Conventional
wisdom suggests that as we go to higher and higher standards of cleanliness
(Class 1000, to Class 100, to Class 10) we should be getting fewer and fewer
defects in the photolithographic structuring of PWBs that are related to
contamination of the yellow room. A look at the ISO (and FED) cleanroom
classification definitions suggests that as we go to cleaner rooms, for all
practical purposes only sub-micron airborne particles are tolerated in limited
concentrations.
The likelihood of a
particle remaining suspended in air depends upon its size, shape and density,
but in general, particles that are larger than 1 micron don’t remain suspended
in air very long, even under conditions of turbulent flow. Now, the particles
that we worry about most, are those that are of a size comparable to the
circuit feature size. For fine line circuits, e.g. 25 micron lines and spaces,
we would worry about 7 micron size or larger particles because they could cause
defect sizes in the range of line and space violation limits (e.g., 30% of
nominal feature size). But these particles are typically not airborne and
therefore not measured. And cleanroom classifications don’t address particles
larger than 5 microns. Not only that, going to more stringent classifications,
i.e. spending more money, only reduces particle count of particles that don’t
matter. It seems to me, what matters is the elimination of dragged-in large
particles on the board surface, and the deposition of large particles that
originate from human interaction.
US FED STD 209E Cleanroom Standards
|
particle/ft³
|
|
Class
|
0.1 µm
|
0.2 µm
|
0.3 µm
|
0.5 µm
|
1 µm
|
5 µm
|
|
1
|
35
|
7
|
3
|
1
|
|
|
|
10
|
350
|
75
|
30
|
10
|
1
|
|
|
100
|
3500
|
750
|
300
|
100
|
10
|
1
|
|
1,000
|
|
|
|
1,000
|
100
|
10
|
|
10,000
|
|
|
|
10,000
|
1,000
|
100
|
|
100,000
|
|
|
|
100,000
|
10,000
|
1,000
|
NOTE: US FED STD 209E was
canceled November 29, 2001.
Reference: http://www.iest.org/publctns/fedstd209.htm
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