Non Aqueous Cleaning

19th February 2006

As long as people and things move inside a clean area, air borne particles will be found deposited on finished and semi finished products. Such particles are usually sub micron in size and either settle on a surface or are drawn there by Electro-static attraction. These particles need to be removed before they cause harm to the reliability and appearance of the products. There are several different wash processes for removing particles, most of which work very well, however in certain circumstances washing is not possible or effective.

Other processes which can be employed use mechanical forces to remove particles, generally with the assistance of some form of relaxant of the bonding electro static forces or by introducing kinetic energy in the form of a mass into the air flow.

Air or Nitrogen Guns

The most common use of this process is the Blow or Spray gun. This type of gun dispenses either Hi pressure clean dry air or for better results dry Nitrogen [N2], which is inert and will not cause other contamination. All wetted parts are PFA, TeflonR , PTFE and polyethylene on the N2 gun which can have a 0.5 um PTFE membrane micro filter. These units rely only upon the mechanical force of the hi pressure air or nitrogen, acting on a particle or surface contaminate to remove it. This is the least effective method and has little impact on electrostatically bonded particles, but is suitable for removing liquids and unattached particles.

Ionizing Guns

Many air borne sub micron particles are attracted to a surface or object by "Electro-static forces". Such particles are generally either positive or negatively charged in relationship to the environment around them. In scientific terms they have either more positive or negative electrodes in orbit than is stable for them. Nature being what it is, it seeks to establish an equilibrium, so the negatively charged particles seek out positively charged opposites and visa versa.

Therefore a charged non-conductor such as the product or work surface will attract air borne particles of opposite polarity, which will then bond to the surface through the electrostatic forces which exist between them, much like the experiment we all did at school where a magnet attracts iron fillings, or the back of an old television set which will have far more attracted particles than could ever arise from settlement alone.

As many of the materials in a cleanroom and a lot of other products are made from non-conducting materials, which are unable to drain away surface charges, the problem can often become very acute.

In these instances a Blow or Spray gun dispensing either high pressure clean dry air or N2, connected to a voltage generator which is capable of generating either positive or negative ions at an emitter point in the air stream, is more effective.

The positive ions in the air stream seek out the negative charged particles on surfaces and products and have the effect of relieving or neutralising the bonding electrostatic forces between particles and surface; the mechanical forces in the hi pressure air flow do the rest. As the power supply produces both negative and positive ions, the effect will be the same on the voltages of the opposite polarity. The guns are made with an aluminium housing and have a 0.45 um disk for optimal cleanliness and are connected to the power unit by an 2 meter shielded and grounded lead. This system is very effective particles which are lightly bonded, but the velocity of the vapour flow combined with the relaxation of the electrostatic forces is insufficient to provide complete cleaning.

Sno Guns

Simply depending on air flow has its limitations as the air's velocity reduces the closer you get to the surface air boundary, where it must eventually become zero, considerably reducing its effectiveness on small particles. More mechanical energy can be introduced into the air flow by adding mass in the form of minute Snow Flakes. This method introduces purified Carbon Dioxide [CO2] expanded from liquid through a hand held gun. The resultant snow flakes physically intercept with attached particles and the kinetic energy employed knocks the particles free. The released particles are then carried away in the vapour stream which sublimes. A similar effect could be achieved by using sand or talc, however these products may cause damage to the surface or product being cleaned, CO2 leaves no residues. Adjustments of the Sno Gun nozzle can produce a mini ice storm through to gentle snow flakes, ideal for cleaning the most delicate products.

By attaching an ionising power supply to the gun, the additional advantages of relaxing the electrostatic forces between the particles and surface can be achieved, greatly enhancing this method of particle removals effectiveness. As CO2 in the liquid phase also acts as a solvent it will remove finger prints and dissolve other organic particles. Most surface contaminates down to 0.1 micron can be effectively removed, leaving the product 100 percent prepared for future use.

- Alan Waterman-Smith