This column is about the philosophy of cleaning. It's about the "philosophical laws" behind cleaning. It's about how those laws describe all the cleaning work that we do, whether in cleanrooms or repair shops. And it's about why you would want to know them.

Four laws or principles of industrial cleaning are common to all cleaning endeavors. They set boundaries on what we can and cannot do, and how well wecan and cannot do that.

KESEY'S LAW OF CLEANING
The most important is one often attributed (falsely, I believe) to the novelist Ken Kesey, author of One Flew Over the Cuckoo's Nest; it is also attributed to someone named Imbesi (and called their “Law of Fifth”). Andit's simple:

*Whenever something becomes clean, something else becomes dirty.

We don't clean the particles from semiconductor surfaces. We don't clean the pyrogens from medical implants. We just move (hopefully) the particles and the pyrogens from where they are to where we want them to be. The particles are suspended in a moving stream of fluid, which can infect another surface. The pyrogens are bound by intermolecular force to a detergent in water, which can also infect another surface.

CLEANING THERMODYNAMICS
The second law of surface cleaning is a modification of the first:

*Soil is like entropy — never destroyed, always created.

A perfectly designed cleaning machine doesn't destroy soil. It doesn't convert it to CO2 and water. It doesn't convert it to protons, neutrons, and electrons. It doesn't convert it to energy. It just moves it from one place to another. This law is also important to you — the more solder you apply to a junction,the more flux residue there will be to be cleaned.

GOOD ENOUGH IS ALL YOU'RE GOING TO GET
As classical thermodynamics teaches that no process is perfectly and totallyreversible, the third law of cleaning is:

* You can never get something completely clean.

If a perfectly clean surface is contacted with a cleaning agent containing one molecule of soil, that soil molecule can be transferred (at some slow rate) to the clean surface and infect it. Then the driving force is reversed, and the soil molecule can be transferred back (at some slow rate) to the cleaning agent. The concept of a limiting driving force explains why perfect cleaning cannot be obtained in any single stage of cleaning work. It's not an abstract concept.

THE UNCERTAINTY PRINCIPLE
The uncertainty principle in cleaning science originates in nuclear physics. Applying Heisenberg's Uncertainty Principle to cleaning operations:

* To get a particle off a surface, first you have to find both.

Cleaning work can be omnidirectional (as with ultrasonic transducers). Cleaning work can also be directional (as with megasonic transducers).

Consider aqueous spray cleaning machines. If the nozzles aren't aimed at the current position of the soil on the parts, you've wasted your work. Worse, when the parts are dried, the soil may be baked on!