Now, I don't know the construction/nature of the tubes used
in scanners, but I myself would be wary of photocopying or
scanning a PC board with an unprotected (i.e. uncovered) EPROM
window face-down on the scanning surface. My reasoning is
as follows;
Although a fluorescent tube in good condition shouldn't emit
much UV, one has to remember the way such a bulb functions.
The excited gas inside the tube emits almost entirely in the
UV spectrum. This is converted to the visible spectrum by the
phospor coating on the inside surface of the tube. Over time,
I have seen some of the coating flake off the inside of old
flourescent tubes, providing a bunch of small UV "windows".
Most UV Eproms begin to erase when exposed to UV wavelengths
shorter than 4000 A (Angstroms). I found the following
statement concerning EPROM sensitivity to UV light on page
10-9 of the National Semiconductor 1984 "CMOS Databook" (and
yes, I _am_ a packrat when it comes to old databooks :-)
concerning the 27C16 (a very common older type of EPROM):
Erasure Characteristics:
"... It should be noted that sunlight and certain types of
fluorescent lamps have wavelengths in the 3000 A - 4000 A
range. Data shows that constant exposure to room-level
flourescent lighting could erase the typical NMC27C16 in
approximately 3 Years, while it would take approximately
1 week to cause erasure when exposed to direct sunlight..."
Also, one must keep in mind that the timeframes listed
above are probably derived from studies of "freshly programmed"
eproms. Most of the ones we would be interested in would
already have a number of years of charge decay under their
belts. I was unable to find a reference to a finite lifetime
for UV Eproms, but I seem to vaguely recall that there is
a limit (~10 Years? sounds short, but I seem to remember
reading it somewhere. Does anyone have this number at hand?)
The end result of all this is that if you have any equipment
you consider valuable, and it has windowed EPROMS, make certain
that the windows are covered with an opague sticker. You might
also consider dumping their contents to a data file for archival
purposes. (and possibly re-programming them to effectively
"refresh" them).
-al-
-acorda_at_1bigred.com
> -----Original Message-----
> From: Tothwolf [mailto:tothwolf_at_concentric.net]
> Sent: Wednesday, May 15, 2002 4:16 PM
> To: cctalk_at_classiccmp.org
> Subject: Re: [CCTALK] [CCTECH] scanners & circuit boards...
>
>
> On Wed, 15 May 2002, Jeff Hellige wrote:
>
> > What's everyone's thoughts on placing circuit boards directly on the
> > bed of a scanner for imaging? Any possibility of damage to
> the board
> > from the light or other parts of the scanner? I've done it before
> > with good results but not with anything truly unique.
>
> I'd be more concerned with accidentally scratching the glass
> bed of the
> scanner due to sharp component leads. I guess that there is a slight
> possibility of degrading the contents of an EPROM if its window isn't
> covered. The fluorescent tube the scanner uses to illuminate the bed
> really shouldn't be emitting too much in the way of UV or
> near UV light
> though. Other than UV erasable components that don't have their window
> covered, I can't think of anything else I'd be concerned about.
>
> -Toth
>
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>
Received on Thu May 16 2002 - 15:13:08 BST