Ultra-fine contaminations not visible to the naked eye are found on all surfaces. Almost always, these contaminations must be removed as a prerequisite for faultless further surface treatment such as:
gluing | printing | painting | bonding | coating | etching
Plasma technology offers solutions for any type of contamination, for any substrate and any post-treatment. In the process, molecular contamination residues are decomposed as well. Different cleaning processes are available for various requirements in the individual case.
How does plasma cleaning work?
Plasma cleaning is an important process of plasma surface technology. Dirt particles are removed by chemical reaction with the ionised gases.
For copper oxide reduction, copper oxides are exposed to a hydrogen gas mixture plasma; here, the oxides are chemically reduced, generating steam.
Plasma
Carbon polymer
Hydrogen
Oxygen
Oxygen
Hydrogen
1. Removing hydrocarbons in the oxygen plasma
Micro-cleaning – Degreasing in oxygen plasma
On nearly all surfaces, there are hydrocarbons as residues of greases, oils, or release agents. These coatings severely reduce the adhesion of other materials during subsequent surface treatment. For this reason, chemical removal of hydrocarbons in the oxygen plasma is a standard treatment before coating, printing or gluing.
The plasma reactions during this cleaning process are illustrated by way of example in the section “Small plasma physics”.
Ions, radicals and UV radiation act together. High-energy UV radiation splits macromolecules. Oxygen radicals, ions and split-off hydrogen radicals occupy free chain ends of the polymer chains to H2O and CO2.
Gaseous in the low-pressure plasma, the degradation products of the hydrocarbons are siphoned off.
On polymeric surfaces, activation by oxygen radicals starts in parallel with the decomposition of surface contamination. This activation is a prerequisite for proper adhesion of non-polar plastics. For details, see Activating materials.
Oil, grease or release agents containing additives cannot always be completely removed in oxygen plasma. Solid oxides may form which adhere to the substrate. If necessary, these can be purified in additional downstream purification processes.
Cleaning in oxygen plasma works on virtually all materials. Often, purified dry air can be used instead of oxygen. Accordingly, hydrocarbons can be removed both in low-pressure plasma and in atmospheric pressure plasma.
2. Mechanical cleaning by micro-sandblasting
Argon plasma
Noble gas plasma is a particularly simple plasma. It consists only of ions, electrons, and noble gas atoms. As the gas is always atomic, there are no radicals and, since noble gases do not react chemically, there are also no reaction products. Because of the kinetic energy of the heavy ions, argon plasma is active nevertheless.
Cleaning
The kinetic energy of the impacting ions chips away at the atoms and molecules forming the coat, gradually decomposing them.
The treatment is effective on virtually all surfaces, and thus on any kind of contamination. Almost all contamination that resists chemical attack can be removed by micro-sandblasting.
As the positively charged ions are accelerated to a negatively charged electrode, plasma excitation occurs in a parallel plate reactor.
Structuring - physical etching
High-energy ions chip away fragments not only off the surface coating but also off the substrate material itself. This results in increased structuring and roughening of the surface on a molecular scale. As with sand blasting or grinding, this leads to an increase in surface area and possibly also to undercuts and improves the adhesion of subsequently applied coatings.
Contrary to chemical etching effects in low-pressure plasma, micro-sandblasting is not isotropic, i.e. it does not act evenly on all surfaces of a component, but mainly in the direction of the electric field because the ions are accelerated in this direction.
3. Reduction of oxide layers
Oxide layers are found on many surfaces. After long storage, most metals tend to form oxides. On many metals, oxide layers form during plasma cleaning in oxygen plasma. These oxide layers interfere with all further processing stages:
- Adhesion of electrical contacts during bonding, soldering
- Poor electrical contact
- Poor adhesion when gluing or painting
Often, solid oxidised deposits are also found on non-metals which sometimes only form during cleaning in an oxygen plasma. Oxide layers often withstand any attack by conventional solvents. Because of their high hardness, it is even difficult to get rid of them mechanically. They are removed by reduction in hydrogen plasma.
