28 Dec
2006
28 Dec
'06
6:56 p.m.
At 05:26 PM 12/28/2006, you wrote:
X-rays can damage ICs. While searching for
information
on possible risks to electronics from x-ray security
screening, I encountered a second-hand reference to a
study to determine the safest X-ray wavelengths and
exposure for nondestructive testing of
high-reliability
ICs. X-rays can inject faults into the silicon
lattice,
which degrade the quality of the transistors. I think
the
exposure that can be tolerated is affected by geometry
(e.g., transistor size), so it might not be as much of
an
issue for older electronics as it is for newer stuff.
Anybody know more?
It was explained to me that the x-rays poked holes between silicon
junctions which reduces conductivity and because of that the power
dissipation kills the device.
A study of the AVR mega103L shows that the device will literally "cook"
itself before it is too damaged to continue operating. It can be used
beyond the "cook" threshold by using a current limiting resistor. This
threshold was somewhere in the 8,000 rad range for the AVR. I have the
full study if you guys are interested in me posting it.
I've read older documents that mention 50,000 rad before a PROM is
erased. So there must be a gate size variable like you said. Most
documents refer to putting the EPROMS right under the port. My board was
36" awa and was exposed to 17.8R/min. When ever you half the distance from
the tube the radiation is increased 4 times (the inverse square law). So
at 2.25" away from the tube we would have somewhere around
4,500R/min. This would make it get erased MUCH faster than at 36" like I
had it.
I've personally had an electronics board partially exposed to a linear
accelerator. The Apple IIe board was scanned at 17.8R/min. The linear
accelerator does a little over 3,000R/min at ONE METER! : ) We were
imaging 8" thick steel blocks.
The board was shielded with a few inches of lead, but its dose after a few
hours of imaging was damaging. The MAX232 chip seemed more damaged than
anything else, we trashed the whole board. It DID actually still work, but
its not worth taking a chance... There were some weird symptoms when ever
the linear accelerator was ramped up, but that could have been from the
dual magnetrons taking 30,000v at 30A pulses to 6MeV. The main power coax
cable is 1.5" diameter carrying 10,000v at 100A, but they said it was very
well shielded.