LCD display splotches - Page 1
Forum section: electronics / general

author: floobydust
author_rank: Super Contributor
timestamp: September 05, 2021, 06:15:12 pm
content: These are reflective 7-segment LCD displays that are showing this abberration as a splotch from the center. It seems to get worse with age. Does anyone know what causes this?I thought the LCD fluid is only where the segments are and the displays work so it's not the fluid.Looking at the display using a polarized filter at different orientations doesn't show anything matching the splotch, it's either all black or the same as using no filter. I thought the polarizers might be causing it.The LCD sandwich is an aluminum backing and two layers of glass glued together, and a plastic polarizing film on the front. Made by AND Japan.During a heat wave this summer, maybe 30°C an ANENG AN8008 display showed the same patternBut it went away when things cooled down. Doubt they're usable in hot weather lol. This is what had me puzzled what is going on with LCD displays.

author: james_s
author_rank: Super Contributor
timestamp: September 05, 2021, 06:17:59 pm
content: I thought the fluid filled the entire gap between the glass sheets and the segments are defined by the transparent electrodes? Certainly in the case of dot matrix displays I can't see how you'd ever keep the fluid contained in the individual pixels, I had assumed segmented displays worked the same way.

author: Benta
author_rank: Super Contributor
timestamp: September 05, 2021, 06:56:38 pm
content: Quote from: james_s on September 05, 2021, 06:17:59 pmI'm with you here. The two glass plates are separated by a distance foil around the circumference, the fluid is fully between the plates.The failure mode here points towards temperature flexing of the plates, perhaps aggravated by atmospheric pressure change, or mechanical pressure on the surface.

author: Gyro
author_rank: Super Contributor
timestamp: September 05, 2021, 07:25:25 pm
content: Yes, it's uneven spacing between the glass sheets. The LCD has to be able to do a 90' shift in light polarisation so that light can make it through (and back) the crossed polariser films front and back.A variation in thickness of the liquid crystal film (or energising the segments) interferes with this, causing darkening.I don't know of any way of de-stressing the glass layers if it's not caused by mechanical pressure in the first place.P.S. I doubt you'd be able to see anything using an external polariser because what's happening is between the two existing layers on the display.

author: floobydust
author_rank: Super Contributor
timestamp: September 05, 2021, 07:38:27 pm
content: Thought maybe it's the glass "flowing" with age? The weakest part of the display is the center.Squishing it can make the LCD segments darken but has no effect on the splotch. Freezing it just seems to have made it worse, one segment was dead (thought zebra strip) and there is now discolouration around it. It might be a chemical reaction between the fluid and electrode. These displays have never been heated up.I was wrong about the construction, apparently it's an even pour across an entire display, one big pool.

author: james_s
author_rank: Super Contributor
timestamp: September 05, 2021, 09:32:52 pm
content: IIRC it's a myth that glass flows with age, very old windows being thicker at the bottom is a result of the process used to make the glass. I have numerous glass objects that are 80+ years old including a few vacuum filled lightbulbs that are even older and none are showing the slightest bit of distortion despite the pressure. Certainly something as modern as an LCD display is not going to have done anything like that.

author: floobydust
author_rank: Super Contributor
timestamp: September 06, 2021, 02:23:44 am
content: Something is causing what looks like dimensional changes and strain marks at the center of the displays.In the interest of science, I took the LCD apart a bit. Removed the rear aluminum reflector and front polarizer, using hot air around 120°C. Note, this might be a great way to learn hot air technique because the LCD fluid turns black when too hot lol.There is some discolouration at electrodes and then using a polarized filter I can see some internal etching between the electrodes. First thing came to mind is it looks like fluorites but not a branching pattern. Who knows what Indium-Tin oxide ITO does with the fluid (N-(4-Methoxybenzylidene)-4-butylaniline) and glass after decades.The one G segment is dead so the electrode must be open circuit.

author: james_s
author_rank: Super Contributor
timestamp: September 06, 2021, 05:04:36 am
content: I do not have a lot of expertise here, but it would not surprise me at all if the problem is oxidation or some other chemical reaction that has caused the electrode material to corrode or otherwise become thicker. Think about what happens when iron rusts, the rust is thicker than the metal it forms on and that causes moving parts to jam up. Given the tiny gap between the glass plates it wouldn't take much added thickness to distort one.