The third dimension of stencil printing - precision & efficiency

According to studies, 50% of all soldering defects in SMD assembly production are still closely linked to the printing of the solder paste. The right choice of a high-precision stencil printer, adapted printing parameters and the optimum precision stencil are therefore crucial for a high yield. High-precision milled stepped stencils offer the most efficient and economical way of applying solder paste.

Step Stencil 3D Christian Koenen — Stepped stencils are used when the amount of paste applied can no longer be adequately regulated via the adjustment of the apertures.

However, a stepped stencil is not simply a stepped stencil:

The volume of the solder paste depot is largely determined by the material thickness of the print stencil used. This means that if components with a height variance in terms of component size or solder paste requirement (inappropriate component mix) are provided on a PCB, the optimum depot volume can no longer be generated with a conventional stencil (insufficient paste quantity).

“Components such as SMD PCB terminals pose particular challenges, as they very often have deviations of up to 100 μm between the contact surfaces of the component and the contact surface,” says Sebastian Bechmann, CTO of the Christian Koenen Group. They require a stencil thickness of around 150 to 200 μm (defective coplanarity). The depots for SMD components of size 0402, on the other hand, are best printed with stencils that are 120 to 150 μm thick. Insufficient paste quantities also frequently occur in the combination of μBGA and connector or SMT components and THR components.

Patented, milled and laser-cut high-precision stepped stencils

Even when Fine-Pitch components are placed close to standard components (mixed configuration), conventional stencils reach their limits and can, for example, cause a bridge short circuit. Particularly for mixed configurations with small grid sizes and a high number of components, a high-precision milled and laser-cut stepped stencil, which is optimally adapted to the layout, is the most efficient and economical option to avoid soldering defects from the outset.

In addition to mixed configuration, there are other relevant indicators that speak in favor of using a stepped stencil. Such as when soldering paste is to be applied on different levels. This is required, for example, when individual components are to be sunk in the PCB or PCB areas of different thicknesses, such as rigid and flexible PCB areas of an assembly, are to be printed in one step (sunken components).

Due to the constant technological development in the field of printed circuit boards and components, new challenges for high-precision printing arise in parallel. If the substrate to be printed has particularly sensitive areas such as bonding surfaces, chip mounting (Attach) or point protrusions due to substrate unevenness, solder resist or protruding via plugs, a conventional printing stencil can damage sensitive areas or cause an unintended jump. “Unwanted, uncontrolled jumps in the stencil image occur more and more frequently after multiple use,” explains Sebastian Bechmann. This can result in more frequent cleaning cycles, printing errors or even damage to the assembly. Stepped stencils offer a solution in these cases: they can be provided with cavities at critical points to protect sensitive areas or compensate for unevenness in the substrate (unevenness on the PCB).

In addition, the CK stepped stencils enable combination printing: some SMT shapes need to be fixed to the PCB so that they cannot stand up or move during the soldering process. With the help of the stepped stencil, the adhesive can be applied directly after printing the solder paste (print on print).

Due to the unbroken trend towards ever higher packing densities, an increased component mix and the use of ever smaller components, the step stencil is expected to become increasingly important in assembly production. The rapidly increasing number of assemblies also presents some of the challenges described above, for which high-precision stepped stencils will be used more and more frequently.

Plasma Coating Stencil Christian Koenen — Stepped stencil with plasma coating: The printing process is significantly improved and thus increases the yield of the production line.

Plasma 3 coating to minimize production costs and increase production yields

More and more users are benefiting from another Christian Koenen innovation: the milled, laser-cut stepped stencil with Plasma 3 coating. This uses plasma-assisted chemical vapor deposition to reproducibly apply a coating on the substrate side and in the apertures, which improves the transfer behavior of the solder paste (defective paste release behavior).

The third generation of plasma coating (Plasma 3.0) is characterized by very low adhesion of the print medium to the inner wall, especially in the smallest apertures. The smearing of the paste on the underside of the stencil is reduced and the number of cleaning cycles can be reduced, which leads to savings in production costs and throughput time. In addition, the coating, which is only a few nanometers thick, is chemically and mechanically resistant.

3D Step Stencil Electronic Production Christian Koenen — Stepped stencils are used to compensate for height differences in the PCB surface, so that an artificial jump and the associated smearing of the underside of the stencil is avoided.

Electronics production with a fully automated printing process

The following real challenges in the precision printing process occur not only individually, but increasingly also in various combinations:

insufficient amount of paste
lack of coplanarity of the component contacts
bridge short-circuit formation
sunken components
unevenness on the printed circuit board
print on print
poor release behavior of the paste from the stencil opening
longer downtimes for stencil cleaning

To support Smart Factory and Industry 4.0 efforts, it is necessary that all production steps can be carried out with the least possible intervention by machine operators. The high precision and customer-specific design of the stepped stencils are absolutely essential so that the fully automated stencil printing process in electronics production can be adapted to ever smaller component sizes, new component shapes and the increased mixing of large and small components. The error rate in stencil printing has been significantly reduced, not least thanks to our employees' many years of experience and the Christian Koenen Group's continuous investment in innovations relating to precision printing tools. As miniaturization and the component mix continue to increase, it is to be expected that investments in creative solutions for the design of printing tools will continue to be necessary. Many of our international customers have confirmed that the customized milled and laser-cut high-precision stencils with Plasma 3 coating are now innovative and reliable solutions for the increasingly complex stencil printing applications in SMD and semiconductor manufacturing.

Summary

Stepped stencils are becoming increasingly important in the production of electronic assemblies and are used when the amount of paste applied can no longer be adequately regulated via the adjustment of the apertures. The printing process is additionally improved and optimized by the plasma coating.

Newsroom

The third dimension of stencil printing - precision & efficiency

However, a stepped stencil is not simply a stepped stencil:

Patented, milled and laser-cut high-precision stepped stencils

Plasma 3 coating to minimize production costs and increase production yields

Electronics production with a fully automated printing process

The third dimension of stencil printing - precision & efficiency

3 steps to progress in electrical engineering

Application Center: Your partner for optimizing printing processes