When the Stencil Meets Semi-Conductor Technology
BY SEBASTIAN BECHMANN, HEAD OF APPLICATION, HIGHTECH STENCILS
Constantly increasing requirements for stencil technology both advance and challenge the continuous improvement process. This applies, above all, when apertures and/or the distance between the apertures are further reduced. This is primarily the case for stencils used in semi-conductor technology.
At Christian Koenen, a solution has been developed for improving the transfer behaviour particularly for constantly smaller apertures or deposit sizes. Here, our in-house designed plasma coating is used, which can be applied to screens, stencils and squeegees. The plasma coating is a classical plasma-enhanced chemical vapour deposition, which has been used as the established method in semi-conductor technology for years and is the state of the art.
But what, actually, is a plasma? In physics and chemistry, plasma refers to the 4th fundamental state of matter. A substance, preferably gases or gas mixtures, is broken down into its basic components. During the decomposition, ions and free charge carriers are produced. Figure 1 illustrates the different fundamental states of matter and the designation of the phase transitions.
What is a plasma-enhanced chemical vapour deposition? In the plasma-enhanced chemical vapour deposition, the process media are broken down into the elementary components. These deposit on the surface and react chemically to form a new compound.
Consequently, the plasma coating permits significant adjustment of the surface characteristics of the stencil. This results in an improved contact angle or respectively reduced adhesive forces on the stencil and in the aperture.
2 illustrates the change in contact angle achieved by our patented coating.
What are the benefits of the process? The complex process permits accurate definition of the layer strength. This measures only a few nanometres on all areas to be coated. Thanks to this thin and homogeneous deposition, the size of the aperture does not make any difference during application on the stencil. The plasma penetrates the smallest apertures producing an even layer. It is also important that no additional allowance is required during laser cutting. Christian Koenen stencils are cut to the highest quality exactly to the customer specifications.
During the process, the strength of the deposited layer is monitored continuously. For this, a special layer thickness measuring device is procured that measures the spectral reflectance. The measuring device calculates the exact layer thickness from several displacement/time measurements of the reflections.
We recommend a contact angle measuring instrument for measuring the function of the deposited layer, which permits reproducible and objective verification of the quality of the contact angle.
The result of a plasma-coated stencil is a significantly improved paste transfer. Nevertheless, we recommend that the limit value for the area ratio of the stencil of ≥ 0.66 should be observed to ensure a stable process. Otherwise, minor variations may occur in the process chain (quality control loop and its influencing variables in accordance with DIN EN ISO 9000 and following). Figure 3 illustrates the improvement of the transfer behaviour, proven in a laboratory test.