Cleanroom technology Was: hard drive data recovery

From: Christian Fandt <>
Date: Tue Jul 14 15:46:50 1998

At 18:51 14-07-98 +0000, you wrote:
>Allison (>>) and Tony (>) wrote:
>>> media you may pit it some or at least remagnetize a small area, disaster
>>> for servo tracks.
>>> FYI/note: Why do ALL drives spin up then load the heads? It's to allow
>> A lot of winchesters land the heads on the platter - and not always in a
>> special 'landing zone'. This implies the heads start flying as the disk
>> gets up to speed.
>> It also implies that the heads can land on the platter without damage. On
>> the other hand, there's a difference between the landing when the drive
>> spins down, and crashing due to hitting a dust particle.
>>> the airflow from the spinning media to establish the clean airflow inside
>>> the drive. Also any dust on the active portion of the media spins off
>>> media and is then captured in a filter.
>> Agreed...
>A lot has been said about dust in hard drives, most of it assuming that
>the dust is dry. But only one person (sorry, can't remember who) has
>talked about other things.
>Remember, not all pollutants are dry. Some (water, spittle) are wet,
>and some are quite sticky (ever parked your car under lime trees?).
>These will mess a hard drive up something cruel!
>Yes, if you take reasonable precautions, you may well be OK. But one
>day, if you haven't got a clean room, you will come to grief...
>BTW, I have two questions. What does HEPA stand for? And (Christian

A HEPA filter is: High Efficiancy Particulate Air filter. A HEPA filter is
built so that 9.97% of particles 0.3 micron and larger are removed from the
air passing through. You see HEPA filters more and more in consumer related
things (at least I have in the States). Example: Air filters in many modern
automobiles, possibly all autos, use a HEPA filter for engine air intake
filtering. Clearances between engine parts are closer nowadays which
improves engine performance and efficiency plus smaller engines are
stressed harder in relatively large modern cars, both thus requiring really
clean air intake.

Also, I think I've seen an advertisement for a fancy grade household vacuum
cleaner advertising it has a HEPA filter on its exhaust. Less allergens
blown about. We use Nilfisk-brand cleanroom vacuum cleaners at work. Made
in Denmark I believe. Cost around US$1000 each but they're meant for use
*in* cleanrooms and *must* not throw junk about from their exhaust.

>will parobably be able to answer this) how do you grade clean rooms? I
>take it it has something to do with the size of the largest particles
>you'll find in the air and/or how many of them you have...

Yes, you are correct re having something to do with size and number of

Cleanroom grading is based on Federal Standard 209E. I have the rev. "D"
document around the office buried somewhere in some papers. This standard
seems to have been adopted outside the USA too. Cleanroom technology has
been around a long time in pharmaceutical manufacturing and medical fields
and adapted to semiconductor manufacturing in the last 20-30 years.

Anyway, here's a short classification table from 209 (hope the formating
comes out okay w/text-only displays!):

Max. number of particles Class: Max. no. of part.
per cubic foot(per liter) English system per cu. ft.(per l.)
0.5 micron and larger (metric system) > 5.0 microns

100 (3.5) 100 (3.5) *
10,000 (350) 10,000 (350) 65 (2.3)
100,000 (3500) 100,000 (3500) 700 (25)

How is this measured? A particle counter is used. These machines typically
use light-scattering to count the particles passing through a special
optical chamber with specially ground curved mirrors.

The class number is obtained by pumping sampled air through the chamber at
a rate of one cu. ft. per minute. A low power He-Ne laser is used in our
particular unit as a stable, coherent light source. Its beam passes through
the chamber at a right angle to the air flow. To the side of the path of
both the airflow and laser beam a detector array is mounted. When an air
particle passes across the beam path, light is reflected off from it to all
of the mirrors and bounces to the detector. Our machine has a linear CCD
array. When many particles pass across the beam this chamber acts as an
opticle integrator and the light from all the particles is 'seen' by the
array. The higher the output from the array, the larger the quantity of

Also, a relative size of the particle is also established. A group of cells
on the array may have light hitting them dependent upon the particle size
to yield an integrated value equivalent to a particular size. The analog
output is run through an A/D converter and processed by one of the
venerable Z80 CPUs and sent to a CRT display and printer. (I told you in
the previous post that the concept was simple yet the device was a little

To sumarize the determination of the count:
Take exactly one cubic foot of air and pass it through the counter during
exactly one minute and count the particles.

Therefore, a class 10,000 cleanroom has less than or equal to 10,000
particles in one cu. ft. of air that you can count during one minute which
are 0.5 microns and larger. Same for class 1000, etc. Typical modern
cleanrooms at chipmakers are class 100 or class 10. There's even a few
*Class 1* cleanrooms around (Dept. of Defense is believed to have had one
of the first built several years ago.)

Some drive makers have advertised they build their drives in class 100 or
class 1000 cleanrooms. That yields excellent quality control in that
failure from physical defects is held much lower.

Imagine this: I recall once reading that the normal air we breathe, when it
is extremely clean and quite clear, has a billion or more particles per cu.
ft. larger than 0.5 micron. The number skyrockets when plant pollen, smoke
from fires, mold spores and many other particle sources are included. I
have allergies to some airborne things and I feel great when I'm working in
the cleanroom. Ours tests to less than class 100 though it is designed for
10,000. No wonder I feel better!

During a dry winter day, as we walk along, there are literally billions of
skin and dust particles shed from our person nearly all of which are too
small to see. Imagine such stuff falling in our modern hard drives where
the flying height of the heads is in the tens of microns range. This is
only one example of the sources of contamination we'd have to watch for in
our homemade cleanrooms/cleanbenches.

>Oh yes. I take it "Tobacco for chewing only" goes without saying ;-)

Just so the spit from yer chaw ain't blown all over! <g>

I guess this turned out to be part of a cleanroom primer after all.

Regards, Chris
-- --
Received on Tue Jul 14 1998 - 15:46:50 BST

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