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November 1998

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Subject:
From:
Eric Christison <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 11 Nov 1998 15:47:41 GMT
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> We have successfully placed 4 off 304 pin Programmable Logic
> Device. However, during vibration and 1/2 metre drop test, the
> legs have deformed in the direction of the drop and some of the
> corner legs fractured at the bend or at the joint to the body.
> These are relatively heavy devices but with 304 legs, we are
> surprised at this result. We cannot redesign, so have to solve
> the problem, can you help?
> We have thought of perhaps glue dotting under the device prior to
> placement, of at the corners after for support.
> Anyone had the same or similar problem, and if so how did you
> solve it?

Bridget,

I suspect you carried out the drop test after the vibration test. If I'm correct I'll build another
assumption on top of that one. I suspect the legs are cracking due to fatigue. If the board flexes a
lot a large device is susceptible to this kind of failure because the relative movement between the
device and the board is large.

Given the above is true have you studied what goes on during vibration? I suggest you carry out a
sinusoidal sweep test between the lower and upper frequencies of the test spec. Mount an accelerometer
on the device and graph the response vs frequency. If you make the excitation amplitude equal to 1g
things will be kept simple. Also, while you're running the test observe what happens with strobe light
by pausing the sweep every time you hit a resonance and slowing down the action to a speed you can
observe. I bet you'll find it interesting!

If you get outputs from the accelerometer greater than, say, 10 times the input at the lowest
resonance of the board (ie with a 1g input to the vibration table the accelerometer sees >10g) at
frequencies below about 80 Hz then you may well have found the problem.

There are two ways to overcome the problem. Increasing the lowest board resonance will reduce the
amplitude of the board/device movement. You could do this by supporting the device near the centre -
this should double the fundamental. Or you could try and stiffen the existing supports.  If the board
has a free edge try adding a stiffener along this edge.

The other option is to move the component to an area of relatively little movement. That's where the
strobe comes in. You'll probably see that the areas of least movement are at the constrained corners
of the board.

Hope this helps.




Eric Christison

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