Oxidation
In oxygen or air plasma, it is possible to oxidise extremely thin metal coats of the thickness of only a few atoms layers. These invisible coats harden and protect the metal from chemical and mechanical attack and further oxidation. They ensure a permanent metallic shine.
Surface oxidation is often carried out in atmospheric pressure plasma.
In many cases, various types of contaminants need to be removed from a surface; accordingly, different cleaning processes are applied in sequence, such as:
1. Removing release agents (hydrocarbons) in the oxygen plasma
2. Micro-mechanical ultra-fine cleaning by micro-sandblasting in argon plasma
or:
1. Degreasing in oxygen plasma
2. Reduction of oxide layers in hydrogen plasma
On the other hand, oxygen purification is directly followed by the activation of non-polar surfaces by means of oxygen radicals. To this end, the process is continued for some time after cleaning. For details, see Activating materials and for even more prolonged reaction, downstream etching of materials.
How can you produce PWIS-free surfaces?
Paint wetting impairment substances – PWIS for short – cause clearly visible defects in the end product, since they prevent an even wetting of the surfaces to be coated. Funnel-shaped impurities and craters appear in the paint layer. Such substances can be silicones, fluorinated (PTFE) substances, certain oils and greases.
The plasma process permanently dissolves all paint wetting impairment substances from the surface as well as from the elastomer itself.
Components made of a large varietyof materials can be cleaned, such as PVC-U, PVC-C, PP, PE, ABS and PVDF as well as metal components.
After cleaning, the components undergo plasma treatment for up to one hour, depending on their degree of contamination. To confirm the success of the treatment and ensure the components are PWIS-free, a PWIS test is carried out after the plasma treatment. The test we use is in compliance with the Volkswagen Test Regulation PV 3.10.7 and is a rapid test for the detection of residual silicones.
All you need is a clean glass plate, acetone, a commercially available spray paint which obviously must be free of silicones. The colour white has proven to be particularly well suited for this test. For the test, the material is placed on the glass plate and rinsed with acetone. After venting of the acetone, the glass plate is sprayed crosswise with the spray paint. After drying of the paint, it is clearly visible whether there are silicone residues on the surface. In these spots, the surface is not wetted by the paint, and so-called crater formation is observed.
Using special processes, however, plasma treatment is also suitable for treating silicone materials. Even silicone rubber can be made PWIS-free.
Removing PWIS substances from component surfaces meant to be coated, innovative and environmentally compatible low-pressure plasma technology can solve a problem which is becoming more and more pressing. Plasma cleaning processes integrated in the production chain have for example the following benefits:
- Reduction of the rework rate
- Reduction of the scrap rate
- Prevention of customer complaints
- Increased reliability of the production process
Diener electronic also offers this process as a surface treatment service. Several plasma systems as well as skilled and experienced staff are at your disposal. Thus, we can ensure optimum surface quality of your components and parts.
FAQ Frequently Asked Questions
What types of impurities can be removed?
All organic, film-like contaminants up to a size of approximately 1 micrometre can be removed using Diener electronic’s plasma systems.
What are the main benefits of treatment with atmospheric pressure plasma?
PlasmaBeam technology is suitable for in-line processes, e.g. plasma cleaning of continuous metal profiles and tubes prior to coating, gluing, bonding or painting.
This technology is suitable for use with robots, i.e. the 2- or 3-dimensional surfaces can be scanned with the plasma beam using robots.
PlasmaBeam enables localised surface cleaning without the need to mask the remaining area; for example, cleaning Al, Au and Cu bond pads prior to wire bonding without coming into contact with the rest of the surface.
Is atmospheric plasma generally recommended for surface cleaning?
Yes, but to a very limited extent. Low-pressure plasma offers significantly greater process flexibility when it comes to cleaning processes.
Can metal chips or dust be removed from components?
Unfortunately, it is not possible to clean metal shavings or dust from the surfaces.
Is low-pressure or atmospheric-pressure plasma the better solution?
When it comes to surface treatment at Diener electronic, there is no such thing as ‘better or worse’, but rather ‘suitable or unsuitable for the application’.
