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protection against vibration

R

Ross Herbert

Jan 1, 1970
0
Hi,
On one of my PCBs i'm using a heavy IC. It is a 70pin gull wing
package. And, the problem is that the pins are breaking near the IC
package. I don't want to use RTV, as it's difficult to remove. What
best I can do to protect this against vibration tests?

Thanks and Regards,
Veeresh

If by "gull wing" you are referring to this package
http://workmanship.nasa.gov/lib/insp/2 books/links/sections/707 Gull-Wing^L Leaded Packages.html

then you might get some hints from NASA Workmanship page 8.02 under
Conformal Coating and Staking (Bonding) from here
http://workmanship.nasa.gov/lib/insp/2 books/frameset.html

My guess is that under vibration the body of the package is moving out
of sync with the pcb vibrations - and with differing amplitudes -
therefore stressing the leads and the soldered joints. Bonding the
body of the IC to the board in accordance with NASA recommendations
may help in ensuring the package moves in sync and with the same
amplitude as the pcb vibrations. A thicker pcb should also help
suppress uneven modal vibrations in the pcb itself.
 
M

MooseFET

Jan 1, 1970
0
If by "gull wing" you are referring to this packagehttp://workmanship.nasa.gov/lib/insp/2%20books/links/sections/707%20G...

then you might get some hints from NASA Workmanship page 8.02 under
Conformal Coating and Staking (Bonding) from herehttp://workmanship.nasa.gov/lib/insp/2%20books/frameset.html

My guess is that under vibration the body of the package is moving out
of sync with the pcb vibrations - and with differing amplitudes -
therefore stressing the leads and the soldered joints. Bonding the
body of the IC to the board in accordance with NASA recommendations
may help in ensuring the package moves in sync and with the same
amplitude as the pcb vibrations. A thicker pcb should also help
suppress uneven modal vibrations in the pcb itself.


The problem is more likely to be the flex of the PCB than anything
else. The resonant frequency of the package vs PCB mas spring system
will be very high. You have to get near the resonant frequency before
you get large deflections.

A PCB supported by standoffs only at the corners can get into many
very bad resonance modes. There is the obvious simple one. The PCB
can form an "S" shape parallel to either edge or running from corner
to corner in either direction. The standoffs can lean back and forth
allowing for modes where the PCB shakes right left, front back and
twistwise.
 
V

veeresh

Jan 1, 1970
0
The problem is more likely to be the flex of the PCB than anything
else. The resonant frequency of the package vs PCB mas spring system
will be very high. You have to get near the resonant frequency before
you get large deflections.

A PCB supported by standoffs only at the corners can get into many
very bad resonance modes. There is the obvious simple one. The PCB
can form an "S" shape parallel to either edge or running from corner
to corner in either direction.
Sir, thanks for the response, I suppose you are suggesting to put more
mounting points in the center. Please elaborate this S shape parallel
to either edga. Sorry for being unable to understanding your point...

The standoffs can lean back and forth
 
M

MooseFET

Jan 1, 1970
0
Sir, thanks for the response, I suppose you are suggesting to put more
mounting points in the center. Please elaborate this S shape parallel
to either edga. Sorry for being unable to understanding your point...

Don't put the extra mounting point exactly in the center.

Trying ASCII art of the edge of the PCB at one extreme of its flex:


****
** ***
** ** ** <--- edge of PCB
! ! *** *** !
! ! **** !
! ! ! <- Stand off

Notice how if the board flexes like this, the standoffs won't do much
to stop it.

If you placed the middle standoff a little off center, the standoff
will stop this mode of vibration.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
use ty wraps or mount a pressure plate to keep it in place RTV is a rubber compound will help not eliminate your problem.
 
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