Cleaning with atmospheric-pressure plasma

Only the devices of the "PlasmaBeam" series are used for the plasma cleaning of surfaces. Plasma APC500 type devices demonstrate virtually no cleaning effects. Therefore, they are essentially not mentioned here.

1. Design and operating principle

2. How does cleaning with atmospheric plasma work?

Plasma cleaning is an important process in plasma surface technology. Dirt particles are removed by chemical reactions with the ionized gases and the compressed air accelerated active gas jet, converted to the gas phase and blown away by the continuous active gas jet effect. The purity levels achievable are relatively high.

For copper oxide reduction, copper oxides are exposed to a hydrogen gas mixture plasma, and thus the oxides are chemically reduced and water is formed. These gas mixtures include Ar/H₂ or N₂/H₂ with a max. percentage of H₂ of less than 5%. For atmospheric-pressure plasma this only works with very high gas consumption.

3. How does atmospheric plasma work during cleaning?

3.1 Plasma cleaning of metals

Some items for treatment are covered with fats, oils, wax and other organic and inorganic contamination (as well as oxide layers).

For certain applications, it may be necessary to achieve absolutely clean and oxide-free surfaces e.g.

• before sputtering
• before painting
• before gluing
• before printing
• before PVD and CVD coating
• for special medical applications
• in analytical sensors
• prior to bonding
• prior to the soldering of printed circuit boards
• for switches etc.

The plasma acts here in two ways:

1. It removes organic coatings (C-containing impurities)

• These are chemically attacked by e.g. oxygen and air
• Then blown away from the surface by the excess pressure.
• Impurities are converted into smaller, stable molecules by the energetic particles in the plasma and can therefore be extracted.

The impurities may only be a few hundred nanometres thick, since the plasma is only capable of removing a few nm per transit.

Fats contain lithium compounds for example. Only the organic components can be removed from them. The same applies to fingerprints. That is why the wearing of gloves is recommended.

2. Reduction of oxides

• The metal oxide reacts chemically with the process gas. The process gas used is a mixture of hydrogen with argon or nitrogen. The thermal effect of the plasma jet can cause further oxidation. Therefore, it is recommended that processes are performed under a protective gas atmosphere (e.g. N₂ or Ar).

3.2 Plasma cleaning of plastics

In the atmospheric pressure plasma treatment, plasma cleaning cannot be separated from plasma activation.

The process gas usually dry and oil-free compressed air is used.

The principle is the same as for plasma cleaning of metals.

3.3 Plasma cleaning of glass and ceramics

The cleaning of glass and ceramics is carried out in the same manner as the cleaning of metals. Compressed air is used as a process gas for cleaning glass.

Cleaning is usually performed with compressed air.

Important parameters that must be taken into account here are spacing, speed and the repetition of the process (multiple treatment is advantageous).

4. Is the plasma jet/active gas jet potential-free?

Yes, the active gas jet of PlasmaBeam has zero or very low electrical potential. PlasmaBeams are therefore often used for the cleaning of electronic components.

The Plasma APC500 device type may be used in the treatment of non-conductive materials. The plasma jet of the Plasma APC500 is not potential-free.

5. What exhaust gases can occur during plasma cleaning?

Oxides of nitrogen NO and NO₂ result. A smaller amount of carbon-containing waste gases are of course also possible (CO₂, CO).

6. How wide is the treatment with the PlasmaBeam?

The treatment width of a nozzle is approximately 8-12 mm. However, the cleaning width must be pretested for each application (e.g. contact angle measurement).

The treatment width is slightly increased with the use of pure oxygen (O₂) or nitrogen (N₂).

7. What treatment speeds can be achieved?

In comparison to activation processes, the cleaning rate is only a few cm per second. An effective cleaning requires a temperature increase on the surface, which is only achievable by means of lower speed.

8. How hot is the jet temperature?

The average plasma jet temperature is approximately 200-250 °C. With the proper distance and speed setting, a surface temperature of around 70 - 80 °C is reached. This technique can therefore be used for all standard materials (metals, ceramics, glass, plastics, elastomers).

9. What is the lifetime of the plasma cleaning using atmospheric plasma?

Here, unfortunately, no reliable numbers can be claimed. The lifetime depends on storage conditions, treatment parameters, and the degree of cleaning.

Examples

1. Humid atmosphere and elevated temperatures (above 20 ° C) drastically reduce the lifetime of the plasma treatment.
2. Multiple treatment increases the lifetime of the treatment.
3. Generally speaking, for metals, glass and ceramic surfaces the following is recommended: Bonding, printing or coating should be performed within one hour after plasma treatment, in order to attain the maximum values.
4. With plastics, the following lifetime applies for the plasma treatment:

• PA (with or without glass fibre reinforcement(): 1 - 2 weeks
• PP, PE: We recommend further treatment within 1 to max. 2 days
• PC: 2 - 5 days
• ABS, PC/ABS: 2 - 5 days

Please note that these are approximate values. Depending on the manufacturer, significant differences can occur through the use of additives and release agents.

10. What are the main benefits for us of treatment using atmospheric-pressure plasma?

PlasmaBeam technology is applicable for in-line processes such as the plasma cleaning of endless metal profiles, pipes before jacketing, gluing, bonding or painting.

This technique can be used with robots, i.e. the 2- or 3-dimensional surfaces can be scanned with the plasma jet with the help of robots.

PlasmaBeam enables local surface cleaning without masking of residual surfaces e.g. cleaning of Al, Au and Cu, bond pads prior to wire bonding, without disturbing the rest of the surface.

11. Which applications are possible?

You can find more information under Applications.

With atmospheric plasma technology, gas is excited by means of a high voltage under atmospheric pressure, such that a plasma is ignited. The plasma is expelled by compressed air from the nozzle. There are two plasma effects:

Activation and precision cleaning is carried out by the reactive particles contained in the plasma jet.

In addition, loose, adherent particles are removed from the surface by the compressed air accelerated active gas jet.

The treatment performance can be affected in different ways by varying the process parameters such as treatment speed and distance from the substrate surface.

The atmospheric plasma processor PlasmaBeam is mainly used for local pretreatment of different surfaces (cleaning, activation):

• Polymers
• Metal
• Ceramic
• Glass
• Hybrid materials

The PlasmaBeam is suitable for robots and can be installed in existing, automated production lines without great effort.