec.html
20200625/http://elform.com/designguide.html
20200625/http://www.lcd-factory.com/html_products/Heat-Seal-Connector-161.html
Oh yeah, and about attaching the heat seal connectors. LCD modules come from the manufacturer without any ribbon cable attached to them at all. The electrical traces are drawn directly on the glass substrate using a transparent conductor, typically indium tin oxide. That’s why if you look carefully at the LCD modules, you see the ribbon cable ends on the glass surface and you can’t see any traces continuing any further. Of course you can’t see any, because they used a transparent conductor. So, first of all, you want to be careful with the LCD end of the attachment not to scratch or rub away the transparent conductive traces on the surface.
20200625/https://en.wikipedia.org/wiki/Indium_tin_oxide
Don’t worry about this material’s melting point, it’s 1526 to 1926 degrees Celsius.
But, the point is, the heat seal connector is indeed heat sealed on both the LCD itself and the printed circuit board. If you hoped you could get away with only needing to reseal on the printed circuit board side, well maybe you’re right, that’s probably where most problems develop. But you might also need to reseal on the LCD glass side too.
But, for the technical parameters, this is what you need to know. You need at least 140 degrees Celsius of heat. Now, here’s where Wikipedia dropped in to be useful. Older ribbon connectors needed higher temperatures, 178 to 180 degrees Celsius… so if you’re wondering why you may have an older calculator manufactured in the 1990s that still works just fine, but a newer calculator manufactured in the 2000s that has got trouble, that would be it. Also, be careful not to exceed 180 degrees Celsius because then air bubbles can break out of the bonding material and make it unstable. The old-style bondings may have been thermoset, but newer bondings are all thermoplastic so they can be reheated for repairs.
The plastic part of the ribbon itself is made out of polyethylene terephthalate plastic (PET) and the conductive traces are usually carbon-based such as graphite. The actual adhesive, well that’s not actually just hot glue, that is electrically anisotropic adhesive, essentially hot glue with conductive particles suspended.
20200625/http://elform.com/ANISO.jpg
All of this being said, a final important point is worth reiterating. If the bonding material epoxy, hopefully it is more temperature resistant and you won’t have problems. But if you do, reheating will not solve your problems because thermoset plastics cannot be melted and reused as such. So, chances are if you do have problems that occurred not from any particularly harsh handling, it is due to the thermoplastic properties causing the material to get soft in high temperatures, to the point you no longer have a good electrical connection. But, the good news here is that you can fix it somewhat easily.
A very important note in the elform page! “HSCs [Heat Seal Connectors] can be tacked into place before final bonding by touching them with a soldering iron set at 160°C.” Yep, you’ve seen it said right there. Alternative connection mechanisms do have their merits. Well, if a soldering iron is okay for tacking into place, so is a hot glue gun, I’m convinced. If you want alternative heat up and press-down methdos, you can try a hot glue gun or soldering iron with aluminum foil, this will give you both heat and a semblance of padding push-down.
Still wondering about the hot glue temperature equation? Oh, sure I am. Okay, so this was something really interesting to find. You can soften some kinds of low-temperature hot glues below 100 degrees Celsius. Wow, the glass transition temperature can be even lower than it is generally touted to be.
20200625/DuckDuckGo does low temperature hot glue soften below 120 celsius
20200625/https://charlesarcheryblog.wordpress.com/tag/what-temperature-hot-melt-glue-soften/
Also, on the other side of affairs, what temperature does a hot glue gun actually operate at? 120 degrees Celsius is the rated melting point of the low-temperature hot glue, not the temperature of the hot glue gun itself. Conceivably, for good performance and easy workability, the low-temperature hot glue gun itself is a little bit higher temperature than this. All the better for using to reheat the bonding material.