Hacker News Reader

Comment by Osmium | Hacker News Reader

The problem isn't with glass, it's with the wrong glass.

If anyone hasn't seen the promotional video for Corning's new Gorilla glass, they should:

http://www.youtube.com/watch?v=wT8xI4PEU8c

There is no reason why a glass back needs to be this fragile. Shattering from thermal shock is absurd, and the glass must be under some kind of stress from how it's secured to the phone. You can rightly disagree with the use of glass in an iPhone, because of course it's better if a phone can survive a drop, but at least it's not going to break just from being there.

View on HN

It's probably more likely that aliens took his phone, broke it, and placed it back on his counter, than the glass having shattered from thermal shock.

Here is a more likely scenario (that my friend the materials engineer gave me):

Gorilla Glass is made by "stuffing" ions into the empty spaces so that the glass is under very high compressive stress. It's really known as "High ion exchange glass" This in turn, gives it very high compressive strength. However, it does have a significant downside that never gets mentioned: Shock absorption and scratches can cause later cracks and shattering. This is just a fact of life. When you place the outer layer under compressive strain, and then begin to compromise part of the outer layer, or begin to stress it, you can eventually cause release of those compressive forces. Once partially compromised, it may seem to be good, but can fail instantly.

For further information and examples, see Prince Rupert's Drops.

http://en.wikipedia.org/wiki/Prince_Ruperts_Drop

>It's probably more likely that aliens took his phone, broke it, and placed it back on his counter, than the glass having shattered from thermal shock.

Completely agreed. That's exactly why I said "shattering from thermal shock is absurd, and the glass must be under some kind of stress from how it's secured to the phone" in the parent comment.

To further your explanation a little, the reason the outermost layer is put into compressive stress is that, when it's scratched, the stress will cause the crack to "close up" and so the glass doesn't shatter. Unfortunately, the interior of the glass is under tensile stress as a result, and so if the scratch happens to be deep enough to reach that tensile-stressed layer then the glass will shatter. Putting the glass in a case which alters this carefully managed stress state, i.e. by trying to attach it to a rim that's acting to pull it slightly apart, is what can cause this kind of failure.

But this is all speculation on my part, and even if true the reality might be more subtle.

Edit: I just found a good article from AnandTech that explains some of this. Take note of the first figure in particular.

http://www.anandtech.com/show/4225/the-ipad-2-review/13

I suppose it is a testament to Corning's marketing that they are held in such high regard, but doesn't the Nexus 4 have the new Gorilla glass? Wired claimed it did: http://www.wired.com/gadgetlab/2012/10/nexus-hands-on-with-g...

It's not unlikely that they only went with Gorilla glass for the screen-side of the device to cut costs, though I know the article says otherwise, so maybe not. They could've also mounted it in a way other than what was designed (i.e. put it under tensile stress).

As for Corning, in fairness to them they were instrumental in the development of this type of glass and own a fair few patents, so it's not just marketing. They developed it originally in the 60s, though I imagine a lot has changed since then.

Scratch resistance and toughness are orthogonal to each other. Even diamonds will crack when applied with a certain force.

From what I understand, Gorilla Glass is only scratch-proof, not tough.

It does, on the front and back [1].

I took a set of keys, a fork and a pocket knife to the front and back glass panels of the Nexus 4 and couldn't get a scratch to show up anywhere. I also slipped and dropped the phone while pulling it out of my pocket this week, and it showed no signs of my fumble.

Until I see a video, I'm not really convinced, tbh. If he has a granite counter-top, and didn't set it down as gently as the author claims, it's completely probable that it cracked. People are incredibly clumsy with phones. Glass doesn't exactly have a high thermal conductivity (nor does granite/linoleum/plastic), I find it difficult to believe the glass would expand/contract enough from body heat/kitchen counter-tops.

[1] http://www.wired.com/reviews/2012/11/google-nexus-4/

People are incredibly clumsy with phones.

Which seems like a pretty good argument for not wrapping them in glass.

Too often in technology we build things that are fragile, and then blame the user when she breaks them. Tools should be designed to work with the user as they are, rather than as we would wish them to be.

I disagree. It's how the glass is attached to the phone.

This is just a theory, but I've discussed it with People Who Know Things (TM) and they all have agreed that it's more than likely a small sliver of plastic or glass (or maybe just a tiny dirt particle) has become lodged in the space between the glass and the phone during fabrication. 9/10 phones don't have these defects, but when you're manufacturing millions of phones 90% still leaves a lot of bad phones.

I've never seen anybody use an iPhone 4 without a bumper due to AntennaGate. The bumper keeps the glass from coming into full contact with a surface, and that's probably why the iPhone 4 seems to be less affected by this problem. The lesson appears to be to always use a bumper with glass backed phone.

I don't think you necessarily need a case to benefit: I don't use a case but slipped on a marble floor once and dropped the phone straight onto the floor, causing it to bounce a few times. The rim of the glass dented (what looks to be a thin plastic coating on the side of the glass?) but nothing shattered.

In any case, IIRC, speculation is that only the screen side of the iPhone uses Gorilla glass, but I'm not sure if that's changed with recent generations. But even without Gorilla glass, you can still use "standard" toughened glass that, correctly mounted, should never shatter just from the sort of thermal shock described in the article.

I have a 4S that slipped from my pant pocket onto a rough concrete floor from sitting height. The rough surface dented the plastic trim, causing the glass to shatter. I believe they went with the plastic trim due to the problems of joining glass and metal. But the plastic trim is the iPhone 4's Achilles' heel.

Not sure whereabouts you're from, but in the UK AntennaGate wasn't particularly concerning and I know plenty of people that kept their iPhones case-free, myself included, without glass shattering issues. Unless they were really, really silly and dropped it onto concrete from high up.

This is complete conjecture. Plenty of people use their 4s 4Ss without bumpers or cases with no problem, including me. It has be a design flaw outside of the base materials involved.

That's anecdotal though. My wife broke her 4S twice. What does that say?

Twice I've dropped my un-cased iPhone 4s from a height of around 1 meter onto a solid tiled floor, and to my surprise not even a scratch.

Maybe I just got lucky, but in my limited experience, the case is not needed to protect the glass.

Most phones will usually survive such a drop. It all depends on how it lands. Phones with a glass back have double the number of ways that they can break when they fall, but it's still less likely to break than not. Drop it enough times and it will eventually break.

But the OP is talking about breakage due to thermal shock, not to physical shock.

The glass on the iPhone is actually quite resilient. The problem is the drop on one of the 4 corners (technically 8). At least that is my theory. All of the broken glasses I've seen have started at one corner and radiate out.

If we want to get anecdotal I used my iPhone 4 from launch with no bumper (even after I got the free one). I even dropped the phone three feet directly on it's top left corner and only the dented the metal band.

The sound design on that video was brilliant.

Reply on news.ycombinator.com