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 variety of 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.
How does plasma cleaning work for metals?
Some components to be treated are covered with grease, oil, wax, silicones (non-PWIS-free, i.e. contaminated with Paint Wetting Impairment Substances) and other organic and inorganic contaminations (including oxide coats).
Certain applications require absolutely clean and oxide-free surfaces, e.g.
- before sputtering,
- before painting,
- before gluing,
- before printing,
- before PVD and CVD coating,
- with specific medical applications,
- with analytic sensors,
- before bonding,
- before soldering printed circuit boards,
- for switches, etc.
Here, the plasma acts in two different ways:
1. It removes organic layers
- These are chemically attacked for example by oxygen and air.
- Underpressure and the surface heating during low-pressure plasma cause some of the contamination to evaporate. With atmospheric pressure plasma, the overpressure blows the contamination off the surface.
- The high-energy particles in the plasma convert the contaminations to smaller, stable molecules fit for extraction/removal.
- Also UV radiation can destroy the contamination and detach it from the surface.
The thickness of the contamination particles must be limited to some micrometres since the plasma can only remove a few nm/s.
Greases contain lithium compounds, for example. Of these, the organic particles only can be removed. This is also true for finger prints. Accordingly, we recommend to wear gloves.
2. It reduces oxides
- Metal oxide reacts chemically with the process gas. Pure hydrogen or an argon-nitrogen mixture is used as process gas.
The processes in low-pressure plasma can also be run in two stages . For example, the items to be treated are first oxidised with oxygen for 5 minutes; then , they are reduced for 5 minutes with the argon-hydrogen mixture (e.g. 90% argon and 10% hydrogen).
How does plasma cleaning work for plastics?
Plasma cleaning of plastics is always accompanied by the activation of the plastic. If plastic is to be cleaned only without activation, simply continue reducing the process parameters until the desired effect has been achieved. However, it is advisable to check whether cleaning only is sufficient to prepare the component for the downstream processes.
Usually, technical oxygen is used as a process gas , but in many cases ambient air is sufficient; atmospheric pressure plasma usually uses dry, oil-free compressed air. Plasma treatment can be repeated as required and causes no toxic exhaust fumes.
The principle corresponds to plasma cleaning of metals.
How does plasma cleaning work for glass and ceramics?
Glass and ceramics are cleaned in the same way as metals. Argon or oxygen are suitable as process gases for glass cleaning, or compressed air for atmospheric pressure plasma.
As a general rule, cleaning is usually done with oxygen plasma .
The other parameters (pressure, generator output, gas flow, treatment duration) depend on the sensitivity of the components to be treated.
Can the weight loss be measured? (low-pressure plasma)
Yes, the etching rate for example can be determined by way of the weight loss using the homogeneity test.
For this test, object carriers covered with PE tape are weighed before and after plasma treatment. The balance then indicates the etching rate.
An analytical balance must be used for weighing because the weight loss is very small.
Is the plasma beam/active gas beam potential-free? (atmospheric pressure plasma)
Yes, the active gas beam of the PlasmaBeam has no or a very small electric potential. Accordingly, PlasmaBeams are often used for cleaning electronic assemblies.
Units of type Plasma APC500 can be used to treat non-conductive materials. The plasma beam of Plasma APC500 is not potential-free.
Which exhaust fumes can occur during plasma cleaning? (atmospheric pressure plasma)
Nitrogen oxides NO and NO2 are generated. Obviously, a smaller amount of carbonic exhaust fumes can also occur (CO2, CO).
What is the treatment width of the PlasmaBeam? (atmospheric pressure plasma)
The treatment width of one nozzle is approx. 8-12 mm. However, the cleaning width for every application must be checked beforehand (e.g. by measuring the contact angle).
Using pure oxygen (O2) or nitrogen (N2) slightly increases the treatment width.
Which treatment rates can be achieved? (atmospheric pressure plasma)
Compared to activation processes, cleaning proceeds at a rate of some cm per second only. For cleaning to be effective, the surface temperature must be increased which can only be achieved at a lower rate.
How high is the jet temperature? (atmospheric pressure plasma)
The mean plasma jet temperature is approximately 200-250 °C. With the proper distance and rate setting, a surface temperature of around 70 - 80 °C is reached. This technology can therefore be used for all standard materials (metals, ceramics, glass, plastics, elastomers).
What is the duration of action of plasma cleaning via atmospheric plasma? (atmospheric pressure plasma)
Unfortunately, it is not possible to give reliable figures here. The duration of action depends on storage conditions, treatment parameters, and the degree of contamination.
Examples:
- Humid atmosphere and elevated temperatures (above 20 ° C) drastically reduce the duration of action of the plasma treatment.
- Multiple treatment increases the duration of action of the treatment.
- In general, the following is recommended for metals, glass and ceramic surfaces: Gluing, printing or coating should be performed within one hour after plasma treatment for maximum success.
- Plasma treatment of plastics has the following duration of action:
- PA (with and without glass fibre reinforcement): 1-2 weeks
- PP, PE: We recommend follow-up treatment within the next 1 to max. 2 days.
- PC: 2-5 days
- ABS, PC/ABS: 2-5 days
Please note that these are approximations. Depending on the manufacturer, significant differences can occur due to the use of additives and release agents.
What are the main benefits of treatment with atmospheric pressure plasma? (atmospheric pressure plasma)
PlasmaBeam technology can be used for inline processes, e. g. plasma cleaning of continuous metal profiles, pipes before cladding, gluing, bonding or painting.
This technology can be used with robots, i.e. robots can scan the 2- or 3-dimensional surfaces with the plasma jet.
PlasmaBeam allows for local surface cleaning without masking of residual surfaces, e.g. cleaning of Al, Au and Cu bond pads prior to wire bonding, without affecting the rest of the surface.
