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Author Topic: Replacing cracked glass (really, more than just a tip)  (Read 1751 times)

Offline Nicabod

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Replacing cracked glass (really, more than just a tip)
« on: June 15, 2013, 03:40:10 PM »

Yrs trly was a fool not to buy a bump absorber. Mine fell face down, about a yard/meter, onto vinyl tile over plywood.

I've been reading about replacing glass before it happened, and have come to some tentative conclusions. These are not necessarily correct, and I'd appreciate corrections. Please consider this a big bunch of maybes that could be bogus!

The digitizer is carried by the surface (underside only?) of the glass, and just about has to consist of quite a few X and Y lines running across and vertically. These are totally-transparent, electrically-conductive patterns, very likely made of indium & tin oxide. They must have oodles of transparent insulating crossovers. Any crack is guaranteed to create an open circuit -- a disconnect.

I've read of separation problems in early N 7's, but mine (Dec 2012) has looked just beautiful. I've also read that the digitizer is bonded to the display. (My display seems totally OK, alas.)

When my glass broke, all the pieces remained adhered, and none rose up above the rest; running a fingernail across the cracks showed that they remained in place remarkably well. Good bonding, eh?

What follows is my best guesses about the bonding between digitizer and display. If you slog through it, you'll see why it would be a better bet to replace both digitizer and display as a unit.

I also had read a comment about a thin (hot?) wire being used to separate the front glass from the top of the display. Perhaps 36, 38, or 40 AWG Nichrome might work. Idea is to melt the adhesive; no hotter than needed, though. However, once the glass is removed, you're likely to have a bumpy surface on the display, unless the adhesive is really thin. Hmmm... Careful use of a hot air gun? There's a risk of destroying the display. (For the hot wire, a low-voltage stepdown transformer fed from a Variac or equivalent is what you'd want.)

I had known about ultraviolet (UV)-cured adhesives, some optically clear, and I'll bet that when Asus had separation problems, that's what they adopted. The adhesive is a liquid to begin with, and cures solid when UV light hits it. Black light might be enough.

If this is indeed the case, the hot wire might work, but once it moves, the adhesive might cool and re-bond. Try pulling a thin Teflon sheet behind the wire? Hand-holding the wire does not seem sensible. A frame would be better. I've mentioned that the display surface might not be flat when done. Perhaps the adhesive is soluble in some industrial solvent, such as that used to bond acrylic, but that might not be true. If so, though, you could clean up the display surface.

You might as well bond the new digitizer with the same type of adhesive. You don't find it at Radio Shack, probably not Frys (pronounced "friss"?), TigerDirect,  Newegg, Mouser, etc. but conceivably Digi-Key. Think optical supplies, not electronic. Edmund?

Once you have some, more than you need, applying just enough takes real talent, I'd judge. No bubbles!!! At the factory, they probably have an automatic dispenser which puts out an exact quantity, and applies it in a pattern (perhaps) that will minimize the likelihood of bubbles. (Wild guess?) Wouldn't be surprised if the digitizer glass is (also) held in a frame, perhaps with a little air cylinder to lower it just so, nicely aligned. Yes, aligned; consider that, too.

If you do it, spillout at the edges needs to be allowed for. Maybe you could wipe it off, carefully. Practice with hmmm... some olive oil, then scrupulously clean it off? No traces left behind, now! Bad idea...

Once you have a bubble-free layer, assuming that black light triggers setting, be scrupulously sure you have the digitizer aligned with the display, then, shine away! There are fluorescent light tubes made with deep violet glass that blocks almost all visible light. The phosphor apparently converts short-wave UV within the tube into long-wave UV, black light, but it apparently puts out a good lot of visible light too, which is absorbed by the glass. Black light is easy on humans; it's used in nightclubs (or was) with fluorescent paints and such. It's the shorter-wavelength UV you need to be very careful about. Afaik, UV-cured adhesives and coatings can use black light; not sure.
Safety UV-blocking goggles and an old EPROM eraser for short wave, but don't try tanning your skin; cover it. That light is bad.

I read somewhere that Asus charges $170 to repair cracked glass (apparently in Grapevine, Texas). If all my speculation is anywhere near the actual procedure, I can't exactly blame them. They might even pay their technicians good money. Why would someone pay so much? To get at the stored data. Seems to me that there's a market for motherboards minus the flash memory chips. Desoldering them from an N 7 with a cracked face is reasonable, but not for amateurs. They have lots of closely-spaced leads, for one. Many people in industry know how to do this; the equipment is too costly for hobbyists, and is likely to use hot air to reflow the solder when attaching.

Business opportunity? After all, there are quite a few tablets out there, and a good supply of inept people, incl. yrs trly.

+=+=+

It seems, and I might be wrong, that selling digitizers alone, intended for replacing broken ones on a Nexus 7, is quite irresponsible. A very few development projects could use them, but engineering samples from the manufacturer serve that purpose. Perhaps their prices will nosedive; dunno.

Agreed, buying an 8 GB N 7 and swapping digitizer and display seems like the way to go. I surely hope there won't be separation problems! Swap the metal edge (bezel?), too. Unfortunate to consider what's left over, nice stuff.

Again, this is speculative, but informed, to some degree. Yrs trly was an electronic tech for most of his life, and an associate editor at Electronic Design magazine in the late 1970s.

Good luck to you!

[nb]



Offline matt

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Re: Replacing cracked glass (really, more than just a tip)
« Reply #1 on: June 15, 2013, 04:51:12 PM »
I had some experience with uv cure inks for silk screening, the solvents used seemed more toxic than any consideration of melanoma, although the lamps were shielded with the parts on a conveyor belt.
I was thinking the digitizer might be bonded at the edges but if the broken glass is staying put there might be a membrane or you could be right on the full surface adhesion. Hoping you can find a cheap unit when the 2nd generation comes out. Good stuff.

 


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