16 Apr
2007
16 Apr
'07
5:44 p.m.
At 5:32 -0500 4/10/07, Sridhar wrote:
Sridhar and Andy,
Good suggestions below. I have a couple of additions, based
on my (limited) experience working on the flight hardware on a couple
of pretty contamination-sensitive projects I've worked on. Sorry for
the delay, I was ( :-) ) working on one of the projects, albeit not
one at the clean-room stage, rather than keeping up with my email.
Basically, the purpose of the clean box is to ensure
that the air you're
working in is clean. The box should be made from something that won't
turn readily into dust, for example, it should *not* be made of
cardboard. Plastic works best.
Painted metal is what we use for practically all of our work.
With the right paint, it can be grounded (through resistors) to
eliminate the chance of ESD; that's hard with plastics. Also, most of
the instruments I've worked on are *very* sensitive to contamination
from hydrocarbons. That affects several of the
statements I make
below, and it precludes most kinds of plastic. Delrin, lexan, and
a
few others with extremely low outgassing properties are still OK, but
there are a lot of plastics that are not allowed in the clean room.
I expect this would be much less of an issue (except for the
ESD considerations) for something other than UV optics or
microchannel detectors.
Also, the box should have at least one clear side. I
used perspex for
the sides and top and thick rigid PVC for the base. It's important for
you to be able to see what you're working on.
Third, it's important to get the box sealed up well, so use plenty of
thick silicone sealant along the corners. And make real sure that the
silicone is completely and thoroughly dry and set up before you use the
clean box. It also couldn't hurt to join the corners on a miter. In
mine, I use a false bottom that holds together with some bolts to make
it easier to open and close the box, so I can get tools and items to be
repaired in and out of the box.
In my experience, sealing is a non-issue. In fact,
practically all of our clean rooms, clean boxes, etc. have open
apertures of one size or another, cracks, vents, etc. What *is*
critical is that the volume have positive pressure maintained inside
of it while any work is going on.
Generally, with a good high-volume but low-pressure supply of
clean air, all the dust is removed (blown out the apertures) within
about 15 minutes of starting the supply.
Of course, if the air supply fails or is shut off, we close
things up pretty well and pretty quick, to keep dust from migrating
back in. We also wipe down surfaces from time to time, so that
whatever dust does manage to accumulate doesn't get stirred up during
work. For wiping down, Isopropyl alcohol (um, propan-2-ol?) and
lint-free wipes ("Kimwipes" brand, in the US) are the standards.
One thing you may not have to worry about is the idea that to
us silicone is a four-letter word. Silicone oils are *very* easy to
inadvertently transport all over the work via accidental contact on a
gloved finger, *very* hard to remove, and fatal to several of the
detectors and optics we use. So we don't allow any of that kind of
material in the clean room as a general principle.
Fourth, it's a good idea to use an inert gas
supply. For reasons of
availability and price, the best choice might be nitrogen.
Danger, Will Robinson! I second Simon's caution - do not allow N2 to
accumulate in the volume you are breathing out of! Even though the
box is "sealed", N2 will be leaking out into the room. If enough of
that happens before the room air is replaced (by an open window, or
whatever) you die. We don't want that.
Bottled
nitrogen won't be completely clean,
A liquid nitrogen dewar is a good supply of clean nitrogen, but that
may not be such a useful suggestion ....
Meantime, with bottled, regulated nitrogen, you may well have issues
with hydrocarbons (if you care about that). Hoses that have been used
downstream (or even upstream) of vacuum pumps can have some of the
lubricating oil on the inside. Many regulators have grease or oil
which can get into the downstream side. If you choose to go this
route, you might consider SCUBA gear and plain compressed air (and a
filter). Most SCUBA equipment and air suppliers are pretty careful
about atomizing oil into divers' lungs, so that can help you.
In general, if it has a noticeable odor, it's got some sort of
hydrocarbon outgassing or contamination, and we would not use it.
Most folks have noses which give them a lot more information than
they pay attention to.
All that said, we don't use any of that for clean-room work. We use
*only* a HEPA (HEPA is a key descriptor, here) filter on the output
end of a good ventilation system. We are reasonably careful about the
input supply (ie no diesel trucks parking right next to the air
inlet), but that's about all. Paxton's comments (hey, if it's good
enough for pizza, it's good enough for a hard drive :-) ) are very
well taken here.
We do use a liquid N2 dewar to supply purge gas which is tubed
directly into some instruments to make sure humidity, fumes from
drying adhesives, diesel fumes from outside, etc. do not get into the
detector and UV optics, but that comes at a really low supply rate
and I don't think any of those considerations are applicable to hard
drives or to most electronics.
so an air filter is probably a good
idea. Don't use a paper one, obviously. I use a glass allergen filter.
A good HEPA filter on a high-volume air supply (ie one that exchanges
air in the work area about once every 15 seconds or so) is really
*all* you need, in my experience. Put the work upstream of the hands
and tools, wipe down the work area with IPA and kimwipes, let the
supply run for an hour or so before you start, and I think you should
be down to a pretty low level of particulates.
And the gas should go through a regulator. If it
comes out too
quickly, it could get supercooled, and that would not be fun. And don't
forget to create a gas outlet with a one-way valve, to prevent outside
air from getting in. If you don't put in an outlet, the gas will find
its own outlet, and that won't be good.
True, but see above. Letting the gas get out any (many) ways is OK,
as long as it's getting out fast enough that none or not much can
come back in.
Obviously, it isn't possible to work in a
completely sealed cube. You
also need a place to put your hands into the box to actually *do* the
work. The most reasonable method for doing this is to attach a pair of
gloves to the box itself. Again, getting a good seal between the box
and the glove is of paramount importance. Use a sealant material which
has enough flexibility to do the job. Also, it would be a good idea to
have a way of changing the gloves without tearing the box apart. I
mounted my gloves to the box on opposite ends of the box so that I can
get my hands to anywhere in the box.
Good for flexibility with the work, bad for staying downstream of the
clean air supply.
If anyone is interested, I can try to grab pictures of some of our
work setups around here. They are probably fancier than what you'd
want, though.
One thing I didn't think of until
after I had built the box is that the positive pressure inside the box
doesn't have to be very high, so you don't have to use gloves that are
very rigid. I used pipefitters' gloves the first time, and I lost a lot
of fine control. I suggest using something much thinner.
Experiment! It's the easiest way to figure this stuff out. And let me
know how it goes. I'd be interested to know. Good luck.
Peace... Sridhar
There are particulate counters, but not cheap. One idea for a test is
to set up your work area, lay a *clean* mirror in it facing up, and
leave it overnight. (or do some non-critical work in the area for a
few hours). Then look at the mirror under a really bright light or
(even better) a UV light. If it still looks clean, you are probably
OK.
I'm hoping Tony D. (and many others) chime in (or have already done)
on this thread; I suspect there's much experience out there more
applicable than mine.
--
- Mark, 210-379-4635
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