NanoWork® stencils, the nanocoated stencils

NanoWork® stencils produced by LaserJob are based on the manufacture of laser-cut SMD stencils. The NanoWork® coating is applied to the underside (PCB side) of the stencil and into the apertures. For this purpose, the cutting burr on the laser exit side must first be removed; this is done by post-processing the cut stencil using a CNC-controlled brush. The squeegee side is not coated to prevent any impact on the rolling behavior of the solder paste. The maximum layer thickness is 2 μm.

Micrograph NanoWork® stencil:

The NanoWork® coating is characterized by anti-adhesive properties that greatly reduce the adhesion of solder paste. This anti-adhesive effect in the apertures results in significantly better contour sharpness and a significantly higher and more consistent transferred solder paste volume. The result is greater process reliability and a lower soldering error rate during the placement and soldering process. Transferred solder paste deposits remain constant throughout the entire printing process. Cleaning cycles on the underside of the stencil are greatly reduced due to the NanoWork® coating, which prevents solder paste from sticking. As a result, more printing cycles can be performed without cleaning steps. With the help of LaserJob's NanoWork® coating, finer structures can be achieved without compromising print quality.

LaserJob is therefore helping to meet the increasing demands for miniaturization:

• optimized aspect ratio and surface area ratio
• significantly fewer cleaning cycles for the underside of the stencil
• better contour sharpness
• excellent release behavior
• consistent solder volume transfer

The following graphic shows the release behavior of a NanoWork® stencil compared to a laser-cut stainless steel stencil, using a BGA component as an example. The apertures of 400 to 250 μm are shown in relation to the surface area. With an area ratio of 0.5, up to 80% of the solder paste volume is transferred with a NanoWork® stencil, with adjusted solder powder grain size. On the other hand, an uncoated laser-cut stainless steel stencil only achieves 50% using solder powder grain size 3 (25–45 μm) and less than 70% using solder powder grain size 4 (20–38 μm). This example clearly illustrates the advantage of a Nano-Work stencil. 

The transfer efficiency of a NanoWork® stencil compared to uncoated laser-cut stainless steel stencils is illustrated using the example of a QFP structure (400 μm). In this case, 16 printing cycles were performed without cleaning the underside of the stencil, and the volume of solder paste transferred was recorded. As the diagram illustrates, a NanoWork® stencil can transfer 100–110% solder paste consistently throughout the entire printing cycle, while the uncoated stencil's transfer volume increases to 130% due to increasing pollution of the underside. The transfer efficiency of the uncoated stencil differs significantly from that of the NanoWork® stencil.

QFP-Struktur (400µm)

Quality control is a top priority at LaserJob. Strict quality monitoring begins as soon as the stainless steel sheets and frames arrive. Each stainless steel sheet is measured to an accuracy of ± 0.5 μm using a thickness gauge. The screen tension is checked on every stretched screen printing frame. Immediately after laser cutting, the aperture size and geometry are verified. A measuring system is used to scan the aperture positions over an area of 400 x 200 mm with an accuracy of 2.5 μm + L /400. The hole contour is determined with a precision of 0.5 μm by a CCD camera using a transmitted light method. The finished stencil is compared with the original data using ScanCheckI+ and checked for congruence.

In order to perform a basic quality check on the NanoWork® coating, images were taken using a scanning electron microscope (SEM) to verify the homogeneity of the coating both in the apertures and on the underside of the stencil. For comparison, an SEM image was taken of a QFP aperture on a laser-cut stainless steel stencil and a NanoWork® stencil.

Laser-cut stencil:

Nanocoated stainless steel stencil:

Energy dispersive X-ray spectroscopy (EDX) reveals that the NanoWork® coating is evenly distributed across the aperture walls, from the bottom to the top edge.

The anti-stick properties of a coated stencil correlate with its surface tension. The surface tension of solid materials is defined by the contact angle of a measuring liquid with the material. In this context, contact angle refers to the angle formed by a drop of liquid on the surface of a solid material relative to that surface.

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An elevated contact angle indicates good non-stick properties, while a low contact angle indicates good wetting and therefore good adhesive properties.

Contact angle measuring is used to monitor the non-stick properties of the Nano-Work coating. The measurement is performed and documented for each template and is computer-controlled with high accuracy and repeatability.

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