CK PLASMA Coating 2.0

BY SEBASTIAN BECHMANN; HEAD OF APPLICATION, CHRISTIAN KOENEN GMBH AND KOENEN GMBH

Constantly rising demands regarding stencil technology promote and necessitate a continuous process of improvement. Particularly when the apertures and/or the spacing between apertures need to be reduced even further. At Christian Koenen, a solution has now been developed to improve transfer behavior, particularly for increasingly smaller apertures or deposit parameters. This solution is based on our in-house PLASMA coating process, which can be used for screens, stencils and squeegees. The Christian Koenen PLASMA Coating 2.0 – or CK PLASMA Coating 2.0 for short – is a classic, state-of-the-art, plasma-enhanced chemical vapor deposition process that has long been established in semi-conductor technology.

But what is PLASMA in reality? PLASMA is defined in physics and chemistry as the 4th state of matter. In this state, substances – preferably gases or gas mixtures – are broken down into their constituent parts. Ions and free charge carriers are produced during this breakdown process. Figure 1 shows the various states of matter and the designations of the phase transitions.

■ Figure 1: States of matter and transition phases.

How does this plasma-enhanced chemical vapor deposition process function? The process media are broken down into their elementary components during the plasma-enhanced chemical vapor deposition process (PECVD). These components are deposited on the surface and react chemically to form a new compound. Figure 2 displays the change in contact angle caused by our patented coating.

■ Figure 2 LEFT Contact angle on the PLASMA coating 104°; RIGHT Contact angle on the reference sample 66°.

■ Figure 2 LEFT Contact angle on the PLASMA coating 104°; RIGHT Contact angle on the reference sample 66°.

What advantages does this procedure offer? The layer thickness can be precisely defined using this complex process. The size of the aperture or thickness of the stencil makes no difference when this thin, homogeneous layer is deposited on the stencil. The PLASMA penetrates the smallest openings and creates a uniform layer. Another advantage is that additional lead time is not required for laser cutting. The Christian Koenen stencils are cut precisely to customer requirements at the highest quality standards.

A huge amount of work has gone into improving the CK PLASMA coating over the past few months as an innovative driver in the sector of screen and stencil manufacturing. This work is characterized by a higher chemical and mechanical resistance in combination with an improvement in printability.

 A chemical and mechanical stress test stand was set up for characterization, with the aim of testing all new layers for their resistance. The test cycle is carried out dry, with paste and with solvent, reflecting the conditions of underside cleaning. The resulting samples are continuously measured during the entire test with the contact angle measuring device and checked optically for damage. Figure 3 shows a part of the test curve.

■ Figure 3: Contact angle change dependent on chemical and mechanical stress test.

The evaluation shows that liquid coatings applied with a cloth, spray or by immersion, break down at the contact angle after just a few cycles and are therefore not resistant. These findings lead to the conclusion that an improvement in printability and/or cleaning is only temporary for volatile layers. If the abrasive effect of the PCB metallization is also taken into account, it can be assumed that the immersion, spray or cloth coatings will degrade even more rapidly. Mechanical abrasion of the material cannot be excluded here, which would, in turn, lead to contamination of the solder joint.

The second main point regarding characterization of the new functional layer concerns printing tests. Large batches were printed in the test setup and documented fully automatically via SPI. The resulting data was also evaluated fully automatically using an in-house macro. Print behavior could be directly compared under the premise that only the surface coating is variable. Christian Koenen GmbH has access to the modern equipment park at the Application Center for printing tests and evaluation. The center is kitted out with state-of-the-art equipment from ASYS, EKRA, ERSA and Koh Young. The Application Center is not just used to continually improve customer processes, but also to drive in-house development even further.

The outcome here is that printability, particularly in the Fine Pitch range, can be improved using the CK PLASMA coating 2.0. This results in a more stable process for the customer, particularly if the IPC limit values (AR limit value ≤ 0.66) are undershot. A printability improvement in the Fine Pitch range of up to 12%, compared to standard stainless steel, was achieved during the printing tests in the Application Center. Figure 4 show the difference in print behavior for a Fine Pitch layout. Apertures of 150 μm x 1250 μm were laser cut in 150 μm stainless steel. The corner radius is 75 μm.

■ Figure 4: RIGHT 3D image of the print pattern on the coated stencil; aperture 150 μm x 1250 μm; 150 μm stainless steel.

■ Figure 4: RIGHT 3D image of the print pattern on the coated stencil; aperture 150 μm x 1250 μm; 150 μm stainless steel.

Another focus in the development of the PLASMA 2.0 coating is the smoothing of the inner walls. Current technology means draws on electropolishing to smooth the inner walls of the apertures. However, this process step is only available for stencils with a material thickness exceeding 80 μm. Our new PLASMA coating supplements our portfolio perfectly: It offers a highly reproducible coating process, in addition to electro polishing, for successfully smoothing the inner walls without any limitations imposed by template material thickness. Figure 5 shows laser-cut apertures in stainless steel, coated with PLASMA 2.0.

■ Figure 5: Laser-cut apertures in stainless steel, coated with PLASMA 2.0.

SUMMARY OF CL PLASMA COATING 2.0 ADVANTAGES

• Improved release behavior

• Greater process safety

• Smooth inner walls, even without electro polishing

• High chemical resistance

• High mechanical resistance

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