The Five-Second Rule
Hunger strikes! After exploring the near- ly-empty fridge, you slather all the peanut butter over the last slice of bread; and promptly drop the whole thing on the
floor, peanut butter side down (naturally). As
your tummy grumbles, you wonder: Does the
five-second rule hold?
While you ponder the possible demise of
your snack, let us consider the implications
for contamination control and critical cleaning. In the case of contamination control, the
issue is in attempting to relate Cleanroom
Classification with deposition of particles on
the surface of the product. A part or component arrives for final assembly in a cleanroom.
Perhaps the parts are received appropriately
pre-cleaned relative to a specified level of cleanliness, sealed in packaging that meets specification. There may be initial receiving inspection
or some other non-destructive test. This means
opening the package and exposing the component to the surrounding environment; as
soon as this happens, there is the potential for
contamination. Can you assume that for a given
exposure time the number of particles deposited
on the part can be directly calculated from the
class of the cleanroom?
Suppose you were doing receiving inspection
in cleanroom A; from years of experience, you
could leave the part sitting out for one minute.
You have the opportunity to do receiving inspection in cleanroom B, where cleanroom B has one
tenth the concentration of particles in the air.
Does that mean you could allow the part to be
exposed for 10 minutes in cleanroom B? And by
the way, why don’t standards indicating levels of
particles on the part mesh with levels of particles
in the air and with AMC (Airborne Molecular
Can you set up the equivalent of a five-second
rule for product exposure based solely on the
class of the cleanroom? The short answer is: no!
Cleanrooms are a tool to help lessen product contamination. A cleanroom in and of itself does not
correct a contaminated product nor does use of a
cleanroom prevent all sources of contamination.
So, given that it is not productive to stare at the
component through the protective packaging, how
do you develop an approach to managing product
exposure to the environment, even to a controlled
It is possible to use fallout prediction to develop
a rational, defendable estimate of allowable product exposure to a given environment. However,
realistic fallout prediction is data-based, and it is
Information about fallout prediction is available in the literature. Parasuraman, et al.1 use
experimental modeling employing a number of
standard test dusts and an aerosol dust generator
along with witness samples to characterize particle
fallout in a cleanroom.
The data varies significantly from a theoretical
model of Hamberg2, derived from the terminal
velocity of particles in still air. Hamberg’s equation, using metric units, is:
ṅ = 4. 8 105 Nc0.773