>
>Yes, because you are right. Taking it beyond what it can possibly survive
>will cause it fail, and  under normal conditions that failure mode might not
>ever be seen. You do need to apply some common sense. There are physical
>limitations, and I do realize that. But, if the crystal is the first thing
>that fails in your HALT test, it's a pretty good bet that field failures
>will also show a failed crystal. Whether or not that came about due to
>vibration. Of course, if vibration is the only thing that causes the crystal
>to fail, then yeah, the excess stress in the test is not really valid.


Case in point.
The crystal's most likely failure mode in this case is actually slamming
into the case walls internally.
This isn't something that's going to happen a little at a lower acceleration.
It won't happen at ALL until you exceed a threshold acceleration.
If the part won't take the stresses it's going to get in the field, then
spec a different part, or protect it from the stresses, but don't take a
part rated for X, and expect it to withstand 2X for half the time (or even
any time with some parts)

>But we don't make delicate lab instruments. We make plumbing tools.
>Construction crews drop tools all the time (from ladders, of course). The
>tools get wet. They get extremely muddy and dusty. They are left in the back
>of a truck in the middle of winter, then taken inside a heated building. Or
>they are left in the truck in the middle of summer, and get taken inside an
>air conditioned building. All of these stresses add up over time and shorten
>the life. Again, we're after time compression. Get them to fail early so you
>understand how they fail, and fix it before it ever gets to a customer.

We had a lot of the same stresses applied to credit card terminals. :)

Well.. Some of the stresses add up, some don't.
Let's take a glass display tube.  You either stress it to breaking, or you
don't.
You can stress it all you like below breaking, and nothing's going to
happen to it.

OTOH, socket pins, and bad solder joints fatigue out really well.


>Perhaps you will see a way to cushion the crystal, or even the whole
>assembly. Perhaps you choose a different crystal, or switch to an RC if
>timing is not critical. And by taking the crystal beyond its specs, you will
>likely encounter something else that breaks.


Again, to a point I agree with you, but then it becomes "well Duh!"
You have to be sure that these additional stresses are actually causing the
same failures that you see in the field. Otherwise, you're just building
extra cost into the product.



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