Openair plasma technology offers numerous application possibilities in the bonding sector. The process is particularly effective for cleaning surfaces. Metals can be cleaned quickly and selectively and thus prepared for wire bonding.
The process developed by Plasmatreat GmbH does not require the use of cleaning media, which saves costs and process times. "The cleaning of metals is achieved through three processes that have different effects and pursue different treatment objectives," says Nico Coenen, Global Business Development Manager Electronics Market at Plasmatreat GmbH, explaining the basic possibilities of plasma treatment. The first process, neutralization, involves the removal of both surface charge and statically bound dirt, such as dust particles. This is done by the charge carriers of the open-air plasma treatment. In the second process, highly volatile components such as moisture and VOCs (volatile organic compounds) are eliminated by evaporation through the thermal effect of the open-air plasma treatment. The final cleaning process is oxidation, which removes organic contaminants by means of oxidation. The reactive species of the open-air plasma break down hydrocarbon chains and break them down into smaller, volatile molecules (up to H2OandCO2).
Proof of successful treatment can be determined using atomic force microscopy, for example, which visualizes the change in the surface. This is a special scanning probe microscope that is used in surface chemistry for mechanical scanning of surfaces and for measuring atomic forces on the nanometer scale. The contact angle measurement can also be used to document the change in surface tension, e.g. caused by a drop of water. The water droplet on a plasma-treated surface changes its wetting properties in such a way that the contact angle and height are reduced, with a larger base and the same volume.
"The cleaning of metals is achieved by three processes that have different effects and pursue different treatment objectives."
This effect is achieved by surface cleaning with open-air plasma. Oxide surfaces in particular, but also contaminants such as flux residues or adhesives, disrupt the bonding process and prevent reliable connections. The open-air plasma removes both surface contamination and the oxide layer, revealing the clean surface of the metal alloy. This is beneficial in semiconductor production, for example, as particularly clean surfaces are required here in order to be able to reliably bond the finest wires. The plasma-treated surface allows bonded materials to form a more stable and larger-area connection.
A similar process is used to remove copper oxides, particularly in semiconductor and LED production. The chemical composition of solids and their surfaces can be determined non-destructively using X-ray photoelectron spectroscopy. After treating the copper, it can therefore be determined that the surface content of copper increases from 3% to 38%, with an accompanying reduction in the carbon content from 43% to 18%. "These changes clearly show that the copper oxide has been reduced and the copper surface area increased," says Coenen. The contact angle measurement also shows a comparable result. This means that bond wires also form reliable connections in this case.
"How well the individual substrates allow stable wire bonding depends on the respective material."
In addition to aluminum and copper substrates, nickel surfaces exhibit similarly good properties after treatment with open-air plasma. This is particularly important in battery production. As nickel oxide acts as a barrier layer that makes bonding with other materials much more difficult, it is essential to clean the nickel surface of oxides. Plasmatreat has therefore developed a special nozzle for this application that meets the general requirements and at the same time complies with the temperature requirements of the process step, i.e. does not exceed the limit value of 50 °C.
"How well the individual substrates allow stable wire bonding depends on the material in question. In any case, the upstream plasma surface treatment improves the application window of the wire bonding process. Wettability and adhesion are optimized," says Coenen.
Plasmatreat has achieved a leading international position in the development of atmospheric plasma technologies and plasma systems for the pre-treatment of material surfaces. The open-air plasma nozzle technology is used worldwide in automated and continuous production processes in almost all industrial sectors. The Plasmatreat Group has technology centers at its headquarters in Germany as well as in the USA, Canada, China and Japan. The company is also represented in more than 30 countries by its own subsidiaries and sales partners.
